Nicolas Auffray
email: nicolas.auffray@univ-mlv.fr

Boris Kolev
email: kolev@cmi.univ-mrs.fr

Michel Petitot
email: michel.petitot@lifl.fr

Invariant-based approach to symmetry class detection

Keywords: Mathematics Subject Classification. 74B05, 15A72. Key words and phrases. Anisotropy, Symmetry classes, Invariant theory. 1 N. AUFFRAY, B. KOLEV, AND M. PETITOT 6. Conclusion 25

come

Introduction Physical motivation

Introduction

Physical motivation. In the theory of linear elasticity, the stress tensor σ and the strain tensor ε are related, at a fixed temperature, by the Hooke's law

σ ij = C ijkl ε kl
If the medium under study is homogeneous, its elastic behavior is fully characterized by a 4th-order elasticity tensor C. The infinitesimal strain tensor ε is defined as the symmetric displacement gradient

ε ij = 1 2 (u i,j + u j,i )
and, assuming that the material is not subjected to volumic couple, the associated stress tensor is the classical symmetric Cauchy's one. Hence, both σ and ε belong to the 6-D vector space S 2 (R 3 ), where S 2 denotes the symmetric tensor product. As a consequence, the elasticity tensor is endowed with minor symmetries:

C ijkl = C jikl = C ijlk .
In the case of hyperelastic materials, the stress-strain relation is furthermore assumed to derive from an elastic potential. Therefore the elasticity tensor is the second-derivative of the potential energy with respect to strain tensor. Thus, as a consequence of Schwarz's theorem, the hyperelasticity tensor possesses the major symmetry:

C ijkl = C klij .
The space of elasticity tensors is therefore the 21-D vector space Ela = S 2 S 2 (R 3 ). To each hyperelastic material corresponds an elasticity tensor C but this association is not unique, there is a gauge group. Indeed, a designation based on the components of C in a fixed reference frame is relative to the choice of a fixed orientation of the material. If this problem is invisible for isotropic materials, it becomes more prominent as the anisotropy increases. For a fully anisotropic (triclinic) material how can it be decided whether two sets of components represent the same material ? To address this problem some approaches have been proposed in the literature so far. Roughly speaking, those propositions are based on the computation of the elasticity tensor spectral decomposition. But despite of being theoretically well-founded, the symmetry class identification relies on the evaluation of the roots multiplicity of the characteristic polynomial. Such a problem is known to be highly ill-posed [START_REF] Zeng | Computing multiple roots of inexact polynomials[END_REF]. Therefore this approach is difficult to handle in practice especially working with noise corrupted data.

In order to avoid the computation of the spectral decomposition an alternative approach based on the invariants (and covariants) of the elasticity tensor is presently considered. Such a way to proceed is in the perspective of Boheler and al. [START_REF] Boehler | On the polynomial invariants of the elasticity tensor[END_REF] original work. It is interesting to notice that the general framework of the present paper is already well-known by the high energy physics community [START_REF] Abud | The geometry of orbit-space and natural minima of Higgs potentials[END_REF][START_REF] Abud | The geometry of spontaneous symmetry breaking[END_REF][START_REF] Sartori | Tools in the orbit space approach to the study of invariant functions: rational parametrization of strata[END_REF]. However, these methods do not seem to have been yet applied in elasticity and especially for a representation as sophisticated as H 4 (R 3 ), which invariant algebra is not free. Reconstruction method based on this invariant-based approach, and numerical comparisons between approaches will be the objects of forthcoming papers. Before introducing in depth the proposed method, we wish to draw a short picture of this approach. Therefore, the next paragraph will be devoted to present the method in a nutshell.

Invariant-based identification in a nutshell. As the material is rotated by an element g(= g j i ) ∈ SO(3), the elasticity tensor C moves under the action of the 3D rotation group SO(3) on the space of elasticity tensors Ela C ijkl → g i p g j q g k r g l s C pqrs . Hence, from the point of view of linear elasticity, the classification of elastic materials can be assimilated to the description of the orbits of SO(3)-action on Ela. Practically, this is a difficult problem because the orbit space of a Lie group action is not a smooth manifold in general. Usually, the orbit space is a singular space which can described by the mean of the isotropic stratification [START_REF] Abud | The geometry of spontaneous symmetry breaking[END_REF] (described in section 1). In the field of elasticity, this isotropic stratification was first established by Forte and Vianello [START_REF] Forte | Symmetry classes for elasticity tensors[END_REF] 1 .

Since the rotation group SO(3) is compact, the algebra of invariant polynomial functions on Ela is finitely generated [START_REF] Weyl | The classical groups[END_REF] and separates the orbits [2, Appendix C]. Therefore, it is possible to find a finite set of polynomial invariants {J 1 , . . . , J N } which define a polynomial function

J : Ela → R N such that J(C) = J(C ′ ) iff C ≈ C ′ .
This way, an SO(3)-orbit can be identified with a point in J(Ela) ⊂ R N . A classical result from algebraic geometry states that the image of a R-vector space under a polynomial map is a semialgebraic set, that is a subset of R N defined by a boolean combination of polynomial equations and inequalities over R [START_REF] Coste | An Introduction to Semialgebraic Geometry[END_REF]. The orbit space is therefore a semi-algebraic set. The invariants {J 1 , . . . , J N } are polynomials in 21 variables (dim Ela = 21). To proceed towards our goal, we will make an extensive use of the decomposition of Ela into SO(3)-irreducible components, also known as the harmonic decomposition (c.f. section 3):

Ela := S 2 S 2 (R 3 ) ≃ 2H 0 (R 3 ) ⊕ 2H 2 (R 3 ) ⊕ H 4 (R 3 )
where H n (R 3 ) is the space of nth-order harmonic tensors on R 3 . Because of the lack of ambiguity, this notation will be shorten H n . It is well known that dim H n = 2n+1 for all n ≥ 0. Historically this decomposition has already been used in the study of anisotropic elasticity tensors [START_REF] Backus | A geometrical picture of anisotropic elastic tensors[END_REF][START_REF] Jerphagnon | The description of the physical properties of condensed matter using irreducible tensors[END_REF][START_REF] Baerheim | Harmonic decomposition of the anisotropic elasticity tensor[END_REF][START_REF] Forte | Symmetry classes for elasticity tensors[END_REF]. We therefore refer to these references for a deeper insight into this topic. It is known [START_REF] Forte | Symmetry classes for elasticity tensors[END_REF] that Ela is divided into 8 conjugacy classes for the SO(3)-action. For each isotropy class [H], we build a linear slice Ela H and we show that the equivalence relation C ≈ C ′ where C, C ′ ∈ Σ [H] reduces to the linear action of a certain monodromy group Γ H on Ela H . The cardinal of Γ H is equal to the number of intersection points of a generic orbit in Σ [H] with the linear slice Ela H . The closure of each stratum Σ [H] , which we denote by Σ [H] and refer to as a closed stratum, is a semialgebraic set which we describe explicitly by a finite number of equations and inequalities on the invariant polynomials {J 1 , . . . , J N }. For each class [H] for which Γ H is finite, we give explicit relations between invariants to check whether C belongs to this class.

Organization of the paper. The first section of this paper (section 1) is devoted to the introduction of the framework uses for this study. There is no claim for any originality here, similar material can be found in [START_REF] Golubitsky | Singularities and groups in bifurcation theory[END_REF][START_REF] Sartori | Tools in the orbit space approach to the study of invariant functions: rational parametrization of strata[END_REF]. We have tried to make this paper accessible with only few prerequisites. The notions of stratification and linear slice are defined, together with a group theoretical method to compute its associated degree. In the second section (section 2), we illustrate these concepts on a simple example: the stratification of the space of quadratic forms on R 3 under the SO(3)-action and we study the stratification of a n-uple of quadratic forms. This constitutes an extension of results on the stratification of 2 quadratic forms. Our results are provided in terms of harmonic tensors, which is not so usual. In section 3, the harmonic decomposition is introduced and the stratification of Ela discussed. The geometrical concepts introduced in section 1 will be enlighten in view of well-known results of anisotropic elasticity. In section 4, we study the strata of H 4 which have finite monodromy group. In each situation the stratification is established and algebraic criteria identifying symmetry classes are provided. These criteria are, up to authors' best knowledge, new. This section will be closed by a bifurcation diagram that sums up the symmetry breaking from H 4 together with the algebraic relations that induce those transitions.

According to the restriction to linear slices, only 6 out of the 8 elastic symmetry classes possess linear slices with finite monodromy group. Therefore, in the present paper, algebraic relations are only provided for the following anisotropic classes : isotropy (

[SO(3)]), cubic ([O]), transverse isotropy ([O(2)]), tetragonal ([D 4 ]), trigonal ([D 3 ]), orthotropic ([D 2 ]
). Nevertheless, the combination of results from section 2 and section 4 completed by an assumption on the genericity of anisotropic of the elasticity tensor understudy (in a sense that will be defined in section 5.) allow the 8 symmetry classes to be identified. This paper will be concluded in section 6 and the extension of the method to a broader class of situations will be discussed.

Main results. The main results of this paper are

• In section 3 a rigorous geometric framework is provided that allows to recover important features of anisotropic elasticity [START_REF] Norris | On the acoustic determination of the elastic moduli of anisotropic solids and acoustic conditions for the existence of symmetry planes[END_REF]. Furthermore we draw a link between the conditions appearing in the well-known Mehrabadi-Cowin theorem [START_REF] Cowin | On the identification of material symmetry for anisotropic elastic materials[END_REF] and the elasticity tensor harmonic decomposition ; • In section 4, five sets of algebraic relations are provided to identify the following symmetry classes in

H 4 : orthotropic ([D 2 ]), trigonal ([D 3 ]), tetragonal ([D 4 ]), transverse isotropic ([SO(2)])

and cubic ([O]

). Furthermore, we provide a parametrization of the corresponding strata by rational expressions involving up to 6 polynomial invariants, namely: J 2 , . . . , J 7 . • As an observation, we also prove that the invariants defined by the coefficients of the Betten polynomial [START_REF] Betten | Irreducible invariants of fourth-order tensors[END_REF] do not separate the orbits of Ela.

Notations. To conclude this introduction let us specify some typographical conventions that will be used throughout this paper. Scalars will be noted with Greek letters ; minuscule letters indicate elements of H 2 , i.e deviator ; bold minuscule letters stand for elements of R 3 ⊗ R 3 , i.e. second order tensors ; elements of H 4 will be indicated by capital letters, meanwhile elasticity tensors (elements of S 2 S 2 (R 3 )) will be noted by bold capital letters. Some minors exceptions to that rule may occur (such as for the stress σ and strain ε tensors) and will therefore be indicated in the text.

Geometry of orbit spaces

In this section, we consider a linear representation

ρ : G → GL(V )
of a compact real Lie group 2 G on a finite dimensional R-vector space V . This action will be noted: g • v := ρ(g)v Unless a real Lie group G acts without fixed point on V , the orbit space V /G is not a differentiable manifold [START_REF] Abud | The geometry of spontaneous symmetry breaking[END_REF]. This is the main reason why the description of the orbit space is difficult in general. However, for a compact Lie group, the algebra of G-invariant polynomials, is known to separate the orbit (c.f. subsection A.5). Using this algebra, V /G can be described as a semialgebraic 3 subset of R N . Let {J 1 , J 2 , . . . , J N } be a set of polynomial invariants which separate the G-orbits. Then, the map

J : v → J 1 (v), J 2 (v), . . . , J N (v)
induces an algebraic homeomorphism between the orbit space V /G and J(V ) ⊂ R N which is a semialgebraic subset of R N . In particular, such a set is a stratified space. It is the union of manifolds of various dimensions, called strata, each of them being the union of orbits of a given isotropy type. This construction is the object of the present section. 2 In the following G will solely represent a compact real Lie group, therefore this precision will mostly be omitted. 3 A semialgebraic set is a subset S of R n defined by a finite sequence of polynomial equations (of the form P (x1, . . . , xn) = 0) and inequalities (of the form Q(x1, . . . , xn)).

1.1. G-orbits. Two vectors v 1 and v 2 are said to be G-related, and we write

v 1 ≈ v 2 if there exists g ∈ G such that v 2 = g • v 1 . The set of all vectors v ∈ V which are related to v by G is called the G-orbit of v and is denoted by G • v: G • v := {g • v | g ∈ G}
The G-orbits are compact submanifolds of V [START_REF] Abud | The geometry of spontaneous symmetry breaking[END_REF][START_REF] Bredon | Introduction to compact transformation groups[END_REF].

