Ralf Hiptmair
email: hiptmair@sam.math.ethz.ch

Peter Robert Kotiuga

AND S ÉBASTIEN Tordeux

SELF-ADJOINT curl OPERATORS

Keywords: curl operator, self-adjoint extension, complex symplectic space, Glazman-Krein-Naimark theorem, co-homology spaces, spectral properties of curl AMS subject classifications. 47F05, 46N20

We study the exterior derivative as a symmetric unbounded operator on square integrable 1-forms on a 3D bounded domain D. We aim to identify boundary conditions that render this operator self-adjoint. By the symplectic version of the Glazman-Krein-Naimark theorem this amounts to identifying complete Lagrangian subspaces of the trace space of H(curl, D) equipped with a symplectic pairing arising from the ∧-product of 1-forms on ∂D. Substantially generalizing earlier results, we characterize Lagrangian subspaces associated with closed and co-closed traces. In the case of non-trivial topology of the domain, different contributions from co-homology spaces also distinguish different self-adjoint extension. Finally, all self-adjoint extensions discussed in the paper are shown to possess a discrete point spectrum, and their relationship with curl curl-operators is discussed.

1. Introduction. The curl operator is pervasive in field models, in particular in electromagnetics, but hardly ever occurs in isolation. Most often we encounter a curl curl operator and its properties are starkly different from those of the curl alone. We devote the final section of this article to investigation of their relationship.

The notable exception, starring a sovereign curl, is the question of stable forcefree magnetic fields in plasma physics. They are solutions of the eigenvalue problem

α ∈ R \ {0} : curl H = αH , (1.1)
posed on a suitable domain, see [START_REF] Chandrasekhar | On force-free magnetic fields[END_REF][START_REF] Jette | Force-free magnetic fields in resistive magnetohydrostatics[END_REF][START_REF] Lundquist | Magneto-hydrostatic fields[END_REF][START_REF]Uber kraftfreie Magnetfelder[END_REF]. A solution theory for (1.1) must scrutinize the spectral properties of the curl operator. The mature theory of unbounded operators in Hilbert spaces is a powerful tool. This approach was pioneered by R.

Picard [START_REF] Picard | Ein Randwertproblem in der Theorie kraftfreier Magnetfelder[END_REF][START_REF]On a selfadjoint realization of curl and some of its applications[END_REF][START_REF]On a selfadjoint realization of curl in exterior domains[END_REF], see also [START_REF] Yoshida | Remarks on spectra of operator rot[END_REF].

The main thrust of research was to convert curl into a self-adjoint operator by a suitable choice of domains of definition. This is suggested by the following Green's formula for the curl operator:

D curl u • v -curl v • u dx = ∂D (u × v) • n dS , (1.2)
for any domain D ⊂ R 3 with sufficiently regular boundary ∂D and u, v ∈ C 1 (D). This reveals that the curl operator is truly symmetric, for instance, when acting on vector fields with vanishing tangential components on ∂D.

On bounded domains D several instances of what qualifies as a self-adjoint curl operators were found. Invariably, their domains were defined through judiciously chosen boundary conditions. It also became clear that the topological properties of D have to be taken into account carefully, see [START_REF]On a selfadjoint realization of curl and some of its applications[END_REF]Thm. 2.4] and [START_REF] Yoshida | Remarks on spectra of operator rot[END_REF]Sect. 4].

In this paper we carry these developments further with quite a few novel twists: we try to give a rather systematic treatment of different options to obtain self-adjoint curl operators. It is known that the curl operator is an incarnation of the exterior derivative of 1-forms. Thus, to elucidate structure, we will mainly adopt the perspective of differential forms.

Further, we base our considerations on recent discoveries linking symplectic algebra and self-adjoint extensions of symmetric operators, see [START_REF]Complex symplectic spaces and boundary value problems[END_REF] for a survey. In the context of ordinary differential equations, this connection was intensively studied by Markus and Everitt during the past few years [START_REF] Everitt | Complex symplectic geometry with applications to ordinary differential equations[END_REF]. They also extended their investigations to partial differential operators like ∆ [START_REF]Elliptic Partial Differential Operators and Symplectic Algebra[END_REF]. We are going to apply these powerful tools to the special case of curl operators. Here, the crucial symplectic space is a Hilbert space of 1-forms on ∂D equipped with the pairing

[ω, η] ∂D := ∂D ω ∧ η .
We find out, that it is the Hodge decomposition of the trace space for 1-forms on D that allows a classification of self-adjoint extensions of curl: the main distinction is between boundary conditions that impose closed and co-closed traces Moreover, further constraints are necessary in the form of vanishing circulation along certain fundamental cycles of ∂D. This emerges from an analysis of the space of harmonic 1-forms on ∂D as a finite-dimensional symplectic space. For all these self-adjoint curls we show that they possess a complete orthonormal system of eigenfunctions.

In detail, the outline of the article is as follows: The next section reviews the connection between vector analysis and differential forms in 3D and 2D. Then, in the third section, we introduce basic concepts of symplectic algebra. Then we summarize how those can be used to characterize self-adjoint extensions through complete Lagrangian subspaces of certain factor spaces. The fourth section applies these abstract results to trace spaces for 1-forms and the corresponding exterior derivative, that is, the curl operator. The following section describes important complete Lagrangian subspaces spawned by the Hodge decomposition of 1-forms on surfaces. The role of co-homology spaces comes under scrutiny. In the sixth section we elaborate concrete boundary conditions for self-adjoint curl operators induced by the complete Lagrangian subspaces discussed before. The two final sections examine the spectral properties of the classes of self-adjoint curls examined before and explore their relationships with curl curl operators. Frequently used notations are listed in an appendix.

2. The curl operator and differential forms. In classical vector analysis the operator curl is introduced as first order partial differential operator acting on vector fields with three components. Thus, given a domain D ⊂ R 3 we may formally consider curl : C ∞ 0 (D) → C ∞ 0 (D) as an unbounded operator on L 2 (D). Integration by parts according to (1.2) shows that this basic curl operator is symmetric, hence closable [START_REF] Weidmann | Linear Operators in Hilbert spaces[END_REF]Ch. 5]. Its closure is given by the minimal curl operator curl min : H 0 (curl, D) → L 2 (D) .

(

N ± := N (curl max ±ı • Id) ⊂ D(curl max ) . (2.4)
Lemma 2.1. The deficiency spaces from (2.4)

satisfy dim N ± = ∞. Proof. Let G ± : R 3 \{0} → C 3,3 be a fundamental solution (dyad) of curl ±ı, that is, G = (curl ∓i)(-1-∇ T ∇)IΦ, where Φ(x) = exp(-|x|)/(4π|x|) is the fundamental solution of -∆ + 1, and ∇ := ( ∂ ∂x1 , ∂ ∂x2 , ∂ ∂x3 ). Then, for any ϕ ∈ C ∞ (R 3 ) |∂D , u(x) := ∂D G(x -y) • ϕ(y) dS(y) , x ∈ D ,
satisfies u ∈ H(curl, D) and curl u ± ıu = 0. From Lemma 2.1 we learn that N ± reveal little about the structure governing selfadjoint extensions of curl. Yet the relationship of curl and differential forms suggests that there is rich structure underlying self-adjoint extensions of curl min .

2.1. Differential forms. The curl operator owes its significance to its close link with the exterior derivative operator in the calculus of differential forms. We briefly recall its basic notions and denote by M an m-dimensional compact orientable manifold with boundary ∂M . If M is of class C 1 it can be endowed with a space of differential forms of degree k, 0 ≤ k ≤ m: Definition 2.2 (Differential k-form). A differential form of degree k (in short, a k-form) and class C l , l ∈ N 0 , is a C l -mapping assigning to each x ∈ M an alternating k-multilinear form on the tangent space T x (M ). We write Λ k,l (M ) for the vector space of k-forms of class C l on M , Λ k,l (M ) = {0} for k < 0 or k > m.

Below, we will usually drop the smoothness index l, tacitly assuming that the forms are "sufficiently smooth" to allow the respective operations.

The alternating exterior product of multilinear forms gives rise to the exterior product ∧ : Λ k (M ) × Λ j (M ) → Λ k+j (M ) by pointwise definition. We note the graded commutativity rule ω ∧ η = (-1) kj η ∧ ω for ω ∈ Λ k , η ∈ Λ j . Further, on any piecewise smooth orientable k-dimensional sub-manifold of M we can evaluate the integral Σ ω of a k-form ω over a k-dimensional sub-manifold Σ of M [START_REF] Cartan | Differentialformen[END_REF]Sect. 4].

This connects to the integral view of k-forms as entities that describe additive and continuous (w.r.t. to a suitable deformation topology) mappings from orientable sub-manifolds of M into the real numbers. This generalized differential forms are sometimes called currents and are studied in geometric integration theory [START_REF] De Rham | Differentiable manifolds. Forms, currents, harmonic forms[END_REF][START_REF] Federer | Grundlehren der mathematischen Wissenschaften[END_REF]. From this point of view differential forms also make sense for non-smooth manifolds.

From the integral perspective the transformation (pullback) Φ * ω of a k-form under a sufficiently smooth mapping Φ :

M → M appears natural: Φ * ω is a k-form on M that fulfills b Σ Φ * ω = Σ ω (2.5)
for all k-dimensional orientable sub-manifolds Σ of M . We remark that pullbacks commute with the exterior product.

If i : ∂M → M stands for the inclusion map, then the natural trace of a k-form ω ∈ Λ k (M ) on ∂M is defined as i * ω. It satisfies the following commutation relations

i * (ω ∧ η) = (i * ω) ∧ (i * η) and d (i * ω) = i * (d ω) .
(2.6)

The key operation on differential form is the exterior derivative

d : Λ k (M ) → Λ k+1 (M ) , (2.7)
which is connected with integration through the generalized Stokes theorem

Σ d ω = ∂Σ ω ∀ω ∈ Λ k,0 (M ) (2.8)
and all orientable piecewise smooth sub-manifolds of M . In fact, (2.8) can be used to define the exterior derivative in the context of geometric integration theory. This has the benefit of dispensing with any smoothness requirement stipulated by the classical definition of d. We have d 2 = 0 and, obviously, (2.8) and (2.7) imply

Φ * • d = d •Φ * .
Since one has the graded Leibnitz formula

d(ω ∧ η) = d ω ∧ η + (-1) k ω ∧ d η ∀ω ∈ Λ k (M ), η ∈ Λ j (M ) (2.9)
exterior derivative and exterior product enter the crucial integration by parts formula

Σ d ω ∧ η + (-1) k Σ ω ∧ d η = ∂Σ i * ω ∧ i * η ∀ω ∈ Λ k (M ), η ∈ Λ j (M ) , (2.10)
where Σ is an orientable sub-manifold of M with dimension k + j + 1 and canonical inclusion i : Σ → ∂Σ.

Metric concepts.

A metric g defined on the manifold M permits us to introduce the Hodge operator

⋆ g : Λ k (M ) → Λ m-k (M ). It gives rise to the inner product on Λ k (M ) (ω, η) k,M := M ω ∧ * g η , ω, η ∈ Λ k (M ) .
(2.11) Thus, we obtain an L 2 -type norm • on Λ k (M ). Completion of smooth k-forms with respect to this norm yields the Hilbert space L 2 (Λ k (M )) of square integrable (w.r.t. g) k-forms on M . Its elements are equivalence classes of k-forms defined almost everywhere on M . Since Lipschitz manifolds possess a tangent space almost everywhere, for them L 2 (Λ k (M )) remains meaningful. As straightforward is the introduction of "Sobolev spaces" of differential forms, see [1, Sect. 1],

W k (d, M ) := {ω ∈ L 2 (Λ k (M )) : d ω ∈ L 2 (Λ k (M ))} , (2.12
(d ω, η) k+1,M = (ω, d * η) k,M ∀ω ∈ W k+1 (d, M ), η ∈ W k 0 (d, M ) , (2.14)
and an explicit calculation shows that

d * = (-1) (mk+1) ⋆ d ⋆ : Λ k+1 → Λ k . (2.15)
Furthermore, d 2 = 0 implies (d * ) 2 = 0. Eventually, the Laplace-Beltrami operator is defined as

∆ M = d d * + d * d : Λ k -→ Λ k . (2.16)
2.3. Vector proxies. Let us zero in on the three-dimensional "manifold" D. Choosing bases for the spaces of alternating k-multilinear forms, differential k-forms can be identified with vector fields with 3 k components, their so-called "vector proxies" [1, Sect. 1]. The usual association of "Euclidean vector proxies" in threedimensional space is summarized in Table 2.1. The terminology honours the fact that the Hodge operators ⋆ : Λ 1 (D) → Λ 2 (D) and Λ 0 (D) → Λ 3 (D) connected with the Euclidean metric of 3-space leave the vector proxies invariant (this is not true in 2D since ⋆ 2 = -1 on 1-forms). In addition the exterior product of forms is converted into the pointwise Euclidean inner product of vector fields. Thus, the inner product (•, •) k,D of k-forms on D becomes the conventional L 2 (D) inner product of the vector proxies. Further, the spaces W k (d, D) boil down to the standard Sobolev spaces H 1 (D) (for k = 0), H(curl, D) (for k = 1), H(div, D) (for k = 2), and L 2 (D) (for k = 3).

Differential form ω

Related function u or vector field u

x → ω(x) u(x) := ω(x) x → {v → ω(x)(v)} u(x) • v := ω(x)(v) x → {(v 1 , v 2 ) → ω(x)(v 1 , v 2 )} u(x) • v 1 × v 2 := ω(x)(v 1 , v 2 ) x → {(v 1 , v 2 , v 3 ) → ω(x)(v 1 , v 2 , v 3 )} u(x) det(v 1 , v 2 , v 3 ) := ω(x)(v 1 , v 2 , v 3 )
Table 2.1 The standard choice of vector proxy u,u for a differential form ω in R 3 . Here, • denotes the Euclidean inner product of vectors in R 3 , whereas × designates the cross product.

Using Euclidean vector proxies, the curl operator turns out to be an incarnation of the exterior derivative for 1-forms. More generally, the key first order differential operators of vector analysis arise from exterior derivative operators, see Figure 2.1. Please note that, since the Hodge operator is invisible on the vector proxy side, curl can as well stand for the operator

curl ←→ ⋆ d : Λ 1 (D) → Λ 1 (D) , (2.17)
which is naturally viewed as an unbounded operator on L 2 (Λ 1 (D)). Thus, (2.17) puts the formal curl operator introduced above in the framework of differential forms on D.