1.2. Isotropy subgroups. The set of transformations of G which leave the vector v fixed forms a closed subgroup of G, which is called the isotropy subgroup (or symmetry group) of v. It will be denoted by G v :

G v := {g ∈ G; g • v = v}
It can be shown that symmetry groups of G-related vectors are conjugate: Lemma 1.1. For any v ∈ V and any g ∈ G, we have

G g•v = g G v g -1
1.3. Isotropy classes. Two vectors v 1 and v 2 are in the same isotropy class and we write v 1 ∼ v 2 if their isotropy subgroups are conjugate in G, that is there exists g ∈ G such that

G v 2 = g G v 1 g -1
Due to lemma 1.1, two vectors v 1 and v 2 which are in the same G-orbit are in the same isotropy class but the converse is generally false. Therefore, the relation "to be in the same isotropy class" is weaker than the relation "to be in the same G-orbit". This observation is summed up by the following implication

v 1 ≈ v 2 ⇒ v 1 ∼ v 2
The conjugacy class of a subgroup H will be denoted by [H]. The conjugacy class of an isotropy subgroup will be called an isotropy class. One can show that there is only a finite number of isotropy classes for a representation of a compact Lie group (see [START_REF] Bredon | Introduction to compact transformation groups[END_REF]). In the field of physics, for the SO(3)-action on tensor, this result is known as the Hermann theorem [START_REF] Herman | Some theorems of the theory of anisotropic media[END_REF][START_REF] Auffray | Démonstration du théorème de Hermann à partir de la méthode Forte-Vianello[END_REF]. On the set of conjugacy classes of closed subgroups of a G, there is a partial order induced by inclusion. It is defined as follows:

[H 1 ] [H 2 ] if H 1 is conjugate to a subgroup of H 2 .
Endowed with this partial order, the set of isotropy classes is a finite lattice 4 . In particular, it has a least element and a greatest element.

1.4. Isotropic stratification. The set of all vectors v in the same isotropy class defined by [H] is denoted Σ [H] and called a stratum. To avoid any misunderstanding it worths to note that a stratum is not a vector space, but a fiber bundle. There is only a finite number of (non empty) strata and each of them is a smooth submanifold of V [START_REF] Abud | The geometry of spontaneous symmetry breaking[END_REF][START_REF] Bredon | Introduction to compact transformation groups[END_REF]. The partial order relation on conjugacy classes induces a (reverse) partial order relation on the strata

[H 1 ] [H 2 ] ⇔ Σ [H 2 ] Σ [H 1 ]
The set of strata inherits therefore the structure of a finite lattice. An orbit G • v is said to be generic if it belongs to the least isotropy class. The generic stratum Σ [H 0 ] , which is the union of generic orbits can be shown to be a dense and open set in V . Moreover, for each other stratum

Σ [H] , we have dim Σ [H] < dim Σ [H 0 ] . The partition V = Σ [H 0 ] ∪ Σ [H 1 ] ∪ • • • ∪ Σ [Hn]
is called the isotropic stratification of (V, ρ). Σ [H 0 ] is the generic stratum. On the opposite side, the stratum which corresponds to the greatest isotropy subgroup is called the minimum stratum.

H := {v ∈ V | h.v = v for all h ∈ H} , is called the fixed point set of H. For each v ∈ V H , G v ⊂ H. One can check that if H 1 ⊂ H 2 then V H 2 ⊂ V H 1 . However, it may happen that V H 2 = V H 1 but H 1 = H 2 .
Furthermore, it should be pointed out that, in general, given H 1 , H 2 in the same conjugacy class [H], we have

V H 2 = V H 1 .
Notice also that a G-orbit belongs to an isotropy class greater than [H] iff it intersects the space V H .

Given a subgroup H of G, the normalizer of H, defined by

N (H) := g ∈ G | gHg -1 = H
is the maximal subgroup of G, in which H is a normal subgroup. It can also be described as the symmetry group of H when G acts on its subgroups by conjugacy.

Lemma 1.3. The space V H is N (H)-invariant. Moreover, if H = G v 0 is the isotropy group of some point v 0 ∈ V , then N (H) is the maximal subgroup which leaves invariant V H .
Proof. Let us first define

K := g ∈ G; g V H = V H Now let g ∈ N (H) and v ∈ V H . Given h ∈ H we have g -1 hg ∈ H and thus g -1 hg • v = v. But then hg • v = g • v and since this is true for all h ∈ H we get that g • v ∈ V H . That is V H is invariant under N (H), therefore N (H) ⊂ K. Suppose now that H = G v 0 for some point v 0 ∈ V . For all g ∈ K and h ∈ H, we have h • g • v 0 = g • v 0 and hence g -1 • h • g • v 0 = v 0 . Therefore, g -1 hg ∈ G v 0 = H.
In other words, g -1 Hg ⊂ H but, since this is also true for g -1 , we have finally g -1 Hg = H and therefore K ⊂ N (H). Hence K = N (H) which concludes the proof. 1.6. Linear slices. One consequence of lemma 1.3 is that, the linear representation ρ : G → GL(V ) induces a linear representation, of N (H) on V H , obtained by the restriction

ρ N (H) : N (H) -→ GL(V H ).
This induced representation is not faithful. However, when H is an isotropy group, its kernel is exactly H and we get a faithful linear representation

ρ Γ H : Γ H -→ GL(V H ) where Γ H := N (H)/H.
Furthermore, in that case, the equivalence relation defined by (V, ρ), when restricted to the set

V H ∩ Σ [H] , is the same as the equivalence relation defined by (V H , ρ N (H) ). Indeed, let v 1 , v 2 in V H , such that G v 1 = G v 2 = H and v 2 = g • v 1 . Then ghg -1 • v 2 = v 2 , for each h ∈ H. Thus ghg -1 ∈ G v 2 = H and g ∈ N (H).
We have therefore reduced (locally, on the stratum Σ [H] ) the problem of describing the orbit space of V mod G by the orbit space of V H mod Γ H . This is especially meaningful when the group Γ H is finite, in which case, we say that V H is a linear slice. Then, each G-orbit intersect V H at most in a finite number of points and the natural map V H → V H /Γ H is a finite ramified covering 5 of V H /Γ H . The degree of V H is defined as the cardinal of Γ H . It is equal to the index [N (H), H] of H in its normalizer N (H). It is also the number of points in a generic fiber of the covering V H -→ V H /Γ H . The group Γ H is called the monodromy group of V H . 5 A ramified covering is a generalization of the concept of a covering map. It was introduced in the theory of Riemann surfaces and corresponds to the situation where a finite (or discrete) group acts on a manifold but with fixed points. A typical example is given by the map z → z n on the complex plane. Outside 0, this map is a covering but 0 is a singularity, a point of ramification. In higher dimension, the set of ramifications points may be more complicated. [START_REF] Bredon | Introduction to compact transformation groups[END_REF] for a more rigorous justification). Thus

Strata dimensions. The restriction of the projection map

π : V → V /G to a stratum Σ [H] π /Σ [H] : Σ [H] → Σ [H] /G (
[H] /G of the bundle (1) is diffeomorphic to (V H ∩ Σ [H] )/Γ H (see
dim Σ [H] /G = dim(V H ∩ Σ [H] )/Γ H = dim V H -dim Γ H dim Σ [H] = dim V H + dim(G/N (H)) = dim V H + dim G -dim N (H) (2)
Mechanical illustrations of the physical contents of those geometric concepts will be given in subsection 3.2.

Remark 1.4. The dimension of V H can be computed using the trace formula for the Reynolds operator (see [START_REF] Golubitsky | Singularities and groups in bifurcation theory[END_REF]). Letting χ ρ be the character of the representation (V, ρ), we have

dim V H = 1 |H| h∈H χ ρ (h) (3)
if H is a finite group (and the preceding formula has to be replaced by the Haar integral over H for an infinite compact group). Explicit analytical formulas based on (3) were obtained in [START_REF] Auffray | Analytical expressions for anisotropic tensor dimension[END_REF] and [START_REF] Golubitsky | Singularities and groups in bifurcation theory[END_REF]. They can be found in Appendix C.

1.8. Implicit equations for closed strata. Let [H] be a fixed isotropy class and Σ [H] ⊂ V the corresponding stratum. Given a finite set of invariant polynomials which separate the orbits of G , the closed stratum Σ [H] can be characterized by a finite set of relations (equations and inequalities) on {J 1 , J 2 , . . . , J N }. To obtain these equations, we fix a subgroup H in the conjugacy class [H] and choose linear coordinates (x i ) 1≤i≤q on V H . Then, we evaluate {J 1 , J 2 , . . . , J N } on V H (as polynomials in the x i ) and we try to obtain implicit equations on the J k for the parametric system

J k = P k (x 1 , . . . x q ), k = 1, . . . , N, P k ∈ R[X 1 , . . . , X q ] (4)
satisfied by the restriction of {J 1 , J 2 , . . . , J N } to V H . This is a difficult task in general (one might consider [START_REF] Cox | Ideals, varieties, and algorithms[END_REF] for a full discussion about the implicitization problem). Moreover, it could happen that the algebraic variety V I defined by such an implicit system is bigger than the variety V P , defined by the parametric system (4). Besides, we are confronted to the difficulty that we work over R which is not algebraically closed. To overcome these difficulties, we observe that the restrictions of {J 1 , J 2 , . . . , J N } to V H are Γ H -invariant and can therefore be expressed as polynomial expressions of some generators σ 1 , . . . , σ r of the invariant algebra of the monodromy group Γ H on V H . This observation reduces the problem to consider first the implicitization problem for the parametric system

J k = p k (σ 1 , . . . , σ r ), k = 1, . . . , N, p k ∈ R[X 1 , . . . , X r ] (5)
To solve this problem, we use a Groebner basis [START_REF] Cox | Ideals, varieties, and algorithms[END_REF] (see subsection A.4) or a regular chain [START_REF] Boulier | Representation for the radical of a finitely generated differential ideal[END_REF].

The main observation is that, in each case we consider, the system (5) leads to a system of relations (syzygies) which characterizes the closed stratum

Σ [H] S j (J 1 , J 2 , . . . , J N ) = 0, j = 1, . . . l S j ∈ R[X 1 , . . . , X N ], (6)
and a system

σ i = R i (J 1 , J 2 , . . . , J N ), i = 1, . . . , r R i ∈ R(X 1 , . . . , X N ), (7)
which express the σ i as rational 6 functions of {J 1 , J 2 , . . . , J N } on the open stratum Σ [H] . Beware, however, that these rational expressions may not be unique. However if some σ i can be written as P 1 /Q 1 as well as P 2 /Q 2 then P 1 Q 2 -P 2 Q 1 belongs to the ideal generated by the S j .

Because the solutions are rational, the fact that the field on which we work is real or complex does not matter at this level. Therefore, for each real solution (J 1 , J 2 , . . . , J N ) of ( 6), it corresponds a unique real solution (σ 1 , . . . , σ r ) given by [START_REF] Baerheim | Harmonic decomposition of the anisotropic elasticity tensor[END_REF]. Nevertheless, we cannot take for granted that a real solution (σ 1 , . . . , σ r ) of ( 7) corresponds to a real point (x 1 , . . . , x q ) in V H . For this, we need to compute an additional system of inequalities on the σ i , or equivalently on the J k , which permits to exclude complex solutions of

σ i = Q i (x 1 , . . . x q ), i = 1, . . . , r, Q i ∈ R[X 1 , . . . , X q ]
However, in the present paper for the linear slices we consider, the only monodromy group that we encounter are 7 1, S 2 and S 3 and the action is the standard one in appropriate coordinates. In these cases, and more generally when the monodromy group is isomorphic to S n and acts by permutation on the coordinates, the invariants σ 1 , . . . , σ n are algebraically independent, r = q = n and x 1 , . . . x n are the roots of the polynomial

p(z) = z n -σ 1 z n-1 + • • • + (-1) n σ n
Therefore, the problem reduces to find conditions on the σ i that ensure that all the roots of p are real. The solution is due to Hermite, [START_REF] Coste | An Introduction to Semialgebraic Geometry[END_REF]. He has proved that the number of distinct real roots of a real polynomial p of degree n is equal to the signature of the Hankel matrix B(p) := (S i+j-2 ) 1≤i,j≤n , where S k := n i=1 (x i ) k is the power sum of the roots of p. In particular, p has real roots if and only if B(p) is non-negative.

In the forthcoming section this method will be applied to establish the stratification of a nuple of quadratic forms. Besides being just an illustration, the results obtained will be of great interest for the symmetry detection of elasticity tensors and used in section 3 and section 5.

The isotropic stratification of a n-uple of quadratic forms

In this section, we will study the isotropic stratification of the space

n k=1 S 2 (R 3 ) = nH 0 ⊕ nH 2 .
For n = 2, that is for a couple of quadratic forms, we get the lower order terms of the harmonic decomposition of Ela (c.f. section 3). Invariant conditions which characterize each symmetry class will be given.