Translated into the language of differential forms, the Green's formula (1.2) becomes a special version of (2.10) for k = j = 1. However, due to (2.13), (1.2) can also be stated as

(⋆ d ω, η) 1,D -(ω, ⋆ d η) 1,D = ∂D i * ω ∧ i * η , ω, η ∈ W 1 (d, D) .
(2.18)

A metric on R 3 induces a metric on the embedded 2-dimensional manifold ∂D. Thus, the Euclidean inner product on local tangent spaces becomes a meaningful

3-forms Λ 3 (D) 2-forms Λ 2 (D) 1-forms Λ 1 (D) 0-forms Λ 0 (D) d d d d * = -⋆ d ⋆ d * = ⋆ d ⋆ d * = -⋆ d ⋆ scalar densities D → R flux density vector fields D → R 3 field intensity vector fields D → R 3 functions D → R grad curl div grad * = -div curl * = curl div * = -grad

Differential forms

Related function u or vector field u

x → ω(x) u(x) := ω(x) x → {v → ω(x)(v)} u(x) • v := ω(x)(v) x → {(v 1 , v 2 ) → ω(x)(v 1 , v 2 )} u(x) det(v 1 , v 2 , n(x)) := ω(x)(v 1 , v 2 )
Table 2.2 Euclidean vector proxies for differential forms on ∂D. Note that the test vectors v, v 1 , v 2 have to be chosen from the tangent space Tx (∂D).

concept and Euclidean vector proxies for k-forms on ∂D, k = 0, 1, 2, can be defined as in Table 2.1, see Table 2.2.

This choice of vector proxies leads to convenient vector analytic expressions for the trace operator i * :

       ω ∈ Λ 0 (D) : i * ω ←→ γu(x) := u(x), u : D → R , ω ∈ Λ 1 (D) : i * ω ←→ γ t u(x) := u(x) -(u(x) • n(x))n(x), u : D → R 3 , ω ∈ Λ 2 (D) : i * ω ←→ γ n u(x) := u(x) • n(x), u : D → R 3 , ω ∈ Λ 3 (D) : i * ω ←→ 0 ,
where x ∈ ∂D. Further, the customary vector analytic surface differential operators realize the exterior derivative for vector proxies, see Figure 2.2. Remark 2. Vector proxies offer an isomorphic model for the calculus of differential forms. However, one must be aware that the choice of bases and, therefore, the description of a differential form by a vector proxy, is somewhat arbitrary. In particular, a change of metric of space suggests a different choice of vector proxies for which the Hodge operators reduce to the identity. Thus, metric and topological aspects are hard to disentangle from a vector analysis point of view. This made us prefer the differential forms point of view in the remainder of the article.

3. Self-adjoint extensions and Lagrangian subspaces. First, we would like to recall some definitions of symplectic geometry. Then, we will build a symplectic 2-forms Λ 2 (∂D)

1-forms Λ 1 (∂D) 0-forms Λ 0 (∂D) d d d * = - * d * d * = - * d * scalar densities vector fields functions grad ∂ curl ∂ grad * = -div ∂ curl * ∂ = curl ∂ = n × grad ∂ Fig. 2.2.
Exterior derivative for Euclidean vector proxies on 2-manifolds space associated to a closed symmetric operator. The reader can refer to [START_REF]Elliptic Partial Differential Operators and Symplectic Algebra[END_REF][START_REF]Complex symplectic spaces and boundary value problems[END_REF] for a more detailed treatment.

3.1. Concepts from symplectic geometry. Symplectic geometry offers an abstract framework to deal with self-adjoint extensions of symmetric operators in Hilbert spaces. Here we briefly review some results. More information is available from [START_REF] Mcduff | Introduction to symplectic topology[END_REF].

Definition 3.1 (Symplectic space). A real symplectic space S is a real linear space equipped with a symplectic pairing

[•, •] (symplectic bilinear form, symplectic product)              [•, •] : S × S -→ R, [α 1 u 1 + α 2 u 2 , v] = α 1 [u 1 , v] + α 2 [u 2 , v], (linearity) [u, v] = -[v, u], ( skew symmetry)
[u, S] = 0 =⇒ u = 0 (non-degeneracy)

(3.1) Definition 3.2. Let L be a linear subspace of the symplectic space S (i) The symplectic orthogonal of L is L ♯ = {u ∈ S : [u, L] = 0}; (ii) L is a Lagrangian subspace, if L ⊂ L ♯ i. e. [u, v] = 0 for all u and v in L; (iii) A Lagrangian subspace L is complete, if L ♯ = L.
In the case of finite dimensional symplectic spaces, symplectic bases offer a convenient way to build complete Lagrangian subspaces, see [START_REF] Everitt | Complex symplectic geometry with applications to ordinary differential equations[END_REF]Example 2].

Definition 3.3. Let (S, [•, •]) be a real symplectic space with dimension 2n (the dimension has to be even so that the pairing

[•, •] can be non degenerate). A symplectic basis {u i } 2n i=1 of S is a basis of S satisfying [u i , u j ] = J i,j with J = 0 n×n I n×n -I n×n 0 n×n (3.2)
Simple linear algebra proves the existence of such bases: Lemma 3.4. For any symplectic space with finite dimension 2n, there exists a (non unique) symplectic basis.

Remark 3. As soon as we have found a symplectic basis {u i } 2n i=1 , it provides many complete Lagrangian subspaces

• the n first vectors {u i } n i=1 of a symplectic basis span a complete Lagrangian subspace.

• the n last vectors {u i } 2n i=n+1 of a symplectic basis span a complete Lagrangian subspace.

• for any

σ : [[1, n]] → [[0, 1]], {u i+σ(i)n } n
i=1 is a complete Lagrangian subspace. We recall some more facts about finite dimensional symplectic spaces Lemma 3.5. Every complete Lagrangian subspace of a finite dimensional symplectic space S of dimension 2n is n-dimensional. Moreover, it possesses a basis that can be extended to a symplectic basis of S.

Application to self-adjoint extensions of a symmetric operator.

Let H be a real Hilbert space and T a closed symmetric linear operator with dense domain D(T) ⊂ H. We denote by T * its adjoint. Let us first recall, see [START_REF] Weidmann | Linear Operators in Hilbert spaces[END_REF], that each self-adjoint extension of T is a restriction of T * , which is classically written as

T ⊂ T s ⊂ T * . (3.3)
Next, introduce a degenerate symplectic pairing on D(T * ) by

[•, •] : D(T * ) × D(T * ) -→ R such that [u, v] = (T * u, v) -(u, T * v). (3.4)
From the definition of T * , the symmetry of T, and the fact T * * = T, we infer that, see [START_REF] Everitt | Complex symplectic geometry with applications to ordinary differential equations[END_REF]Appendix],

[u + u 0 , v + v 0 ] = [u, v], ∀u 0 , v 0 ∈ D(T), ∀u, v ∈ D(T * ), u ∈ D(T * ), [u, v] = 0, ∀v ∈ D(T * ), =⇒ u ∈ D(T). (3.5)
As a consequence, we obtain a symplectic factor space, see Appendix of [START_REF] Everitt | Complex symplectic geometry with applications to ordinary differential equations[END_REF],

Lemma 3.6. The space S = D(T * )/D(T), [•, •] is a symplectic space. The graph norm on D(T * ) induces a factor norm on S and, due to (3.5), the symplectic pairing [•, •] is continuous with respect to this norm

|[u, v]| 2 ≤ u 2 + T * u 2 • v 2 + T * v 2 ∀u ∈ D(T * ), v ∈ D(T * ) ,
Let L ⊕ D(T) denotes the pre-image of L under the factor map D(T * ) → S. Corollary 3.7. The symplectic orthogonal V ♯ of any subspace V of S is closed (in the factor space topology). Any linear subspace L of S defines an extension T L of T through

T ⊂ T L := T * |L⊕D(T) ⊂ T * . (3.6)
This relationship allows to characterize self-adjoint extensions of T by means of the symplectic properties of the associated subspaces L. This statement is made precise in the Glazman-Krein-Naimark Theorem, see Theorem 1 of [START_REF] Everitt | Complex symplectic geometry with applications to ordinary differential equations[END_REF]Appendix]. Theorem 3.8 (Glazman-Krein-Naimark Theorem symplectic version). The mapping L → T L is a bijection between the space of complete Lagrangian subspaces of S and the space of self-adjoint extensions of T. The inverse mapping is given by

L = D(T L )/D(T) . (3.7)
4. Symplectic space for curl. Evidently, the unbounded curl operators introduced in Section 2 (resorting to the vector proxy point of view) fits the framework of the preceding section and Theorem 3.8 can be applied. To begin with, from (2.1) and (2.2) we arrive at the symplectic space

S curl := H(curl, D)/H 0 (curl, D) . (4.1)
By (1.2) it can be equipped with a symplectic pairing that can formally be written as

[u, v] ∂D := ∂D (u(y) × v(y)) • n(y) dS(y) , (4.2)
for any representatives of the equivalence classes of S curl . From (4.1) it is immediate that S is algebraically and topologically isomorphic to the natural trace space of H(curl, D). By now this trace space is well understood, see the seminal work of Paquet [START_REF] Paquet | Problemes mixtes pour le systeme de Maxwell[END_REF] and [START_REF]Traces theorems on non-smmoth boundaries for functional spaces related to Maxwell equations: An overwiew[END_REF][START_REF] Buffa | On traces for functional spaces related to Maxwell's equations. Part I: An integration by parts formula in Lipschitz polyhedra[END_REF][START_REF]On traces for functional spaces related to Maxwell's equations. Part II: Hodge decompositions on the boundary of Lipschitz polyhedra and applications[END_REF][START_REF] Buffa | On traces for H(curl, Ω) in Lipschitz domains[END_REF] for the extension to generic Lipschitz domains. To begin with, the topology of S curl is intrinsic, that is, with

D ′ := R 3 \ D, the norm of S c curl := H(curl, D ′ )/H 0 (curl, D ′ ) (4.3)
is equivalent to that of S curl ; both spaces are isomorphic algebraically and topologically. This can be proved appealing to an extension theorem for H(curl, D). Moreover, the pairing [•, •] ∂D identifies S curl with its dual S ′ curl : Lemma 4.1. The mapping

S curl → S ′ curl , u → {v → [u, v] ∂D } is an isomorphism. Proof. Given u ∈ S curl , let u ∈ H(curl, D) solve curl curl u + u = 0 in D , γ t u = u on ∂D . (4.4) Set v := curl u ∈ H(curl, D) and v := γ t v ∈ S curl . By (1.2) [u, v] ∂D = D curl u • v -curl v • u dx = D | curl u| 2 + |u| 2 dx = v H(curl,D) u H(curl,D) ≥ v S curl u S curl , as v H(curl,D) = u H(curl,D) . We immediately conclude sup v∈S curl |[u, v]| v S curl ≥ u S curl .
The trace space also allows a characterization via surface differential operators. It relies on the space H 1 2 t (∂D) of tangential surface traces of vector fields in (H 1 (D)) 3 and its dual H t (∂D)) ′ . Then one finds that, algebraically and topologically, S curl is isomorphic to

S curl ∼ = H -1 2 (curl ∂ , ∂D) := {v ∈ H -1 2 t (∂D) : curl ∂ v ∈ H -1 2 (∂D)} . (4.5)
The intricate details and the proper definition of curl ∂ can be found in [START_REF] Buffa | On traces for H(curl, Ω) in Lipschitz domains[END_REF]. When we adopt the perspective of differential forms, the domain of curl max is the Sobolev space H 1 (d, D) of 1-forms. Thus, S curl has to be viewed as a trace space of 1-forms, that is, a space of 1-forms (more precisely, 1-currents) on ∂D. In analogy to (2.12) and (4.5) we adopt the notation

S curl ∼ = W -1 2 ,1 (d, ∂D) . (4.6)
Please observe, that the corresponding symbol for the trace space of H 0 (d, D) will be W -1 2 ,0 (d, ∂D) (and not W In light of (2.18), the symplectic pairing on W -1 2 ,1 (d, ∂D) can be expressed as

[ω, η] ∂D := ∂D ω ∧ η , ω, η ∈ W -1 2 ,1 (d, ∂D) . (4.7) Whenever, W -1 2 ,1 (d, ∂D
) is treated as a real symplectic space, the pairing (4.7) is assumed. The most important observation about (4.7) is that the pairing [•, •] is utterly metric-free! Now we can specialize Theorem 3.8 to the curl operator. We give two equivalent versions, one for Euclidean vector proxies, the second for 1-forms: Theorem 4.2 (GKN-theorem for curl, vector proxy version). The mapping which associates to L ⊂ H -1 2 (curl ∂ , ∂D) the curl operator with domain

D(curl L ) := {v ∈ H(curl, D) : γ t (v) ∈ L}
is a bijection between the set of complete Lagrangian subspaces of H -1 2 (curl ∂ , ∂D) and the self-adjoint extensions of curl min .

Theorem 4.3 (GKN-theorem for curl, version for 1-forms). The mapping which associates to L ⊂ W -1 2 ,1 (d, ∂D) the ⋆ d operator with domain

D(⋆ d L ) := {η ∈ W 1 (d, D) : i * η ∈ L}
is a bijection between the set of complete Lagrangian subspaces of W -1 2 ,1 (d, ∂D) and the self-adjoint extensions of ⋆ d defined on W 1 0 (d, D). We point out that the constraint i * η ∈ L on traces amounts to imposing linear boundary conditions. In other words, the above theorems tell us, that self-adjoint extensions of curl min will be characterized by demanding particular boundary conditions for their argument vector fields, cf. [START_REF]On a selfadjoint realization of curl and some of its applications[END_REF].

Remark 4. Thanks to (2.6) the symplectic pairing on W -1 2 ,1 (d, ∂D) commutes with the pullback. Thus, if D, D ⊂ R 3 are connected by a Lipschitz homomorphism Φ : D → D, we find that Φ * : Λ 1 (∂D) → Λ 1 (∂ D) provides a bijective mapping between the complete Lagrangian subspaces of W -1 2 ,1 (d, ∂D) and of W -1 2 ,1 (d, ∂ D). Thus, pulling back the domain of a self-adjoint extension of curl min on D to D will give a valid domain for a self-adjoint extension of curl min on D. In short, self-adjoint extensions of curl min are invariant under bijective continuous transformations. This is a very special feature of curl, not shared, for instance, by the Laplacian -∆.