2.1.

The isotropic stratification of one quadratic form. The lattice of isotropy classes for the tensorial representation of SO(3) on the space of symmetric 3-matrices,

S 2 (R 3 ), is a total order [D 2 ] [O(2)] [SO(3)] . Since the least isotropy class, [D 2 ], is not trivial, the fixed point set S 2 (R 3 ) D 2 (
which corresponds to the subspace of diagonal matrices) is a global linear slice. Each SO(3)-orbit intersects this slice (a symmetric matrix is diagonalizable in a direct orthogonal frame). This linear slice is of dimension 3 and degree 6 (see Appendix B). Generically, an orbit intersects this slice in exactly 6 points which correspond to the permutation of the eigenvalues λ 1 , λ 2 , λ 3 . The monodromy group N (D 2 )/D 2 ≃ S 3 acts on {λ 1 , λ 2 , λ 3 } by permutation. The stratification problem is reduced to the computation of the number of distinct eigenvalues, namely (3, 2, 1). The invariant algebra of (S 2 (R 3 ) D 2 , S 3 ) is the free algebra generated by the 3 symmetric elementary functions σ 1 , σ 2 , σ 3 . 6 The fact that the solutions are rational will not be justified here. We just observe that this is the case for all classes we have treated so far in this article. 7 Where Sn is the the group of permutations acting on n elements (symmetric group).

According to Hermite (see [START_REF] Coste | An Introduction to Semialgebraic Geometry[END_REF]) the characteristic polynomial of a has 3 distinct real roots iff the quadratic form

B(a) :=   3 2 σ 1 σ 2 2 σ 1 2 σ 1 2 -2 σ 2 σ 1 σ 2 -3 σ 3 σ 2 σ 1 σ 2 -3 σ 3 -2 σ 1 σ 3 + σ 2 2   is positive definite. The three principal minors of B are given by    ∆ 1 = 3, ∆ 2 = 2σ 2 1 -6σ 2 , ∆ 3 = -27 σ 3 2 + 18 σ 1 σ 2 -4 σ 1 3 σ 3 -4 σ 2 3 + σ 1 2 σ 2 2 .
These minors are precisely those which have been introduced by Hermite to count the number of real roots of a real polynomial of degree 3. ∆ 3 is the discriminant of the polynomial

∆ 3 = (λ 1 -λ 2 ) 2 (λ 2 -λ 3 ) 2 (λ 1 -λ 3 ) 2 , whereas ∆ 2 = (λ 1 -λ 2 ) 2 + (λ 2 -λ 3 ) 2 + (λ 1 -λ 3 )
2 vanishes when all the roots are equal. They give directly the classification we are looking for, which is summarized in table 1.

H N (H) Γ H card Γ H strata Σ H dim V H dim Σ [H] /G dim Σ H D 2 O S 3 6 ∆ 3 > 0 3 3 6 O(2) O(2) 1 1 ∆ 3 = 0, ∆ 2 > 0 2 2 4 SO(3) SO(3) 1 1 ∆ 3 = 0, ∆ 2 = 0 1 1 1
Table 1. Isotropy classes for the representation of SO(3) on symmetric 3-matrices.

2.2. The isotropic stratification of n quadratic forms. In this subsection, we consider a n-uple of quadratic forms (a 1 , . . . , a n ). The lattice of isotropy classes is the following totally ordered set (see [START_REF] Olive | Isotropy classes of SO(3) representations[END_REF] for a general algorithm to compute isotropy classes)

[1] [Z 2 ] [D 2 ] [O(2)] [SO(3)]
The stratification is summarized in table 2. To establish the bifurcation conditions we need

H N (H) Γ H card Γ H dim V H dim Σ [H] /G dim Σ [H] 1 SO(3) SO(3) ∞ 6n 6n -3 6n Z 2 O(2) O(2) ∞ 4n 4n -1 4n + 2 D 2 O S 3 6 3n 3n 3n + 3 O(2) O(2) 1 1 2n 2n 2n + 2 SO(3) SO(3) 1 1 n n n
Table 2. Isotropy classes for the representation of SO(3) on a n-uple of quadratic forms.

to introduce the commutator of two quadratic forms. Given two quadratic forms (a, b) on the euclidean space R 3 , the commutator [a, b] := abba is a skew-symmetric endomorphism Ω, which can be represented, in the oriented vector space R 3 , by a vector ω. We will denote by ω → Ω := j(ω) this linear isomorphism between vectors of R 3 and skew-symmetric second order tensors on R 3 . Recall that for every g ∈ SO(3) and ω ∈ R 3 , we have j(gω) = gj(ω)g t . In particular ω(a, b)

:= j -1 ([a, b])
is a covariant of degree 2 of the pair (a, b).

Theorem 2.1. The characterization of isotropy classes in n k=0 S 2 (R 3 ) is summarized below: (1) A n-uple (a 1 , . . . , a n ) has at least isotropy [Z 2 ] iff there exists a non-vanishing vector ω ∈ R 3 and real numbers λ k,l such that

ω(a k , a l ) = λ k,l ω, 1 ≤ k, l ≤ n, a k ω ∧ ω = 0, 1 ≤ k ≤ n, ( 8
)
where ∧ is the classical vector product of

R 3 . (2) A n-uple (a 1 , . . . , a n ) has at least isotropy [D 2 ] iff ω(a k , a l ) = 0, 1 ≤ k, l ≤ n. (9)
(3

) A n-uple (a 1 , . . . , a n ) has at least isotropy [O(2)] iff 25 tr(a 2 k ) 3 = 54 tr(a 3 k ) 2 , 1 ≤ k ≤ n, [tr(a k a l )] 2 = tr(a 2 k ) tr(a 2 l ) , 1 ≤ k, l ≤ n, (10)
where

a k = dev(a k ) is the deviatoric part of a k . (4) A n-uple (a 1 , . . . , a n ) has isotropy [SO(3)] iff a k = 0, 1 ≤ k ≤ n, (11)
where

a k = dev(a k ) is the deviatoric part of a k . Proof. (1) If a n-uple (a 1 , . . . , a n ) has at least isotropy [Z 2 ]
, its elements share a common principal axis (the axis of the non trivial rotation σ in Z 2 ). Let ω be a unit vector parallel to this axis. Then ω belongs to the kernel of the commutator of each pair (a k , a l ) and thus ω(a k , a l ) is proportional to ω. Relations ( 8) are a consequence of these facts. Conversely, if relations (8) are satisfied, then ω is a common eigenvector of the n-uple (a 1 , . . . , a n ). In that case, the isotropy group of the pair (a, b) is at least in the class [Z 2 ].

(2) If a n-uple (a 1 , . . . , a n ) has at least isotropy [D 2 ], then there exists a g ∈ SO(3) which brings all the a k to a diagonal form. Thus they commute. Conversely, if the a k commute, they can be diagonalized simultaneously and the isotropy group of the n-uple (a 1 , . . . , a n ) is at least in the class [D 2 ].

(3) A n-uple (a 1 , . . . , a n ) has at least isotropy [O(2)] iff all the a k can be diagonalized simultaneously and, moreover, iff their respective deviator has (in a well chosen basis) a diagonal form proportional to diag(1, 1, -2). The first relation of equations [START_REF] Bona | Characterization of Elasticity-Tensor Symmetries Using SU(2)[END_REF] expresses that the deviator of a k has a double eigenvalue, the second relation expresses that the deviators a k and a l are proportional (Cauchy-Schwarz relation). Conversely, if equations [START_REF] Bona | Characterization of Elasticity-Tensor Symmetries Using SU(2)[END_REF] are satisfied, then all the deviators a k commute and each of them has a double eigenvalue. It is then possible to find a basis in which all the a k are proportional to diag(1, 1, -2). Therefore the n-uple (a 1 , . . . , a n ) has at least isotropy [O [START_REF] Abud | The geometry of spontaneous symmetry breaking[END_REF]].

(4) The last assertion is trivial.

3. The space of elasticity tensors 3.1. The harmonic decomposition of elasticity tensors. To establish the isotropic stratification of a tensor space, the first step is to decompose it into a direct sum of elementary pieces. For the SO(3)-action, this decomposition is known as the harmonic decomposition. The SO(3)-irreducible pieces are symmetric and traceless tensors (i.e. harmonic tensors). The space of elasticity tensors admits the following harmonic decomposition [START_REF] Backus | A geometrical picture of anisotropic elastic tensors[END_REF][START_REF] Baerheim | Harmonic decomposition of the anisotropic elasticity tensor[END_REF][START_REF] Forte | Symmetry classes for elasticity tensors[END_REF][START_REF] Forte | Restricted invariants on the space of elasticity tensors[END_REF]:

Ela ≃ 2H 0 ⊕ 2H 2 ⊕ H 4 (12)
Therefore, each C ∈ Ela can be written as C = (λ, µ, a, b, D) where λ, µ ∈ H 0 , a, b ∈ H 2 and D ∈ H 4 and such that for all g ∈ SO(3)

C = ρ(g) • C ⇐⇒ λ = λ, μ = µ, ā = ρ 2 (g) • a, b = ρ 2 (g) • b, D = ρ 4 (g) • D
Elements of the set (λ, µ, a, b, D) are covariants (c.f. subsection A.2) to C : (λ, µ) are invariants, (a, b) are ρ 2 -covariants, and D is ρ 4 -covariant. The explicit harmonic decomposition is well known, given an orthonormal frame (e 1 , e 2 , e 3 ) of the Euclidean space, we get [START_REF] Boehler | On the polynomial invariants of the elasticity tensor[END_REF]:

C ijkl = λδ ij δ kl + µ(δ ik δ jl + δ il δ jk ) + δ ij a kl + δ kl a ij + δ ik b jl + δ jl b ik + δ il b jk + δ jk b il + D ijkl (13)
where the metric is written q ij := δ ij . This formula can be inverted to obtain the 5 harmonic tensors (λ, µ, a, b, D) from C. On Ela = S 2 S 2 (R 3 ), there are only two different traces:

d ij = (tr 12 C) ij := 3 k=1 C kkij , v ij = (tr 13 C) ij := 3 k=1 C kikj .
where d and v are known as, respectively, the dilatation tensor and the Voigt tensor [START_REF] Cowin | On the identification of material symmetry for anisotropic elastic materials[END_REF][START_REF] Cowin | Properties of the anisotropic elasticity tensor[END_REF].

Starting with [START_REF] Coste | An Introduction to Semialgebraic Geometry[END_REF] we get

d = (3λ + 2µ)q + 3a + 4b, v = (λ + 4µ)q + 2a + 5b. ( 14
)
Taking the traces of each equation, one obtains:

tr(d) = 9λ + 6µ, tr(v) = 3λ + 12µ
and, finally:

λ = 1 15 (2 tr(d) -tr(v)), µ = 1 30 (-tr(d) + 3 tr(v)) a = 1 7 (5 dev(d) -4 dev(v)), b = 1 7 (-2 dev(d) + 3 dev(v))
where we write dev(a) := a -1 3 tr(a) q the deviatoric part of a 2nd-order tensor. The isotropy classes of elasticity tensors have been computed in [START_REF] Forte | Symmetry classes for elasticity tensors[END_REF][START_REF] Bona | Characterization of Elasticity-Tensor Symmetries Using SU(2)[END_REF] and the totally anisotropic class [START_REF] Abud | The geometry of orbit-space and natural minima of Higgs potentials[END_REF] (also know as the triclinic class). We refer to Appendix B for the precise definitions of these groups. This stratification is a consequence of the index symmetries of elasticity tensors. It is not a general result that encompass all kind of 4th-order tensors. For example, the photo-elasticity tensors space which is a 4th-order tensor space solely endowed with minor symmetries is divided into 12 symmetry classes [START_REF] Forte | Symmetry classes and harmonic decomposition for photoelasticity tensors[END_REF].

3.2.

The isotropic stratification of Ela. The stratification of Ela is provided in the table 3. The dimensions of Σ [H] and Σ [H] /G are obtained using the formulas (2) of subsection 1.7.

H N (H) Γ H card Γ H dim V H dim Σ [H] /G dim Σ [H] 1 SO(3) SO(3) ∞ 21 18 21 Z 2 O(2) O(2) ∞ 13 12 15 D 2 O S 3 6 9 9 12 D 3 D 6 S 2 2 6 6 9 D 4 D 8 S 2 2 6 6 9 O(2) O(2) 1 1 5 5 7 O O 1 1 3 3 6 SO(3) SO(3) 1 1 2 2 2
Table 3. Isotropy classes for Ela.

And dim V H was computed using the trace formula (3) and the explicit formulas provided Appendix C. V H : The fixed-point set V H is the vector space of tensors having at least H as isotropy group. Its dimension equals the dimension of the matrix space used to represent them. The space V H can be constructed for any closed subgroup H, not only for an isotropy subgroup. In elasticity, for instance, fixed-point sets have been constructed for Z 3 , Z 4 (which are not isotropy subgroups). This is why it was believed for a long time that there exists 10 type of elasticity. The construction of such matrix spaces is detailed, for example, in [START_REF] Cowin | Eigentensors of linear anisotropic elastic materials[END_REF] for classical elasticity, and extended to second order elasticity in [START_REF] Auffray | Derivation of anisotropic matrix for bi-dimensional strain-gradient elasticity behavior[END_REF]. It worthwhile emphasizing the fact that for a non-isotropy subgroup, V H is solely a fixed-point set and in no way a slice. It is geometrically meaningless for the description of the stratification, that is the geometry of the orbit space. Σ [H] : The set of tensors which symmetry group is conjugate to H forms a stratum, or a symmetry class according to Forte and Vianello [START_REF] Forte | Symmetry classes for elasticity tensors[END_REF]. We got the following dimension relation dim Σ [H] = dim V H + e H where e H = dim SO(3)/N (H) is the number of Euler angles needed to define the appropriate coordinate system in which the elastic tensor has the minimum number of elastic constants [START_REF] Norris | On the acoustic determination of the elastic moduli of anisotropic solids and acoustic conditions for the existence of symmetry planes[END_REF]. The strata dimension corresponds to what Norris, for example, calls the number of independent parameters characterizing a tensor. Σ [H] /G : This is the orbit space of Σ [H] , i.e. each point of this space represents the G-orbit of a tensor which symmetry group is conjugate to H. The intrinsic properties of a tensor are characterized by its orbit. In a slice V H the image of the orbit is given by monodromy group, and we have the dimensional relation dim

V H = dim Σ [H] /G + dim Γ H . Therefore when Γ H is discrete, Σ [H]
/G and V H have the same dimension, this is the situation for most of elasticity symmetry classes. At the opposite, when Γ H is continuous, the dimension of the orbit is strictly smaller than the one of the slice, in elasticity this case occurs for 1, Z 2 .

Remark 3.1. It is known for long time that for the symmetry classes 1, Z 2 , there is no canonical way to obtain a basis in which the number of non-zero components is minimal. More precisely, an element of the orbit of such a tensor in an appropriate fixed point set V H does not have necessarily the minimal number of non-zero components and thus, such a basis has to be computed arbitrarily. This is due to the fact that in these cases, Γ H is continuous. For the trivial class, [START_REF] Abud | The geometry of orbit-space and natural minima of Higgs potentials[END_REF], V H ≃ V and there is no natural way to obtain a normal form. The monodromy group Γ H ≃ SO(3) and there are three arbitrary degree of freedom to choose a nice basis. For the other cases, V H is smaller than V but the number of non-zero components of a tensor in V H can usually be lowered arbitrarily. The monodromy Γ H ≃ O(2) and there is one arbitrary degree of freedom to lower the number of non-zero components. In each cases, it appears clearly that one can properly choose a basis in which those angular parameters equal zeros, such a basis is non necessarily unique. In such a basis the number of independent parameters is lowered and we obtain the minimum number of elastic constants, as discussed, for example, by Norris [START_REF] Norris | On the acoustic determination of the elastic moduli of anisotropic solids and acoustic conditions for the existence of symmetry planes[END_REF]. This minimum number of elastic constants is equal to the orbit dimension.

The results appearing in table 3 are well-known in elasticity, but their constructions are usually ad-hoc and do not stand on rigorous mathematical foundations. Therefore, we hope to provide such a firm geometric construction. Conversely, as the geometrical concepts we use are abstract, these well-known mechanical results help to illustrate their physical contents. As it has been said before the space of elasticity tensors can be decomposed in the following way:

Ela ≃ 2H 0 ⊕ 2H 2 ⊕ H 4 (15)
The section 4 will be devoted to the stratification of H 4 . The stratification of 2H 0 ⊕ 2H 2 is a consequence of the results obtained in the subsection 2.2 in the case n = 2. In such a case a direct link between the bifurcation conditions given (2.1) and the ones appearing in the Cowin-Mehrabadi theorem can be drawn.

d ij n j = δn i , and v ij n j = νn i .
These conditions are nothing but the two first ones of the Cowin-Mehrabadi (CM) theorem for the existence of a symmetry plane. Let summarize this theorem as it is formulated in [START_REF] Ting | Generalized Cowin-Mehrabadi theorems and a direct proof that the number of linear elastic symmetries is eight[END_REF].

Theorem 3.2 (Cowin-Mehrabadi).
Let n be a unit vector normal to a plane of material symmetry, if m is any vector on the symmetry plane a set of necessary and sufficient conditions for n to be normal to the symmetry plane is

C ijkk n j = (C pqtt n p n q )n i , C isks n k = (C pqrq n p n r )n i , C ijks n j n s n k = (C pqrt n p n q n r n t )n i , C ijks m j m s n k = (C pqrt n p m q n r m t )n i .
The two first conditions of this theorem can be rephrased in terms of the second order covariants (a, b) of the harmonic decomposition. The bifurcation condition (8) provides a method to find a good candidate to be the normal of a symmetry plane ω = [a, b]. In the same spirit the third and fourth conditions are indeed conditions on D. Therefore the theorem can be reformulated in term of harmonic covariant using the condition (8): Theorem 3.3 (Cowin-Mehrabadi). Let C = (λ, µ, a, b, D) be an elasticity tensor. Let suppose there exist a non null-vector ω = [a, b], this vector is a vector normal to a plane of material symmetry, if there exists ω ⊥ verifying ω ⊥ • ω ⊥ = 0 and such that the following conditions are satisfied

ω ∧ (D : ω ⊗ ω)ω = 0, ω ∧ (D : ω ⊥ ⊗ ω ⊥ )ω = 0.

The isotropic stratification of H 4

In this section, the isotropic stratification of H 4 will be established. This space is isomorphic to the higher order component of the harmonic decomposition of Ela. Invariant conditions characterizing each finite monodromy symmetry class will be given. These results provided new sets of necessary conditions to identify higher order symmetry classes, and will be used in section 5.

4.1. Lattice of isotropy. The isotropy classes of the 9-D space H 4 are the same as the 8 classes of Ela. The corresponding lattice 8 is illustrated in figure 1. With the convention that a group at the starting point of an arrow is conjugate to a subgroup of the group pointed by the arrow. 8 It should be noticed that it is not possible to realize this lattice of conjugacy classes as a lattice of subgroups (for the inclusion relation). In particular, if each conjugacy class in figure 1 is replaced by the corresponding subgroup defined in Appendix B, then almost all the arrows corresponds to an inclusion relation but not the arrow from (4) to (2) : D3 is not a subgroup of O but is conjugate to a subgroup of O. . For each of them, explicit results will be provided. The algebraic procedure used to obtain their solutions was explained in subsection 1.8.

H N (H) Γ H card Γ H dim V H dim Σ [H] /G dim Σ [H] 1 SO(3) SO(3) ∞ 9 6 9 Z 2 O(2) O(2) ∞ 5 4 7 D 2 O S 3 6 3 3 6 D 3 D 6 S 2 2 2 2 5 D 4 D 8 S 2 2 2 2 5 O(2) O(2) 1 1 1 1 3 O O 1 1 1 1 4 SO(3) SO(3) 1 1 0 0 0 Table 4.
Isotropy classes for the tensorial representation of SO(3) on H 4 .

4.2.

Global parametrization. On an euclidean vector space, any bilinear form can be represented by a symmetric linear operator. This applies in particular to an elasticity tensor C which may be considered as a quadratic form on S 2 (R 3 ), where the Euclidean structure is induced by the standard inner product on R 3 . It is therefore customary to represent an elasticity tensor C by its corresponding symmetric linear operator C on S 2 (R 3 ) (relatively to the canonical scalar 9 It is quite remarkable that for quadratic forms in every dimension n, the generic stratum has non trivial isotropy and that the corresponding linear slice has finite monodromy Sn. This is the main reason why every orbit has a natural normal form, namely a diagonal representation (up to the finite monodromy group Sn). In comparison, the main difficulty in linear elasticity is the fact that for a generic elasticity tensor, the isotropy group is trivial and there is no natural normal form.

product a, b = tr(ab)). The corresponding matrix representation [START_REF] Thomson | On six principal strains of an elastic solid[END_REF][START_REF] Cowin | Eigentensors of linear anisotropic elastic materials[END_REF] is given by

C =        
       
where c mn are the components of the elasticity tensor in an orthonormal frame. We use the conventional rule to recode a pair of indices (i, j) (i, j = 1, 2, 3) by a integer m = α(i, j) (m = 1, 2, . . . 6), where α(i, i) := i for 1 ≤ i ≤ 3 and α(i, j) := 9 -(i + j) for i = j. Given a general tensor D ∈ H 4 , we denote by D the corresponding symmetric linear operator. In the canonical basis of R 3 , D has the following matrix representation, depending on 9 real parameters:

D = D 11 √ 2 D 12 √ 2 D T 12 2 D 22
where T represents the transposition,

D 11 =   -h 9 -h 8 h 9 h 8 h 9 -h 9 -h 7 h 7 h 8 h 7 -h 8 -h 7   D 12 =   -h 5 -h 6 h 2 h 1 h 5 -h 2 -h 4 h 3 h 6 h 4 -h 1 -h 3   ,
and

D 22 =   h 7 -h 1 -h 3 -h 2 -h 4 -h 1 -h 3 h 8 -h 5 -h 6 -h 2 -h 4 -h 5 -h 6 h 9  
Using this parametrization, D has the following invariance properties:

(i) Z 2 -invariant iif h 2 = h 4 = h 5 = h 6 = 0 , (ii) D 2 -invariant iif h k = 0 pour 1 ≤ k ≤ 6 , (iii) D 3 -invariant iif h 1 = h 2 = h 3 = h 4 = h 6 = 0 et h 7 = h 8 = -4h 9 , (iv) D 4 -invariant iif h k = 0 pour 1 ≤ k ≤ 6 et h 7 = h 8 , (v) O(2)-invariant iff h k = 0 pour 1 ≤ k ≤ 6 et h 7 = h 8 = -4h 9 , (vi) O-invariant iff h k = 0 pour 1 ≤ k ≤ 6 et h 7 = h 8 = h 9 , (vii) SO(3)-invariant iff h k = 0 pour 1 ≤ k ≤ 9 .
In the following subsections, the fundamental invariants of 4-th order harmonic tensors will be used. As these results are not well-known we will, in the next lines, summarize their expressions as obtained by Boelher and al. [START_REF] Boehler | On the polynomial invariants of the elasticity tensor[END_REF]. For technical details and historical considerations, we therefore refer to this publication, and the references therein.

d 2 = tr 13 (D 2 ) d 3 = tr 13 (D 3 ) d 4 = d 2 2 d 5 = d 2 Dd 2 d 6 = d 3 2 d 7 = d 2 2 Dd 2 d 8 = d 2 2 D 2 d 2 d 9 = d 2 2 Dd 2 2 d 10 = d 2 2 D 2 d 2 2 ( 16
)
are covariant to D. The nine fundamental invariants are defined by:

J k := tr(d k ), k = 2, . . . , 10.
The first 6 invariants J 2 , . . . , J 7 are algebraically independent. The last 3 ones J 8 , J 9 , J 10 are linked to the formers by polynomial relations. These fundamental syzygies were computed in [START_REF] Shioda | On the graded ring of invariants of binary octavics[END_REF].

Cubic symmetry ([O]).

A O-invariant tensor D ∈ H 4 has the following matrix representation:

D =         8 δ -4 δ -4 δ 0 0 0 -4 δ 8 δ -4 δ 0 0 0 -4 δ -4 δ 8 δ 0 0 0 0 0 0 -8 δ 0 0 0 0 0 0 -8 δ 0 0 0 0 0 0 -8 δ         (17)
where δ ∈ R. The evaluation of the invariants ( 16) on this slice gives the following parametric system:

J 2 = 480 δ 2 , J 3 = 1920 δ 3 , J 4 = 76800 δ 4 , J 5 = 0, J 6 = 12288000 δ 6 , J 7 = 0, J 8 = 0, J 9 = 0, J 10 = 0.
We notice that the parameter 4δ = J 3 /J 2 is a rational invariant. The linear slice corresponding to the octahedral group is of degree 1, which was already known from the algebraic argument explained in Appendix B.

3 J 4 = J 2 2 , J 5 = 0, 30 J 3 2 = J 2 3 , 9 J 6 = J 2 3 , J 7 = 0, J 8 = 0, J 9 = 0, J 10 = 0. ( 18
) D is in the cubic class [O] if moreover J 2 (D) = 0.
In that case, it admits the normal form [START_REF] Cox | Ideals, varieties, and algorithms[END_REF] where 4δ := J 3 (D)/J 2 (D).