5. Hodge theory and consequences. Now we study particular subspaces of the trace space W -1 2 ,1 (d, ∂D). We will take for granted a metric on ∂D that induces a Hodge operator ⋆.

The Hodge decomposition.

Let us first recall the well-known Hodge decomposition of spaces of square-integrable differential 1-forms on ∂D. For a more general exposition we refer to [START_REF] Morrey | Multiple integrals in the calculus of variations[END_REF]:

Lemma 5.1. We have the following decomposition, which is orthogonal w.r.t. the inner product of L 2 (Λ 1 (∂D)):

L 2 (Λ 1 (∂D) = dW 0 (d, ∂D) ⊕ ⋆ d W 0 (d, ∂D) ⊕ H 1 (∂D) .
Here, H 1 (∂D) designates the finite-dimensional space of harmonic 1-forms on ∂D:

H 1 (∂D) := {ω ∈ L 2 (Λ 1 (∂D)) : d ω = 0 and d ⋆ω = 0} .
(5.1)

In terms of Euclidean vector proxies, the space L 2 (Λ 1 (∂D) is modelled by the space L 2 t (∂D) of square integrable tangential vector fields on ∂D. Then, the decomposition of Lemma 5.1 reads

L 2 t (∂D) = grad ∂ H 1 (∂D) ⊕ curl ∂ H 1 (∂D) ⊕ H 1 (∂D) , H 1 (∂D) := {v ∈ L 2 t (∂D) : curl ∂ v = 0 and div ∂ v = 0} .
The Hodge decomposition can be extended to W -1 2 ,1 (d, ∂D) on Lipschitz domains, as was demonstrated in [9, Sect. 5] and [START_REF] Buffa | Hodge decompositions on the boundary of a polyhedron: The multiconnected case[END_REF]. There the authors showed that, with a suitable extension of the surface differential operators, that

H -1 2 (curl ∂ , ∂D) = grad ∂ H 1 2 (∂D) ⊕ curl ∂ H 3 2 (∂D) ⊕ H 1 (∂D) , (5.2)
where, formally,

H 3 2 (∂D) := ∆ -1 ∂D H -1 2 * (∂D) , H -1 2 * (∂D) := {v ∈ H -1 2 (∂D) : ∂Di v dS = 0} .
(5.3)

with ∂D i the connected components of ∂D.

For C 1 -boundaries this space agrees with the trace space of H 2 (D).

The result (5.2) can be rephrased in the calculus of differential forms: Theorem 5.2 (Hodge decomposition of trace space). We have the following orthogonal decomposition

W -1 2 ,1 (d, ∂D) = d W -1 2 ,0 (d, ∂D) ⊕ ⋆ d W 3 2 ,0 (∂D) ⊕ H 1 (∂D) , (5.4)
whith

W 3 2 ,0 (∂D) := ∆ -1 ∂D ϕ ∈ W -1 2 ,0 (∂D : ϕ, 1 ∂Di = 0 . (5.5)
with ∂D i the connected components of ∂D.

The first subspace in the decomposition of Theorem 5.2 comprises only closed 1-forms, because

d d W -1 2 ,0 (d, ∂D) = 0 . (5.6)
The second subspace contains only co-closed 1-forms, since

d * ⋆ d W 3
2 ,0 = 0 .

(5.7)

The Hodge decomposition hinges on the choice of the Hodge operator ⋆. Consequently, it depends on the underlying metric on ∂D.

Lagrangian properties of the Hodge decomposition.

We find that the subspaces contributing to the Hodge decomposition of Theorem 5.2 can be used a building blocks for (complete) Lagrangian subspaces of W -

[d ω, d η] ∂D = ∂D d ω ∧ d η = ∂D ω ∧ d 2 η = 0 .
(5.9)

Proposition 5.4. The linear space

⋆ d W 3 2 ,0 (∂D) is a Lagrangian subspace of W -1 2 ,1 (d, ∂D).
Proof. The proof is the same as above, except that one has to use that ⋆ is an isometry with respect to the inner product induced by it:

[⋆ d ω, ⋆ d η] ∂D = ∂D ⋆ d ω ∧ ⋆ d η = - ∂D d ω ∧ d η = - ∂D ω ∧ d 2 η = 0 . (5.10)
In a similar way we prove the next proposition. Proposition 5.5. The space of harmonic 1-forms H 1 (∂D) is symplectically orthogonal to d W -1 2 ,0 (d, ∂D) and ⋆ d W

ω = d ω 0 + ⋆ d ω ⊥ + ω H and η = d η 0 + ⋆ d η ⊥ + η H . (5.11)
Note that the forms ω 0 and ω ⊥ are not unique since the kernels of d and ⋆ d are not empty (they contain the piecewise constants on conected components of ∂D).

Taking into account the symplectic orthogonalities stated in Propositions 5.3, 5.4, and 5.5

[d ω 0 , d η 0 ] ∂D = [⋆ d ω ⊥ , ⋆ d η ⊥ ] ∂D = [d ω 0 , η H ] ∂D = [⋆ d ω ⊥ , η H ] ∂D = [ω H , d η 0 ] ∂D = [ω H , ⋆ d η ⊥ ] ∂D = 0 (5.
12) we see that we can compute the symplectic pairing on W -1 2 ,1 (d, ∂D) according to

[ω, η] ∂D = [d ω 0 , ⋆ d η ⊥ ] ∂D + [⋆ d ω ⊥ , d η 0 ] ∂D + [ω H , η H ] ∂D .
(5.13)

It can also been expressed in terms of the L 2 -inner product (more precisely, its extension to duality pairing) as Since the set of harmonic vector fields is stable with respect to the Hodge operator (note that ⋆⋆ = -1 for 1-forms on ∂D)

[ω, η] ∂D = (d ω 0 , d η ⊥ ) 1,∂D -(d ω ⊥ , d η 0 ) 1,∂D + [ω H , η H ] ∂D . ( 5
     η ∈ H 1 (∂D) =⇒ η ∈ L 2 (Λ 1 (∂D)), d η = 0, d ⋆η = 0 =⇒ ⋆η ∈ L 2 (Λ 1 (∂D)), d ⋆(⋆η) = 0, d(⋆η) = 0 =⇒ ⋆η ∈ H 1 (∂D), (5.16)
Thus we find that the pairing [•, •] ∂D is non-degenerate on H 1 (∂D):

[ω H , η H ] ∂D = 0, ∀η H ∈ H 1 (D) =⇒ [ω H , ⋆ω H ] ∂D = (ω H , ω H ) 1,∂D = 0 . (5.17)
Lemma 5.6. The space of harmonic 1-forms H 1 (∂D) is a symplectic space with finite dimension when equipped with the symplectic pairing

[•, •] ∂D . It is a finite- dimensional symplectic subspace of W -1 2 ,1 (d, ∂D).
6

. Some examples of self-adjoint curl operators. Starting from the Hodge decomposition of Theorem 5.2, we now identify important classes of self-adjoint extensions of curl. We rely on a generic Riemannian metric on ∂D and the associated Hodge operator.