We observe that ∀J 2 , J 3 ∈ R, δ ∈ R. Therefore we do not need to add any inequality to the syzygy system [START_REF] Dubrovin | Modern geometry-methods and applications. Part I[END_REF].

D =         3 δ δ -4 δ 0 0 0 δ 3 δ -4 δ 0 0 0 -4 δ -4 δ 8 δ 0 0 0 0 0 0 -8 δ 0 0 0 0 0 0 -8 δ 0 0 0 0 0 0 2 δ         (19)
where δ ∈ R. The evaluation of the invariants ( 16) on this slice gives:

J 2 = 280 δ 2 , J 3 = 720 δ 3 , J 4 = 32800 δ 4 ,
J 5 = 80000 δ 5 , J 6 = 4528000 δ 6 , J 7 = 17600000 δ 7 , J 8 = 211200000 δ 8 , J 9 = 3872000000 δ 9 , J 10 = 46464000000 δ 10

The parameter δ = 7 J 3 /18 J 2 is a rational invariant. The linear slice corresponding to the group O(2) is of degree 1, which was already known (see Appendix B).

98 J 4 = 41 J 2 2 , 63 J 5 = 25 J 3 J 2 , 3430 J 3 2 = 81 J 2 3 , 1372 J 6 = 283 J 2 3 , 882 J 7 = 275 J 2 2 J 3 , 4802 J 8 = 165 J 2 4 , 12348 J 9 = 3025 J 2 3 J 3 , 67228 J 10 = 1815 J 2 5 . ( 20
)
D is in the transversely isotropic class [O(2)] if moreover J 2 (D) = 0. In that case, it admits the normal form [START_REF] Forte | Symmetry classes for elasticity tensors[END_REF] where δ = 7 J 3 (D)/18 J 2 (D).

As for [O] the expression of δ ensures that, for all J 2 , J 3 ∈ R, the solution δ is real. Therefore no inequality is needed. This fact is indeed general to any linear slice of degree 1.

D =         3 δ δ -4 δ - √ 2σ 0 0 δ 3 δ -4 δ √ 2σ 0 0 -4 δ -4 δ 8 δ 0 0 0 - √ 2σ √ 2σ 0 -8 δ 0 0 0 0 0 0 -8 δ -2 σ 0 0 0 0 -2 σ 2 δ         (21)
where (δ, σ) ∈ R 2 . The action of the monodromy group S 2 is given by:

δ → δ, σ → -σ .
The evaluation of the invariants ( 16) on this slice gives:

J 2 = 280 δ 2 + 16 σ 2 J 3 = 144 δ 5 δ 2 -σ 2 J 4 = 32800 δ 4 + 2720 σ 2 δ 2 + 88 σ 4 J 5 = 32 δ -σ 2 + 50 δ 2 2
J 6 = 4528000 δ 6 + 436800 σ 2 δ 4 + 20640 σ 4 δ 2 + 496 σ 6

J 7 = 320 δ 22 δ 2 + σ 2 -σ 2 + 50 δ 2 2 J 8 = 3840 δ 2 22 δ 2 + σ 2 -σ 2 + 50 δ 2 2 J 9 = 3200 δ -σ 2 + 50 δ 2 2 22 δ 2 + σ 2 2 J 10 = 38400 δ 2 -σ 2 + 50 δ 2 2 22 δ 2 + σ 2 2
We notice that the parameter:

δ = - 1 4
J 5 J 2 2 -3 J 4
is a rational invariant, therefore for all J 2 , J 4 , J 5 ∈ R, the parameter δ is real. However, the minimal equation satisfied by σ is of degree 2

J 2 = 280 δ 2 + 16 σ 2 . ( 22
)
A condition is needed for σ to be real. This condition, which is σ 2 ≥ 0, is given by the following inequality on J 2 , J 4 , J 5 :

2J 2 (J 2 2 -3 J 4 ) 2 -35J 2 5 ≥ 0 ( 23
)
Having computed δ, the equation ( 22) has two roots with opposite signs. The linear slice corresponding to the group D 3 is of degree 2, which was already known (see Appendix B).

192 J 6 = -51 J 2 3 + 216 J 2 J 4 + 10 J 3 2 36 J 7 = -2 J 2 2 J 3 + 6 J 3 J 4 + 27 J 2 J 5 768 J 4 2 = -99 J 2 4 + 552 J 2 2 J 4 + 10 J 2 J 3 2 + 240 J 3 J 5 240 J 8 = -33 J 2 4 + 96 J 2 2 J 4 + 30 J 2 J 3 2 + 40 J 3 J 5 576 J 4 J 5 = -41 J 2 3 J 3 + 120 J 2 J 3 J 4 + 216 J 2 2 J 5 + 30 J 3 3 1152 J 9 = -99 J 2 3 J 3 + 296 J 2 J 3 J 4 + 648 J 2 2 J 5 + 10 J 3 3 1440 J 5 2 = -11 J 2 5 + 32 J 2 3 J 4 -70 J 2 2 J 3 2 + 240 J 2 J 3 J 5 + 240 J 3 2 J 4 8640 J 10 = -891 J 2 5 + 2592 J 2 3 J 4 + 730 J 2 2 J 3 2 + 2160 J 2 J 3 J 5 + 240 J 3 2 J ( 24
)
together with inequality [START_REF] Geymonat | Symmetry classes of piezoelectric solids[END_REF]. D is in the trigonal class [D 3 ] if moreover 3 J 4 -J = 0 and 98 J 4 -41 J 2 2 = 0. In that case, it admits the normal form [START_REF] Forte | Restricted invariants on the space of elasticity tensors[END_REF] where

δ = - 1 4 J 5 J 2 2 -3 J 4
and where σ is the positive root of J

D =         -σ + 3 δ σ + δ -4 δ 0 0 0 σ + δ -σ + 3 δ -4 δ 0 0 0 -4 δ -4 δ 8 δ 0 0 0 0 0 0 -8 δ 0 0 0 0 0 0 -8 δ 0 0 0 0 0 0 2 σ + 2 δ         (25)
where (δ, σ) ∈ R 2 . The monodromy group is defined by:

δ → δ, σ → -σ.
The evaluation of the invariants ( 16) on this slice gives:

J 2 = 8 σ 2 + 280 δ 2 J 3 = 48 δ σ 2 + 15 δ 2 J 4 = 32 σ 4 + 960 δ 2 σ 2 + 32800 δ 4 J 5 = 128 δ (5 δ -σ) 2 (5 δ + σ) 2 J 6 = 128 σ 6 + 5760 σ 4 δ 2 + 86400 σ 2 δ 4 + 4528000 δ 6 J 7 = 512 δ 55 δ 2 + σ 2 (5 δ -σ) 2 (5 δ + σ) 2 J 8 = 6144 δ 2 55 δ 2 + σ 2 (5 δ -σ) 2 (5 δ + σ) 2 J 9 = 2048 δ (5 δ -σ) 2 (5 δ + σ) 2 55 δ 2 + σ 2 2 J 10 = 24576 δ 2 (5 δ -σ) 2 (5 δ + σ) 2 55 δ 2 + σ 2 2
We notice that the parameter δ = -1 4

J 5 J 2
2 -3 J 4 is a rational invariant, therefore for all (J 2 , J 4 , J 5 ) ∈ R 3 , δ ∈ R. However, the minimal equation satisfied by σ is of degree 2

J 2 = 8 σ 2 + 280 δ 2 (26)
As for [D 3 ], the condition

2J 2 (J 2 2 -3 J 4 ) 2 -35J 2 5 ≥ 0 ( 27
)
has to be added in order to ensure σ to be real. Having computed δ, the equation ( 26) has two roots with opposite signs. The linear slice corresponding to the group D 4 is of degree 2, which was already known (see Appendix B). Proposition 4.9. A harmonic tensor D ∈ H 4 is in the closed stratum Σ [D 4 ] if and only if the invariants J 2 (D) • • • J 10 (D) satisfy the following polynomial relations

6 J 6 = -3 J 2 3 + 9 J 2 J 4 + 20 J 3 2 3 J 7 = J 2 2 J 3 -3 J 3 J 4 + 3 J 2 J 5 6 J 4 2 = -3 J 2 4 + 9 J 2 2 J 4 + 20 J 2 J 3 2 -20 J 3 J 5 5 J 8 = -3 J 2 4 + 6 J 2 2 J 4 + 30 J 2 J 3 2 -5 J 3 J 5 3 J 4 J 5 = 7 J 2 3 J 3 -15 J 2 J 3 J 4 + 3 J 2 2 J 5 -60 J 3 3 6 J 9 = 5 J 2 3 J 3 -13 J 2 J 3 J 4 + 6 J 2 2 J 5 -20 J 3 3 5 J 5 2 = -2 J 2 5 + 4 J 2 3 J 4 + 10 J 2 2 J 3 2 -20 J 2 J 3 J 5 + 30 J 3 2 J 4 15 J 10 = -9 J 2 5 + 18 J 2 3 J 4 + 85 J 2 2 J 3 2 -30 J 2 J 3 J 5 + 15 J 3 2 J 4 ( 28
)
together with inequality [START_REF] Hilbert | Theory of algebraic invariants[END_REF]. D is in the tetragonal class

[D 4 ] if moreover 3 J 4 -J 2 2 = 0 and 5 J 3 2 -8 J 2 J 4 -70 J 2 3 = 0.
In that case, it admits the normal form [START_REF] He | Characterization of the anisotropic materials capable of exhibiting an isotropic Young or shear or area modulus[END_REF] where

δ = - 1 4
J 5 J 2 2 -3 J 4
and where σ is the positive root of J 2 = 8 σ 2 + 280 δ 2 .

Remark 4.10. All 2nd-order tensors in H 2 covariant to D are multiple of diag(1, 1, -2) and are eigenvectors of D corresponding to the eigenvalue 12δ.

Orthotropic symmetry ([D 2 ]

). A D 2 -invariant tensor D ∈ H 4 has the following matrix representation:

D =         -λ 2 -λ 3 λ 3 λ 2 0 0 0 λ 3 -λ 3 -λ 1 λ 1 0 0 0 λ 2 λ 1 -λ 1 -λ 2 0 0 0 0 0 0 2λ 1 0 0 0 0 0 0 2λ 2 0 0 0 0 0 0 2λ 3         (29)
where (λ 1 , λ 2 , λ 3 ) ∈ R 3 . The monodromy group is the group S 3 acting on (λ 1 , λ 2 , λ 3 ) by permutation. The evaluations of the invariants ( 16) on this slice are S 3 -invariant. They can therefore be expressed as polynomial functions of the elementary symmetric polynomials (σ 1 , σ 2 , σ 3 ) defined by:

σ 1 := λ 1 + λ 2 + λ 3 , σ 2 := λ 1 λ 2 + λ 1 λ 3 + λ 2 λ 3 , σ 3 := λ 1 λ 2 λ 3 .
More precisely, we have:

J 2 = -14 σ 2 + 8 σ 2 1 J 3 = -6 σ 1 σ 2 + 24 σ 3 J 4 = 40 σ 1 σ 3 -112 σ 2 1 σ 2 + 68 σ 2 2 + 32 σ 4 1 J 5 = 64 σ 2 1 σ 3 -12 σ 2 σ 3 -16 σ 3 1 σ 2 + 28 σ 1 σ 2 2 J 6 = -344 σ 3 2 + 192 σ 3 1 σ 3 -24 σ 2 3 -672 σ 4 1 σ 2 + 1008 σ 2 1 σ 2 2 + 128 σ 6 1 -504 σ 1 σ 2 σ 3 J 7 = -432 σ 2 1 σ 2 σ 3 + 384 σ 4 1 σ 3 + 104 σ 2 2 σ 3 -96 σ 1 σ 2 3 -64 σ 5 1 σ 2 + 192 σ 3 1 σ 2 2 -248 σ 1 σ J 8 = 608 σ 3 1 σ 2 σ 3 + 80 σ 4 2 -768 σ 5 1 σ 3 + 192 σ 2 1 σ 2 3 + 72 σ 2 σ 2 3 + 288 σ 4 1 σ 2 2 -416 σ 2 1 σ 3 2 + σ 1 σ 2 2 σ 3 J 9 = -5248 σ 4 1 σ 2 σ 3 + 2880 σ 2 1 σ 2 2 σ 3 + 1328 σ 1 σ 2 σ 2 3 + 144 σ 3 3 + 2304 σ 6 1 σ 3 -1152 σ 3 1 σ -880 σ 3 2 σ 3 -256 σ 7 1 σ 2 + 1024 σ 5 1 σ 2 2 -2304 σ 3 1 σ 3 2 + 2160 σ 1 σ 4 2 J 10 = 10752 σ 5 1 σ 2 σ 3 -1280 σ 3 1 σ 2 2 σ 3 -5664 σ 1 σ 3 2 σ 3 -2688 σ 2 1 σ 2 σ 2 3 -800 σ 5 2 -4608 σ 7 1 σ 3 + 2304 σ 4 1 σ 2 3 -1344 σ 2 2 σ 2 3 -288 σ 1 σ 3 3 + 1536 σ 6 1 σ 2 2 -4224 σ 4 1 σ 3 2 + 3104 σ 2 1 σ 4 2
As it can be observed, the parametric system obtained for [D 2 ] is rather complicated. Nevertheless, the evaluation of J 2 , . . . , J 10 on the slice and their expressions using σ 1 , σ 2 , σ 3 is simple using computer. By computing the elimination ideal of a Groebner basis for the ideal generated by

J k -p k (σ 1 , σ 2 , σ 3 ), k = 2, . . . , 7,
we obtain a set of 6 syzygies.