6.1. Self-adjoint curl associated with closed traces. In this section we aim to characterize the complete Lagrangian subspaces L of W -1 2 ,1 (d, ∂D) (equipped with [•, •] ∂D ) which contain only closed forms:

L ⊂ Z -1 2 ,1 (∂D) := {η ∈ W -1 2 ,1 (d, ∂D) : d η = 0} . (6.1)
Hodge theory (see Theorem 5.2) provides the tools to study these Lagrangian subspaces, since we have the following result: Lemma 6.1. The set of closed 1-forms in W -1 2 ,1 (d, ∂D) admits the following direct (orthogonal) decomposition

Z -1 2 ,1 (∂D) = d W -1 2 ,0 (d, ∂D) ⊕ H 1 (∂D) . (6.2) Proof. For ω ∈ Z -1 2 ,1 (∂D), ⋆ d ω ⊥ of (5.11) satisfies d(⋆ d ω ⊥ ) = 0, d ⋆(⋆ d ω ⊥ ) = 0, (⋆ d ω ⊥ ) H = 0 (6.3)
which implies that ⋆ d ω ⊥ = 0 and yields the assertion of the lemma. The next result is important, as it states a necessary condition for the existence of Lagrangian subspaces included in Z -1 2 ,1 (∂D). Lemma 6.2. The space Z -1 2 ,1 (∂D) includes its symplectic orthogonal d W -1 2 ,0 (d, ∂D).

Proof. Recall from Definition 3.2 that the symplectic orthogonal of Z -1 2 ,1 (∂D) is defined as the set

{ω ∈ W -1 2 ,1 (d, ∂D) : [ω, η] ∂D = 0, ∀η ∈ Z -1 2 ,1 (∂D)} . (6.4)
Using Theorem 5.2 for ω = d ω 0 + ⋆ d ω ⊥ + ω H and Lemma 6.1 for η = d η 0 + η H , we have with (5.13):

[ω, η] ∂D = [⋆ d ω ⊥ , d η 0 ] ∂D + [ω H , η H ] ∂D . (6.5)
This implies with η = ⋆ω H = η H (here we use the stability of H 1 (∂D) with respect to the Hodge operator)

[ω, ⋆ω H ] ∂D = [ω H , ⋆ω H ] ∂D = ∂D ω H ∧ ⋆ω H = 0 =⇒ ω H = 0 , (6.6)
and, for η

= d η 0 with η 0 = ω ⊥ ∈ W 3/2,0 (∂D) [ω, d ω ⊥ ] ∂D = [⋆ d ω ⊥ , d ω ⊥ ] ∂D = - ∂D d ω ⊥ ∧ ⋆ d ω ⊥ =⇒ d ω ⊥ = 0 . (6.7)
Hence, we have ω = d ω 0 (and ω H = 0). The converse holds due to (6.5). Lemma 6.2 tells us that, when restricted to the subspace of closed forms, the bilinear pairing [•, •] ∂D becomes degenerate. More precisely, on the subset of closed forms, one can use the splitting (6.5) and evaluate

[•, •] ∂D on Z -1 2 ,1 (∂D) according to [ω, η] ∂D = [ω H , η H ] ∂D , ∀ω, η ∈ Z -1 2 ,1 (∂D) . (6.8)
Hence, this pairing depends only on the harmonic components. Thus another message of Lemma 6.2 is that [•, •] ∂D furnishes a well-defined non-degenerate symplectic pairing, when considered on the co-homology factor space

H 1 (∂D, R) = Z -1 2 ,1 (∂D))/ d W -1 2 ,0 (d, ∂D) . (6.9)
This space is algebraically, topologically and symplectically isomorphic to H 1 (∂D), the space of harmonic 1-forms, see Section 5.3. This means that all the complete Lagrangian subspaces L of W -1 2 ,1 (d, ∂D) contained in Z -1 2 ,1 (∂D) are related to complete Lagrangian subspaces L H of H 1 (∂D) (or equivalently to the complete Lagrangian subspace L H of H 1 (∂D, R)) by

L = d W -1 2 ,0 (d, ∂D) ⊕ L H (or, equivalently, L H = L/ d W -1 2 ,0 (d, ∂D)) .
(6.10) Thus, we have proved the following lemma (the symplectic pairing [•, •] ∂D is used throughout) Lemma 6.3. There is a one-to-one correspondance between the complete Lagrangian subspaces L of the symplectic space W -1 2 ,1 (d, ∂D) satisfying

L ⊂ Z -1 2 ,1 (∂D) (6.11)
and the complete Lagrangian subspaces L H of H 1 (∂D). The bijection is given by (6.10). Theorem 4.3 and Lemma 6.3 lead to the characterization of the self-adjoint curl operators whose domains contain only functions with closed traces. Theorem 6.4. There is a one-to-one corresondance between the set of all selfadjoint curl operators ⋆ d S satisfying

D(⋆ d S ) ⊂ ω ∈ W 1 (d, D) i * ω ∈ Z -1 2 ,1 (∂D) (6.12)
and the set of complete Lagrangian subspaces L H of H 1 (∂D). They are related according to

D(⋆ d S ) = ω ∈ W 1 (d, D) i * ω ∈ d W -1 2 ,0 (d, ∂D) ⊕ L H . (6.13)
Obviously, the constraint

i * ω ∈ d W -1 2 ,0 (d, ∂D) ⊕ L H (6.14)
is a boundary condition, since it involves only the boundary of the domain D. In addition, we point out that no metric concepts enter in (6.13), cf. Section 4. Remark 5. Now, assume the domain D to feature trivial topology, that is, the genus of D is zero, and the space of harmonic forms is trivial. Theorem 6.4 reveals that there is only one self-adjoint ⋆ d with domain containing only forms with closed traces

D(⋆ d S ) = ω ∈ W 1 (d, D) d (i * ω) = 0 . (6.15)
In terms of vector proxies, this leads to the self-adjoint curl operator with domain

D(curl S ) = u ∈ W 0 (curl, D) curl(u) • n = 0 on ∂D , (6.16)
which has been investigated in [START_REF]On a selfadjoint realization of curl in exterior domains[END_REF][START_REF] Yoshida | Remarks on spectra of operator rot[END_REF]. In case D has non-trivial topology, then dim(H 1 (∂D)) = 2g = 0 and one has to examine the complete Lagrangian subspaces of H 1 (∂D), which is postponed to Section 6.3.

Self-adjoint curl based on co-closed traces.

In this section we seek to characterize those Lagrangian subspaces L of W -1 2 ,1 (d, ∂D) that contain only coclosed forms, ie.

L ⊂ ω ∈ W -1/2,1 (d, ∂D) : d ⋆ ω = 0 .
(6.17)

The developments are parallel to those of the previous section, because, as is illustrated by (5.13), from a symplectic point of view, the subspaces of closed and co-closed 1forms occuring in the Hodge decomposition of Theorem 5.2, are symmetric. For the sake of completeness, we give the details, nevertheless. Lemma 6.5. The subspace of co-closed 1-forms of W -1/2,1 (d, ∂D) admits the following orthogonal decomposition which implies that d ω 0 = 0 and proves (6.18).

ω ∈ W -1/2,1 (d, ∂D) : d ⋆ ω = 0 = ⋆ d W 3/2,0 (∂D) ⊕ H 1 (∂D) . ( 6
The next result is important as it states a necessary condition for the existence of Lagrangian subspaces comprising only co-closed forms.