-1350

J 3 J 7 -840 J 4 J 6 + 465 J 2 2 J 6 + 270 J 5 2 + 720 J 2 J 3 J 5 + 747 J 2 J 4 2 -170 J 3 2 J 4 -564 J 2 3 J 4 + 70 J 2 2 J 3 2 + 84 J 2 = 0 (30a) -1620 J 4 J 7 + 810 J 2 2 J 7 + 360 J 5 J 6 -1110 J 2 J 3 J 6 + 999 J 2 J 4 J 5 + 960 J 3 2 J 5 -549 J 2 3 J 5 -972 J 3 J 4 2 + 1638 J 2 2 J 3 J 4 -80 J 2 J 3 3 -312 J 2 4 J 3 = 0 (30b) 4050 J 5 J 7 -25650 J 2 J 3 J 7 -14310 J 2 J 4 J 6 + 9600 J 3 2 J 6 + 7965 J 2 3 J 6 + 9450 J 3 J J 5 + 10530 J 2 2 J 3 J 5 + 1134 J 4 3 + 11259 J 2 2 J 4 2 -12330 J 2 J 3 2 J 4 -9018 J 2 4 J 4 + 400 J 3 4 + 3270 J 2 3 J 3 2 + 1350 J 2 = 0 (30c) -12150 J 2 J 3 J 7 + 3600 J 6 2 -11610 J 2 J 4 J 6 + 9750 J 3 2 J 6 + 4410 J 2 3 J 6 + 8505 J 3 J 4 J 5 + 3645 J 2 2 J 3 J 5 + 1458 J 4 3 + 5670 J 2 2 J 4 2 -10710 J 2 J 3 2 J 4 -4104 J 2 4 J 4 + 400 J 3 4 + 2580 J 2 3 J 3 2 + 576 J 2 6 = 0 (30d) 1800 J 6 J 7 -10800 J 2 J 4 J 7 + 4800 J 3 2 J 7 + 4950 J 2 3 J 7 + 4020 J 3 J 4 J 6 -8370 J 2 2 J J 6 + 162 J 4 2 J 5 + 7371 J 2 2 J 4 J 5 + 2880 J 2 J 3 2 J 5 -3483 J 2 4 J 5 -9216 J 2 J 3 J 4 2 + 640 J 3 3 J 4 + 11946 J 2 3 J 3 J 4 -720 J 2 2 J 3 3 -2160 J 2 5 J = 0 (30e) 60750 J 7 2 + 178200 J 3 J 4 J 7 -546750 J 2 2 J 3 J 7 + 3780 J 4 2 J 6 -246780 J 2 2 J 4 J 6 + 348000 J 2 J 3 2 J 6 + 137025 J 2 4 J 6 + 116640 J 2 J 3 J 4 J 5 -75600 J 3 3 J 5 + 223560 J 2 3 J 3 J 5 + 29808 J 2 J 4 3 + 82170 J 3 2 J 4 2 + 177660 J 2 3 J 4 2 -438390 J 2 2 J 3 2 J 4 -148014 J 2 5 J 4 + 17200 J 2 J 3 4 + 102000 J 2 4 J 3 2 + 22221 J 2 = 0 (30f)
If these relations are satisfied, the elementary symmetric polynomials σ 1 , σ 2 and σ 3 can be expressed, on the open stratum defined by

6 J 6 -9 J 2 J 4 -20 J 3 2 + 3 J 2 3 = 0,
as rational expressions of J 2 , . . . , J 7 . More precisely, we have

σ 2 = 4 7 σ 1 2 - 1 14 J 2 , σ 3 = 1 24 J 3 + 1 7 σ 1 3 - 1 56 σ 1 J 2 ,
and

σ 1 = - 9 3 J 7 -3 J 2 J 5 + 3 J 3 J 4 -J 2 2 J 3 2 6 J 6 -9 J 2 J 4 -20 J 3 2 + 3 J 2 3 .
Then λ 1 , λ 2 , λ 3 are recovered (up to their monodromy group) as the roots of the third degree polynomial in λ p(λ) = λ 3 -σ 1 λ 2 + σ 2 λ -σ 3 But to ensure that given J 2 , . . . , J 7 ∈ R, we can find λ 1 , λ 2 , λ 3 ∈ R, the following inequalities have to be satisfied

∆ 3 = σ 2 1 σ 2 2 -4σ 3 1 σ 3 + 18σ 1 σ 2 σ 2 3 -27σ 2 3 ≥ 0 , as well as ∆ 2 = 2σ 2 1 -6σ 2 ≥ 0 .
The discriminant ∆ 3 can be rewritten, using J 2 , . . . , J 6 , as

∆ 3 = 6 J 6 -9 J 2 J 4 -20 J 3 2 + 3 J 2 3
432 .

whereas ∆ 2 can be rewritten, using J 2 , . . . , J 7 , as

∆ 2 = N 2 14 6 J 6 -9 J 2 J 4 -20 J 3 2 + 3 J 2 3 2
where

N 2 = 3645 J 7 2 + 7290 J 2 J 5 J 7 -7290 J 3 J 4 J 7 + 2430 J 2 2 J 3 J 7 -36 J 2 J 6 2 + 108 J 2 2 J 4 J 6 + 240 J 2 J 3 2 J 6 -36 J 2 4 J 6 -3645 J 2 2 J 5 2 + 7290 J 2 J 3 J 4 J 5 -2430 J 2 3 J 3 J 5 -3645 J 3 2 J 4 2 -81 J 2 3 J 4 2 + 2070 J 2 2 J 3 2 J 4 + 54 J 2 5 J 4 -400 J 2 J 3 4 -285 J 2 4 J 3 2 -9 J 2 7 .
We will summarize these results in the following proposition.

obtained from J k -p k (σ 1 , σ 2 , σ 3 ), where we have substituted the σ i by their corresponding rational expression, and we have cleared the denominators. The inequalities

6 J 6 -9 J 2 J 4 -20 J 3 2 + 3 J 2 3 ≥ 0 ( 32
)
as well as 3645

J 7 2 + 7290 J 2 J 5 J 7 -7290 J 3 J 4 J 7 + 2430 J 2 2 J 3 J 7 -36 J 2 J 6 2 + 108 J 2 2 J 4 J 6 + 240 J 2 J 3 2 J 6 -36 J 2 4 J 6 -3645 J 2 2 J 5 2 + 7290 J 2 J 3 J 4 J 5 -2430 J 2 3 J 3 J 5 -3645 J 3 2 J 4 2 -81 J 2 3 J 4 2 + 2070 J 2 2 J 3 2 J 4 + 54 J 2 5 J 4 -400 J 2 J 3 4 -285 J 2 4 J 3 2 -9 J 2 7 ≥ 0 (33)
are also required. D is in the orthotropic class [D 2 ] if moreover inequality (32) is strict. In that case, it admits the normal form (29) where λ 1 , λ 2 , λ 3 are the roots of the third degree polynomial

p(λ) = λ 3 -σ 1 λ 2 + σ 2 λ -σ 3 .
Remark 4.12. Even if it is the core of the method, it worths to note that, instead of λ 1 , λ 2 , λ 3 , the 3 numbers σ 1 , σ 2 , σ 3 are uniquely defined on the open stratum: they are invariants of D and can be written as rational expressions of (J 2 , . . . , J 7 ). It should be noticed that it is also possible to express σ 1 , σ 2 , σ 3 as rational functions of only (J 2 , . . . , J 5 ) on an open subset of Σ [D 2 ] . We will, however, not give these formulas here.

Remark 4.13. All second rank tensors in H 2 covariant to D commute together and the commutators of two of any of them vanishes.

4.8. Bifurcation conditions for tensors in H 4 . An important observation has to be made.

Mathematical results [START_REF] Shioda | On the graded ring of invariants of binary octavics[END_REF] tell us that the invariant algebra of H 4 is generated by the 9 fundamental invariants (J 2 , . . . , J 10 ). In this set 6 invariants (J 2 , . . . , J 7 ) are algebraically independent, meanwhile the others are linked to the formers by polynomial relations. As discussed before, it is a well-known fact that this algebra separates the orbits. But as demonstrated exhaustively in this section, for any class with a finite monodromy group 10 , the first 6 invariants (J 2 , . . . , J 7 ) are necessary and sufficient to separate the orbits inside each class. This observation can be summed-up by the following important theorem.

Theorem 4.14. For the following isotropy classes: the cubic, the transversely isotropic, the trigonal the tetragonal (and the trivial isotropic) classes in H 4 (R 3 ), the first 4 invariants (J 2 , . . . , J 5 ) separate the orbits inside the class. For the orthotropic class, the first 6 invariants (J 2 , . . . , J 7 ) separate the orbits inside the class.

We will now summarized the previous results by giving bifurcation conditions on the J k to explicit how to "travel" from a given isotropy class to another. More precisely, so far, we have given necessary and sufficient conditions to belong to a closed stratum Σ [H] with finite monodromy group. As we have seen, Σ [H] is characterized by a set of polynomial relations (syzygies) involving the invariants J k . These relations define implicit equations which characterize the closed stratum 11 . The set of relations that characterizes a given (closed) stratum generates an ideal (c.f. subsection A.4), I [H] in R[J 2 , . . . , J 10 ], and we have

[H 1 ] [H 2 ] ⇒ I [H 2 ] ⊂ I [H 1 ]
The bifurcation conditions, detailed on figure 2, correspond to relations (generators) which need to be added to

I [H 1 ] to obtain I [H 2 ] .
Remark 4.15. These bifurcation relations were obtained with the following algorithm. Starting with some subgroup H and the corresponding fixed point space V H , we look for linear relations needed to define the subspace V K of V H , where H ⊂ K. Given such a relation α = 0 (in fact, in all the cases considered, only one relation was needed), we compute a Γ H -invariant polynomial

p α := γ∈Γ H γ • α , provided Γ H is finite. It is then clear that p α vanishes on a point v ∈ V H if and only if v belongs to V K or V γKγ -1
, for some γ ∈ Γ H , that is if and only if v has at least isotropy [K]. This Γ H -invariant polynomial is then expressed using the J k . 4.9. Relation with the characteristic polynomial. To conclude this subsection, some links that exists between the invariants of H 4 and the characteristic polynomial of D will be investigated. The first link is formulated by the following proposition: Proposition 4.16. Let D ∈ H 4 . The coefficients of the characteristic polynomial of D are SO(3)-invariants of D. They can be expressed in terms of the invariants J 2 , . . . , J 5 defined above. We have

χ D (z) = z 6 - 1 2 J 2 z 4 - 1 3 J 3 z 3 + 1 5 J 2 2 -2 J 4 z 2 + 2 25 ( J 2 J 3 -3 J 5 ) z. ( 34
)
10 i.e. for all symmetry classes, except the monoclinic and the triclinic classes 11 Notice that a rigorous and complete classification of these closed strata would require also some real inequalities that we have not completely detailed here. Proof. Notice first that z = 0 is always an eigenvalue of D. Indeed, if q stands for the standard metric on R 3 , we have Dq = tr 12 (D) = 0. The coefficient of z k in χ D is a homogeneous invariant polynomial of degree 6 -k for 1 ≤ k ≤ 4. Therefore, it can be expressed as a polynomial in J 2 (D), • • • J 5 (D). The computation of this last expression reduces therefore to identify some real coefficients. This can be done by specializing on particular values or on some particular strata where it is easy to compute, the cubic or the transverse isotropic stratum, for instance.

Besides the characteristic polynomial, Betten [START_REF] Betten | Irreducible invariants of fourth-order tensors[END_REF] has introduced a two variables, invariant polynomial of the elasticity tensor. It is defined as

B C (λ, µ) = det(C -C λ,µ )
where

C λ,µ =         λ + 2 µ λ λ 0 0 0 λ λ + 2 µ λ 0 0 0 λ λ λ + 2 µ 0 0 0 0 0 0 2 µ 0 0 0 0 0 0 2 µ 0 0 0 0 0 0 2 µ        
is the totally isotropic tensor. Notice that B D (0, µ) = χ D (2 µ) and that B C is of degree lower than 1 in λ (indeed, this can be observed by subtracting the first line to the second line and to the third line in the determinant). The supposed interest of this polynomial is to obtain more invariants than the ones of the characteristic polynomial. However, it seems that it does not bring any new invariants for a tensor D ∈ H 4 (R 3 ). Indeed, we have the following result.

Lemma 4.17. The Betten polynomial of a harmonic tensor D ∈ H 4 (R 3 ) is related to its characteristic polynomial by the formula

B D (λ, µ) = (3 λ + 2 µ) χ r D (2 µ)
where χ D (λ) = λ χ r D (λ).

Proof. Let L k (1 ≤ k ≤ 6) denote the lines of the determinant det(D -C λ,µ ). For a harmonic tensor D, we have

3 k=1 d p,k = 0, 1 ≤ p ≤ 6.
Thus, substituting L 1 → L 1 + L 2 + L 3 in the determinant det(D -C λ,µ ), we get

B D (λ, µ) = (3 λ + 2 µ) P (λ, µ)
but since B D (λ, µ) is of degree lower than 1 in λ we have P (λ, µ) = P (0, µ). Now for λ = 0 we have B D (0, µ) = χ D (2 µ) = (2 µ) P (0, µ) from which we get P (0, µ) = χ r D (2 µ). This achieves the proof. Corollary 4.18. The invariants defined by the coefficients of the Betten polynomial do not separate the orbits of Ela.

Proof. If it was the case, it would also separate the orbits of H 4 but then lemma 4.17 and proposition 4.16 would imply that J 2 , J 3 , J 4 , J 5 separate the orbits of H 4 , which is false.

Towards an effective determinacy of elasticity classes

In this last section, an application of the concepts previously introduced will be given. This application will concern the identification of the symmetry class of a tensor which is not expressed in one of its natural bases. As our objective is mainly to illustrate the tools we propose, we will only provide a set of sufficient conditions. This approach is based on a hypothesis of genericity for the anisotropic tensors. Such hypothesis, which amounts to exclude any exceptional case, will be detailed and criticized in the next subsection. Therefore, in order to avoid any misunderstanding, our aim is not to provide a complete answer to identification problem. A full treatment of this question would require a detailed analysis of each particular non generic case that may occur, and constitutes a full and complex study on its own. As the core of this first paper is focused on the symmetry class identification in terms of H 4 invariants, we have rather choose a simpler problem to treat.

Generic anisotropic elasticity tensors.

A well-known result of anisotropic elasticity [START_REF] Forte | Symmetry classes for elasticity tensors[END_REF] states that the symmetry group of an elasticity tensor is the intersection of the symmetry group of each one of its irreducible components. In other terms: [START_REF] Auffray | Démonstration du théorème de Hermann à partir de la méthode Forte-Vianello[END_REF]. In the most general situation, all the former tensors are independent and therefore a complete analysis of the intersection of symmetry group has to be conducted. Such a problem is a complex one, and is a subject of a study on its own. For the physical situations considered here, a simplifying hypothesis will be made. As the tensor symmetry is a consequence of the material symmetries, a weak coupling between the tensors of the set (a, b, D) will be considered. To that end let us consider the triplet of quadratic forms L := (a, b, d 2 ), where d 2 is the fundamental covariant of degree 2 introduced in ( 16). We have the following direct lemma Lemma 5.1.

G C = G a ∩ G b ∩ G D since we trivially have G λ = G µ = SO
G C = G L ∩ G D Proof. As d 2 is covariant to D, G D ⊆ G d 2 and therefore G D ∩ G d 2 = G D . Thus G L ∩ G D = G a ∩ G b ∩ G d 2 ∩ G D = G a ∩ G b ∩ G D = G C
The hypothesis that will be made here is the following

Hypothesis 5.2 (Material Genericity Assumption (MGA)). If [G L ] [D 2 ] then G L = G d 2
Such an hypothesis is equivalent to the following:

if [G L ] [D 2 ] then G d 2 ⊆ G a and G d 2 ⊆ G b .
From a physical point of view, this means that the symmetries of a and b are compatible with the ones of D in the sense that adding G a and G b to the intersection will not lower G D . In other words : If this genericity assumption fails, and without a deeper study, all it can be said is G C ⊆ G D . From a mathematical point of view, studying these cases are necessary to close the problem. On the other hand, from a material point of view and for a first approach, we believe that the genericity assumption to hold true for a broad class of classical materials. Anyway situations where this hypothesis may fail can be conceived:

Lemma 5.3. G L = G d 2 ⇒ G C = G D Proof.
Tailored materials: As the symmetry class of an elasticity tensor is driven by the architecture of the microstructure, it can be conceived to tailor the microstructure in order to give the material some non-conventional properties [START_REF] He | Characterization of the anisotropic materials capable of exhibiting an isotropic Young or shear or area modulus[END_REF]. Therefore for such a specific design the genericity can not be assumed. Such kind of very specific architecture might not only be found in engineered materials, but also in some biomaterials. Damaged materials: Genericity may also fail for damaged-materials, according to certain model [START_REF] François | A damage model based on Kelvin eigentensors and Curie principle[END_REF] the symmetry breaking induces by the damage process may result in non generic anisotropic properties. Therefore, bearing in mind the aforementioned limitations induced by the genericity assumption, we can propose, in the following section, an identification algorithm.

Symmetry class identification.

Based on the results obtained in this paper, the algorithm provided figure 3 allows to identify the symmetry class under the MGA assumption. Concerning this algorithm some remarks are in order.

• The first step of the algorithm is based on the symmetry analysis of the triplet of quadratic forms L. This symmetry identification is based on the conditions given in subsection 2.2. As indicated on the diagram if G L = Z 2 , the symmetry class of C is decided using the Cowin-Mehrabadi criterion for the non vanishing vector ω, obtained from case (1) of Theorem 2.1 applied to the triplet L. This test is denoted by CM (ω)? in figure 3. • For classes strictly greater than [D 2 ] we use the syzygy relations given in section 4, to check the isotropy class of D. Some tests can be avoided a priori (and will thus not be included in figure 3) by the following observations which result from lemma 5.4 combined with the bifurcation conditions given in figure 2.

(

) If G L = O(2) = G d 2 , then the isotropy class of D cannot belong to {[O], [SO(3)]}. ( 1
) If G L = SO(3) = G d 2 , then J 2 2 -3J 4 = 0 and the isotropy class of D cannot belong to {[D 3 ], [D 4 ], [O(2)]}. Lemma 5.4. If d 2 = λId then J 2 2 -3J 4 = 0. Proof. If d 2 is a scalar multiple of the identity, J 2 2 = 9λ 2 and J 4 = tr(d 4 ) = tr(d 2
2 ) = 3λ 2 . The conclusion follows.

Conclusion

In this paper a method aiming at identifying the symmetry class of an elasticity tensor has been provided. Conversely to other approaches based on the spectral decomposition of the elasticity tensor, this method is based on the specific relations between invariants in each symmetry ). Completed by a genericity assumption on the elasticity tensor under study this approach was shown to be able to identify the 8 possible elasticity classes.

We consider this paper as a first step towards a more systematic uses of invariant-based methods in continuum mechanics. Using the geometrical framework introduced in this paper some extensions of the current method can be envisaged :

• The first point would be to get rid of the genericity assumption we made, such a point constitutes a full study on its own ; • We aim at extending the current sygyzy-characterization to group having continuous monodromy group ; • To propose a complete set of normal forms of elasticity tensor constructed using this invariants approach ; • To tackle the increasing degree of the algebraic relations for low symmetry group, it might be interesting to characterize classes in terms of covariants. Such an approach was conducted here in the context of classical elasticity, but the framework used allows a direct extension of this method to other kinds of anisotropic tensorial behaviors. It can be suited for the study of anisotropic features of piezoelasticity [START_REF] Geymonat | Symmetry classes of piezoelectric solids[END_REF], flexoelasticity [START_REF] Quang | The number and types of all possible rotational symmetries for flexoelectric tensors[END_REF] or strain-gradient elasticity [START_REF] Le Quang | Symmetry groups and symmetry classes for sixth-order tensors[END_REF]. Furthermore, and in a more experimental view, we aim at testing our approach on experimental situations. This would allow to compare the proposed approach with the more classical ones.

Appendix A. Geometric concepts A.1. Normal form. A normal form is a map NF : V → V such that v 1 ≈ v 2 if and only if NF(v 1 ) = NF(v 2 ).
The existence of a computable normal form allows to decide whether two vectors v 1 and v 2 belong to the same G-orbit.

Remark A.1. In practice, one contents itself usually with a normal form defined up to a finite group. For instance, one is satisfied with a diagonal matrix as "the" normal form for the action of the rotation group on the space of symmetric matrices. However this diagonal form is not unique. It is defined up to the permutation of the diagonal entries.

A.2. Covariants. Given a second representation (V ′ , ρ ′ ) of the same group G, we say that a linear map ϕ :

V → V ′ is equivariant if the following diagram V ϕ ----→ V ′ ρ(g)     ρ ′ (g) V ϕ ----→ V ′ commutes for each g ∈ G.
The composition of two equivariant maps is equivariant.

Given v ∈ V the element ϕ(v) will be said to be covariant to v. It is clear that every element v ∈ V is covariant to itself (with ϕ = Id). However, this relation is not symmetric. Indeed, if v ′ is covariant to v, the converse is not necessarily true (v is not necessarily covariant to v ′ ) unless the linear map ϕ : V -→ V ′ is bijective. In this case, we say that the representations (V, ρ) and (V ′ , ρ ′ ) are isomorphic or equivalent.

When the representation (V ′ , ρ ′ ) is trivial, i.e. when ρ ′ (g) = Id for all g ∈ G, the covariant element ϕ(v) will be called an invariant. We have then

v ≈ v ′ ⇒ ϕ(v) = ϕ(v ′ ).
A.3. Polynomial invariants. The linear action of the group G on the R-vector space V extends naturally to the vector space of polynomial functions defined on V with values in R. This extension is given by (g • P )(v) := P (g -1 • v) for every polynomial P ∈ R[V ] and every vector v ∈ V . The set of all invariant polynomials is a sub-algebra of the algebra R[V ] of all polynomials defined on V . We denote this sub-algebra by R[V ] G and call it the invariant algebra.

When G is a compact Lie group (or more generally a reductive group), this algebra R[V ] G is of finite type [START_REF] Hilbert | Theory of algebraic invariants[END_REF]. This means first that it is possible to find a finite set of invariant polynomials J 1 , . . . , J N which generate (as an algebra) R[V ] G . But, in general, this algebra is not free, which means that it is not always possible to extract from J 1 , . . . , J N a subset of generators which are algebraically independent 12 . The set of polynomial relations on J 1 , . . . , J N (also known as syzygies) is an ideal of R[J 1 , . . . , J N ] that is finitely generated. Such a set of generators is called an integrity basis for R[V ] G . Of course it is not unique and moreover its cardinal may change from one basis to another. A.4. Ideal and Groebner basis. Consider a subset V of k n (where k = R or C) defined implicitly by a finite set of polynomial equations p 1 , . . . p r . The fundamental observation is that the set I of polynomials p ∈ k[x 1 , . . . , x n ] such that p(x) = 0 for each point x ∈ S (where x = (x 1 , . . . , x n )) satisfies the following properties:

(i) 0 ∈ I, (ii) If p, q ∈ I, then p + q ∈ I, (iii) If p ∈ I and q ∈ k[x 1 , . . . , x n ], then pq ∈ I. Such a subset I of k[x 1 , . . . , x n ] (or more generally any commutative ring) is called an ideal. In the example above the ideal I is (finitely) generated by p 1 , . . . p r (i.e. every p ∈ I can be written as a finite sum s j=1 f j p j where f j ∈ k[x 1 , . . . , x n ]) and we write I =< p 1 , . . . p r >. To solve a system of linear equations in k n , a standard algorithm is to reduce the matrix of the system to a triangular form (a normal form, even if not unique). A Groebner basis of an ideal • I is the icosahedral group, the (orientation-preserving) symmetry group of a icosahedra or dodecahedron. It has order 60. • 1 is the trivial subgroup, containing only the unit element. The lattice of conjugacy classes of closed subgroups of SO(3) is completely described [START_REF] Golubitsky | Singularities and groups in bifurcation theory[END_REF] by the following inclusion of subgroups (which generates order relations between conjugacy classes) Finally, consider D n for some n ≥ 2. Because D n is subgroup of index 2 of D 2n , we have D 2n ⊂ N (D n ). Suppose first that n > 2 and let g ∈ N (D n ). Then gr 2π/n g -1 is a rotation of angle ±2π/n and its axis is therefore the z-axis. Hence g(e 3 ) = ±e 3 . Besides, gσg -1 is a rotation of order 2 whose axis is not the z-axis (otherwise we would have g(e 1 ) = ±e 3 ). Therefore we have where R e j denotes the rotation by angle π around e j . For each element g of the octahedral group O, we have g(e i ) = ±e j and hence gR e i g -1 = R e j .