Lemma 6.6. The symplectic orthogonal of the subspace of co-closed forms of

W -1/2,1 (d, ∂D) is ⋆ d W 3/2,0 (∂D).
Proof. Use the definition of the symplectic orthogonal as

ω ∈ W -1/2,1 (d, ∂D) : [ω, η] ∂D = 0 ∀η co-closed . ( 6
] ∂D = [ω H , ⋆ω H ] ∂D = ∂D ω H ∧ ⋆ω H = 0 =⇒ ω H = 0, and, for η = ⋆ d η ⊥ with η ⊥ ∈ W 3 2 ,0 (∂D) [ω, ⋆ d η ⊥ ] ∂D = [d ω 0 , ⋆ d η ⊥ ] ∂D = ∂D d ω 0 ∧ ⋆ d η ⊥ = 0 =⇒ d ⋆(d ω 0 ) = 0 .
Moreover, one has d(d ω 0 ) = 0 and (d ω 0 ) H = 0, which shows that d ω 0 = 0. Hence, we have ω = ⋆ d ω ⊥ (d ω 0 = 0 and ω H = 0). The other inclusion holds due to (6.21).

Remark 6. Formally, in the proof we have used

η = ⋆ d η ⊥ with η ⊥ = ω 0 which shows that [ω, η] ∂D = [d ω 0 , ⋆ d ω 0 ] ∂D = ∂D d ω 0 ∧ ⋆ d ω 0 =⇒ d ω 0 = 0. (6.22)
However, the lack of regularity of ω 0 does not allow this straightforward computation. When restricted to the space of co-closed forms, the bilinear pairing [•, •] ∂D becomes degenerate. However, due to Lemma 6.6, it is a non-degenerate symplectic product on the co-homology factor space.

ω ∈ W -1/2,1 (d, ∂D) : d ⋆ω = 0 ⋆ d W 3/2,0 (∂D) , (6.23)
which can be identified with H 1 (∂D). Indeed, also on the subset of co-closed forms, one can evaluate [•, •] ∂D by means of (6.8). Lemma 6.7. The complete Lagrangian subspaces L of W -1/2,1 (d, ∂D) containing only co-closed forms are one-to-one related to the complete Lagrangian subspaces L H of H 1 (∂D) by

L = ⋆ d W 3/2,0 (∂D) ⊕ L H . (6.24)
Theorem 4.3 and Lemma 6.7 lead to the characterization of the self-adjoint curl operators based on coclosed forms: Theorem 6.8. There is a one to one correspondance between the set of all selfadjoint operators ⋆ d S satisfying

D(⋆ d S ) ⊂ ω ∈ W 1 (d, ∂D) : d ⋆(i * ω) = 0 (6.25)
and the set of complete Lagrangian subspaces

L H of H 1 (∂D) equipped with [•, •] ∂D .
The underlying bijection is

D(⋆ d S ) = ω ∈ W 1 (d, D) : i * ω ∈ ⋆ d W 3/2,0 (∂D) ⊕ L H . (6.26) Remark 7.
Let D be a domain with trivial topology. Then there is only one self-adjoint operator ⋆ d whith domain containing only forms whose traces are coclosed

D(⋆ d S ) = ω ∈ W 0 (d, Ω) : d ⋆ (i * ω) = 0 . (6.27)
In terms of Euclidean vector proxies, we obtain the self-adjoint curl operator with domain

D(curl S ) = u ∈ H(curl, D) : div ∂ (γ t (u)) = 0 on ∂D . (6.28)
On the contrary, if D has non trivial topology, then one has to identify the complete Lagrangian subspaces of H 1 (∂D). This is the topic of the next section.

6.3. Complete Lagrangian subspaces of H 1 (∂D). The goal is to give a rather concrete description of the boundary conditions implied by (6.13) and (6.26). Concepts from topolgy will be pivotal.

To begin with we exploit a consequence of the long Mayer-Vietoris exact sequence in co-homology [START_REF] Bott | Differential Forms in Algebraic Topology[END_REF], namely the algebraic isomorphisms [START_REF]Topological duality in three-dimensional eddy-current problems and its role in computeraided problem formulation[END_REF]

H 1 (∂D) ∼ = H 1 (∂D; R) ∼ = i * in H 1 (D; R) + i * out H 1 (D ′ ; R) . (6.29)
Here, H 1 (D) is the co-homology space Z 1 (d, D)/ d W 0 (d, D), and i in : ∂D → D, i out : ∂D → D ′ stand for the canonical inclusion maps. We also point out that [START_REF]Topological duality in three-dimensional eddy-current problems and its role in computeraided problem formulation[END_REF] 1

2 dim H 1 (∂D; R) = dim H 1 (D; R) = dim H 1 (D ′ ; R) = g , (6.30)
where g ∈ N 0 is the genus of D.

Next, we find bases of H 1 (D; R) and H 1 (D ′ ; R) using the Poincaré duality between co-homology spaces and relative homology spaces 1 In [START_REF] Kotiuga | On making cuts for magnetic scalar potentials in multiply connected regions[END_REF], it is shown that these lattices as Abelian groups are torsion free, and that representatives of homology classes can be realized as orientable embedded surfaces. More precisely, we can find 2g compact orientable embedded Seifert surfaces, or "cuts"

H 1 (D; R) ∼ = H 2 (D, ∂D; R) . ( 6
S i , S ′ i , 1 i g , (6.34)
such that their equivalence classes under appropriate homology relations form bases for the following associated lattices2

{ S i } g i=1 for H 2 (D, ∂D; Z), { S ′ i } g i=1 for H 2 (D ′ , ∂D; Z) (6.35)
and

{ ∂S ′ i , ∂S i } g i=1 for H 1 (∂D; Z). (6.36)
In other words, the boundaries ∂S i , ∂S ′ i provide fundamental non-bounding cycles on ∂D.

In [START_REF]Topological duality in three-dimensional eddy-current problems and its role in computeraided problem formulation[END_REF], it was established that the set of surfaces { S i } g i=1 ∪ { S ′ i } g i=1 can be chosen so that they are "dual to each other". Here, this duality is expressed through the intersection numbers of their boundaries, see Chapter 5 of [START_REF] Gross | Electromagnetic Theory and Computation: A Topological Approach[END_REF]. Lemma 6.9. The set of surfaces { S i } g i=1 ∪ { S ′ i } g i=1 can be chosen such that the intersection pairing on H 1 (∂D; Z) can be reduced to (1 ≤ i, j ≤ g)

Int( ∂S i , ∂S ′ j ) = δ i,j , Int( ∂S ′ i , ∂S j ) = -δ i,j . (6.37)
Furthermore, when the boundaries of these surfaces are "pushed out" of their respective regions of definition, we get curves in the complementary region

∂S ′ i -→ C i , ∂S i -→ C ′ i . (6.38)
The homology classes of these curves form bases for homology lattices as follows

{ C i } g i=1 for H 1 (D; Z), and
{ C ′ i } g i=1 for H 1 (D ′ ; Z). (6.39)
This paves the way for a construction of bases of the co-homology spaces on D and D ′ [23]: Lemma 6.10. The co-homology classes generated by the closed 1-form in the sets defined for

1 ≤ i ≤ g κ i ∈ L 2 (Λ 1 (D)) : d κ i = 0 and Cj κ i = δ ij for 1 ≤ j ≤ g κ ′ i ∈ L 2 (Λ 1 (D ′ )) : d κ i = 0 and C ′ j κ ′ i = δ ij for 1 ≤ j ≤ g form bases of H 1 (D; R) and H 1 (D ′ ; R), respectively.
For instance, κ i can be obtained as the piecewise exterior derivative of a 0-form on D \ S i that has a jump of height 1 across S i . An analoguous statement holds for κ ′ i with S i replaced with S ′ i . More precisely, one has for 1