Z n ⊂ D n ⊂ O(2) (n ≥ 2),
Therefore O ⊂ N (D 2 ). Using again the maximality argument, we conclude that N (D 2 ) = O.

For the computation of the quotient groups, all of them are obvious but N (Z n )/Z n and N (D 2 )/D 2 . Concerning the first quotient group, we remark that every element in O(2) can be written uniquely as σ ε r where ε = 0, 1 and r ∈ SO(2) is a rotation around the z-axis. Consider now the map

φ : O(2) → O(2), σ ε r → σ ε r n .
One can check that this map is a surjective, group morphism and that its kernel is exactly Z n . This establish the isomorphism between O(2)/Z n and O(2). For the second quotient group, we recall that there is a well-known isomorphism from the symmetry group of the cube O onto the group of permutations of the diagonals of the cube, that is S 4 . Under this isomorphism, the subgroup D 2 of O is sent to D 2 := {e, (12)(34), (13)(24), (23)(14)} , which is a normal subgroup of S 4 . Let S 3 be the subgroup of S 4 which fixes the element 4. Then S 4 is the semi-direct product of D 2 by S 3 . In particular, the quotient S 4 /D 2 is isomorphic to S 3 . This subgroup S 3 corresponds geometrically to the subgroup of O generated by a rotation of angle 2π/3 around a diagonal and a rotation of angle π around any diagonal perpendicular to the first one, that is a conjugate of the group D 3 .

  . There are exactly 8 classes: the isotropic class [SO(3)], the cubic class [O], the transversely isotropic class [O(2)], the trigonal class [D 3 ], the tetragonal class [D 4 ], the orthotropic class [D 2 ], the monoclinic class [Z 2 ]

3. 3 .

 3 The Cowin-Mehrabadi condition. The information contained in the Voigt and dilatational tensors[START_REF] Cowin | Properties of the anisotropic elasticity tensor[END_REF] solely concerns the covariants (λ, µ, a, b). As a consequence any statement on d and v can be translated in terms of (λ, µ, a, b), and conversely.The condition (8) for (a, b) to have at least isotropy [Z 2 ] means that ω = [a, b] is a common eigenvector of (a, b). If we note n = ω, it is a straightforward computation to show that this condition on (a, b) is equivalent to

Figure 1 .

 1 Figure 1. The lattice of isotropy classes for H 4 .

Proposition 4 . 1 .

 41 Let D ∈ H 4 . The 2nd-order tensors d 2 , . . . , d 10 :

Remark 4 . 2 .

 42 Notice that the first invariant J 2 (D) = D, D is the squared norm of D, it corresponds to the squared Frobenius norm of D. In particular, J 2 (D) = 0 if and only if D = 0.

Proposition 4 . 3 .

 43 A harmonic tensor D ∈ H 4 is in the closed stratum Σ [O] if and only if the invariants J 2 (D) • • • J 10 (D) satisfy the following polynomial relations

Remark 4 . 4 . 4 . 4 .

 4444 All 2nd-order tensors in H 2 covariant to D vanish. Transversely isotropic symmetry ([O(2)]). A O(2)-invariant tensor D ∈ H 4 has the following matrix representation:

Proposition 4 . 5 .

 45 A harmonic tensor D ∈ H 4 is in the closed stratum Σ [O(2)] if and only if the invariants J 2 (D) • • • J 10 (D) satisfy the following polynomial relations

Remark 4 . 6 . 4 . 5 .

 4645 All 2nd-order tensors in H 2 covariant to D are multiple of diag(1, 1, -2) and are eigenvectors of D corresponding to the eigenvalue 12δ. Trigonal symmetry ([D 3 ]). A D 3 -invariant tensor D ∈ H 4 has the following matrix representation:

Proposition 4 . 7 .

 47 A harmonic tensor D ∈ H 4 is in the closed stratum Σ [D 3 ] if and only if the invariants J 2 (D) • • • J 10 (D) satisfy the following polynomial relations

Proposition 4 . 11 .

 411 A harmonic tensor D ∈ H 4 is in the closed stratum Σ [D 2 ] if and only if the invariants J 2 (D), • • • , J 10 (D) satisfy the polynomial relations (30), completed with the 3 equations S k (J 2 , . . . J 10 ) = 0, k = 8, 9, 10

Figure 2 .

 2 Figure 2. Bifurcation paths for finite monodromy isotropy classes in H 4 .

From the lemma 5 . 1 ,

 51 we have G C = G L ∩ G D . Therefore, according to the hypothesis G C = G d 2 ∩ G D , and as d 2 is covariant to D the intersection reduces to G C = G D .

Figure 3 .

 3 Figure 3. Symmetry identification for a generic tensor in Ela.

Z

  n ⊂ Z m and D n ⊂ D m , (if n divides m), Z 2 ⊂ D n (n ≥ 2), Z n ⊂ SO(2) ⊂ O(2) (n ≥ 2),and the arrows in figure4which complete the lattice, taking account of the exceptional subgroups O, T, I (beware that figure 4 cannot be realized by an inclusion diagram between representatives of these conjugacy classes).

Figure 4 .

 4 Figure 4. Exceptional classes in the lattice of conjugacy classes of closed subgroup of SO(3). Remark B.1. Notice that O and I are maximal subgroups of SO(3) which both contained T.Recall that the normalizer of a subgroup H of a group G, denoted by N (H) is the biggest subgroup of G in which H is normal. It is defined byN (H) := g ∈ G; gHg -1 = H .Proposition B.2. The normalizers of the subgroups in the list (35) are given below.N (SO(3)) = SO(3), N (O(2)) = O(2), N (SO(2)) = O(2), N (O) = O, N (I) = I, N (T) = O, N (1) = SO(3), N (Z n ) = O(2) for n ≥ 2, N (D n ) = D 2n for n ≥ 3,and N (D 2 ) = O. Moreover, the quotient groups are given byN (SO(3))/SO(3) = 1, N (O(2))/O(2) = 1, N (SO(2))/SO(2) = Z 2 , N (O)/O = 1, N (I)/I = 1, N (T)/T = Z 2 , N(1)/1 = SO(3), N (Z n )/Z n = O(2) for n ≥ 2, N (D n )/D n = Z 2 for n ≥ 3,

gσg - 1 =

 1 r 2kπ/n σr -2kπ/n = r 4kπ/n σ, for some k ∈ {0, 1, . . . , n -1}. Now if g(e 3 ) = e 3 , we have g = r θ and necessarily 2θ = 4kπ/n. If g(e 3 ) = -e 3 we have g = σr θ and necessarily 2θ = 4kπ/n. In both cases, this means that g ∈ D 2n and we conclude that N (D n ) = D 2n . Suppose now that n = 2, in which case D 2 is the Klein group D 2 := {e, R e 1 , R e 2 , R e 3 } ,

  Remark 1.2. Notice that the closure of a stratum Σ [H] , denoted Σ [H] and called the closed stratum associated to [H], corresponds to vectors v ∈ V such that v has at least isotropy [H], whereas Σ [H] corresponds to vectors v ∈ V such that v has exactly isotropy [H]. 1.5. Fixed point sets and normalizers. Let H be any subgroup of G. The set of vectors v ∈ V which are fixed by H V

  Notice that the set V H ∩ Σ[H] is the subspace of vectors in V H which isotropy class is exactly[H]. It is an open and dense set in V H , i.e almost all elements of V H are in V H ∩ Σ [H] . These vectors correspond to the generic elements in V H . Now, since the equivalence relation defined by (V, ρ), when restricted to the space V H , is the same as the equivalence relation defined by (V H , ρ Γ H ), the basis Σ

	1)
	is a fiber bundle with fiber G/H, i.e. a space that locally looks like Σ [H] /G × G/H. Therefore, we have
	dim Σ [H] = dim(G/H) + dim(Σ [H] /G)
	where dim(G/H) = dim G -dim H.

Table 3

 3 These results of elasticity enlighten the physical contents of the geometric concepts introduced in section 1.

sums up the different dimensions that are classically associated to anisotropic elasticity tensors in the literature. All these dimensions are linked by the formula

dim Σ [H] = dim Σ [H] /G + dim Γ H + e H ,

where Σ [H] /G is the orbit space for tensors in isotropy class [H], Γ H = N (H)/H is the monodromy group and e H = dim G/N (H). Notice that e H ≃ [H] is the set of distinct subgroups of G which are conjugate to H.

The concept of symmetry class used by Forte and Vianello is equivalent to the notion of an isotropic stratum.

The lattice of conjugacy classes of all closed subgroups of SO(3) is described in Appendix B.

Polynomials P1, . . . , PN are said to be algebraically independent over R if the only polynomial in N variables which satisfies Q(P1, . . . , PN ) = 0 is the zero polynomial.

I is a set of generators for I which is a normal form for an algebraic system, in a certain sense analog to a triangular system for a set of linear equations. It permits to compute and answer effectively a lot of problems related to ideals k[x 1 , . . . , x n ] and algebraic systems. It solves, in particular, the implicitization problem. We refer to [START_REF] Cox | Ideals, varieties, and algorithms[END_REF] for an excellent exposition on Groebner basis (which is very accessible for non specialists).

A.5. Separation of orbits. Given an algebra A of invariant functions defined on V , we say that A separates the orbits if given two vectors v 1 , v 2 ∈ V 2 which are not on the same orbit, it is always possible to find a function f ∈ A such that f (v 1 ) = f (v 2 ). In general, when the group G is not compact, the algebra of polynomial invariants does not separate the orbits 13 . However, it is remarkable that for any compact group, the algebra of real invariant polynomials always separates the orbits. This result is attributed to Weyl [START_REF] Weyl | The classical groups[END_REF].

Now, let K 1 and K 2 be two distinct G-orbits in V . The following function

Since the group G is compact, the orbits are compact and it is possible to find a closed ball B in V which contains both orbits. We apply then the Stone-Weierstrass theorem which states that real polynomial functions are dense in the space of continuous functions defined on B (for the sup norm). Therefore, a polynomial function P on V can be found such that

Now, averaging this polynomial

where µ is the Haar measure over G, we obtain a G-invariant polynomial P which separates the orbits K 1 and K 2 .

Appendix B. Normalizers of closed subgroups of SO(3)

Every closed subgroup of SO( 3) is conjugate to one of the following list [START_REF] Dubrovin | Modern geometry-methods and applications. Part I[END_REF][START_REF] Golubitsky | Singularities and groups in bifurcation theory[END_REF] SO(3), O(2), SO(2), D n (n ≥ 2), Z n (n ≥ 2), T, O, I, and 1

where:

• O( 2) is the subgroup generated by all the rotations around the z-axis and the order 2 rotation σ : (x, y, z) → (x, -y, -z) around the x-axis. • SO(2) is the subgroup of all the rotations around the z-axis.

• Z n is the unique cyclic subgroup of order n of SO(2), the subgroup of rotations around the z-axis. • D n is the dihedral group. It is generated by Z n and σ : (x, y, z) → (x, -y, -z).

• T is the tetrahedral group, the (orientation-preserving) symmetry group of a tetrahedron.

It has order 12. • O is the octahedral group, the (orientation-preserving) symmetry group of a cube or octahedron. It has order 24. 13 This can be illustrated by the example of the adjoint action of the general linear group GL(n, C) on its Lie algebra gl(n, C). Each orbit is described by a unique Jordan form but the algebra of polynomial invariants, which is generated by tr(M ), tr(M 2 ), . . . tr(M n ) where M ∈ gl(n, C), is not able to distinguish between two different Jordan forms which have the same diagonal part.

Appendix C. Trace formulas for V H Let (V, ρ) be a finite dimensional linear representation of SO(3), which admits the following harmonic isotypic decomposition :

where α k is the multiplicity of the space H k in the decomposition. Using the trace formula (3) the following explicit formulas can easily be obtained [START_REF] Golubitsky | Singularities and groups in bifurcation theory[END_REF][START_REF] Auffray | Analytical expressions for anisotropic tensor dimension[END_REF] dim