≤ i ≤ g ∃ψ i ∈ W 0 (d, D) : κ i = d ψ i on D \ S and [ψ i ] Sj = δ i,j (6.40)
∃ψ ′ i ∈ W 0 (d, D) : κ ′ i = d ψ ′ i on D ′ \ S ′ and [ψ ′ i ] S ′ j = δ i,j (6.41)
with [•] Γ denoting the jump across Γ. Lemma 6.11. For 1 m, n g, we have

a) ∂D i * in (κ m ) ∧ i * in (κ n ) = 0, b) ∂D i * out (κ ′ m ) ∧ i * out (κ ′ n ) = 0. (6.42)
Proof. To establish a) we rewrite the integral as one over D, as the following calculation shows

     ∂D i * in (κ m ) ∧ i * in (κ n ) = ∂D i * in (κ m ∧ κ n ) = D d(κ m ∧ κ n ) = D (d κ m ) ∧ κ n -κ m ∧ (d κ n ) = 0.
Similarly, b) follows from an analogous calculation where ∂D = -∂D ′ with forms defined on D ′ . Lemma 6.12. For 1 i, j g, we have

∂D i * in (κ i ) ∧ i * out (κ ′ j ) = δ i,j . (6.43)
Proof. Let us represent the 1-form κ i by means the 0-form ψ i , which jumps across S i , see (6.40). Taking into account that d i * out κ ′ j = 0, we get

i * in κ i ∧ i * out κ ′ j = d i * in ψ i ∧ i * out κ ′ j = d i * in ψ i ∧ i * out κ ′ j . (6.44)
Applying Stokes Theorem leads to (one has to take care of the orientation)

∂D i * in κ i ∧ i * out κ ′ j = ∂Si i * in ψ ∧ i * out κ ′ ∂Si + ∂S ′ j i * in ψ ∧ i * out κ ′ ∂Si . (6.45)
By (6.40), we get

∂D i * in κ i ∧ i * out κ ′ j = ∂Si κ ′ j + 0. (6.46)
Since ∂S i ∈ C ′ i , the result follows from (6.38) and Lemma 6.10. Remark 8. When the cuts do not satisfy (6.37), a generalization of Lemma 6.12 takes the form

∂D i * in (κ i ) ∧ i * out (κ ′ j ) = Int( ∂S i , ∂S ′ j ) . (6.47)
with Int( ∂S i , ∂S ′ j ) the intersection number of ∂S i and ∂S ′ j , see [START_REF] Gross | Electromagnetic Theory and Computation: A Topological Approach[END_REF]. Now, take (6.37) for granted. Write κ H,i , κ ′ H,i , 1 ≤ i ≤ g, for the unique harmonic 1-forms, i.e., κ

H,i , κ ′ H,i ∈ H 1 (∂D), such that i * in κ i = κ H,i + d α , i * out κ ′ i = κ ′ H,i + d β , (6.48)
for some α, β ∈ L 2 (Λ 0 (∂D)). Combining Lemmas 6.11 and 6.12 gives the desired symplectic basis of the space of harmonic 1-forms on ∂D: Lemma 6.13. The set {κ H,i , κ ′ H,i } g i=1 is a symplectic basis of H 1 (∂D). Obviously, since the trace preserves integrals and integrating a closed form over a cycle evaluates to zero, the 1-forms κ H,i and κ ′ H,i inherit the integral values over fundamental cycles from κ i and κ ′ i , cf. Lemma 6.10:

∂Sj κ H,i = δ ij , ∂S ′ j κ H,i = 0 , ∂S ′ j κ ′ H,i = δ ij , ∂Sj κ ′ H,i = 0 . (6.49)
Lemma 6.14. Given interior and exterior Seifert surfaces S i , S ′ i , the conditions (6.49) uniquely determine a symplectic basis {κ H,1 , . . . , κ H,g , κ ′ H,1 , . . . , κ ′ H,g } of H 1 (∂D).

Proof. If there was another basis complying with (6.49), the differences of the basis forms would harmonic 1-forms with vanishing integral over any cycle. They must vanish identically.

Given a symplectic basis, we can embark on the canonical construction of complete Lagrangian subspaces of H 1 (∂D) presented in Remark 3. We start from a partition

I ∪ I ′ = {1, . . . , g} , I ∩ I ′ = ∅ . (6.50)
Owing to Lemma 6.12 and (6.37) the symplectic pairing [•, •] ∂D has the matrix representation

0 g×g I g×g -I g×g 0 g×g ∈ R 2g,2g , (6.51)
with respect to the basis

{κ H,i } i∈I ∪ {-κ ′ H,i } i∈I ′ ∪ {-κ ′ H,i } i∈I ∪ {κ H,i } i∈I ′ (6.52)
of H 1 (∂D). Thus,

L H := span{κ H,i } i∈I ∪ {-κ ′ H,i } i∈I ′ (6.53)
will yield a complete Lagrangian subspace of H 1 (∂D). By theorems 6.4 and 6.8, L H induces self-adjoint curl = ⋆ d operators. From Lemma 6.1, Lemma 6.5 and (6.49) we learn that their domains allow the characterization

D(curl s ) := ω ∈ W 1 (d, D) : d(i * ω) = 0, ∂Sj ω = 0, j ∈ I, ∂S ′ j ω = 0, j ∈ I ′ (6.54)
in the case of closed traces, and

D(curl s ) := ω ∈ W 1 (d, D) : d ⋆(i * ω) = 0, ∂Sj ω = 0, j ∈ I, ∂S ′ j ω = 0, j ∈ I ′ , (6.55)
in the case of co-closed traces, respectively. In fact, the choice I ′ = ∅ together with closed trace is the one proposed in [START_REF] Yoshida | Remarks on spectra of operator rot[END_REF] to obtain a self-adjoint curl.

7. Spectral properties. Having constructed self-adjoint versions of the curl operator, we go on to verify whether their essential spectrum is confined to 0 and their eigenfunctions can form a complete orthonormal system in L 2 (D). These are common important features of self-adjoint partial differential operators.

The following compact embedding result is instrumental in investigating the spectrum of curl s . Related results can be found in [START_REF] Weber | A local compactness theorem for Maxwell's equations[END_REF] and [START_REF]An elementary proof for a compact imbedding result in generalized electromagnetic theory[END_REF].

Theorem 7.1 (Compact embedding). The spaces, endowed with the W 1 (d, D)norm,

X 0 :={ω ∈ W 1 (d, D) : d * ω = 0, i * (⋆ω) = 0} and X ⊥ :={ω ∈ W 1 (d, D) : d * ω = 0, d ⋆(i * ω) = 0}
are compactly embedded into L 2 (Λ 1 (D)).

Remark 9. In terms of Euclidean vector proxies these spaces read

X 0 ={v ∈ H(curl, D) : div v = 0, γ n u = 0} , X ⊥ ={v ∈ H(curl, D) : div v = 0, div ∂ (γ t u) = 0}
where the constraint div ∂ (γ t u) = 0 should be read as "orthogonality" to grad ∂ H 1 2 (∂D) in the sense of the Hodge decomposition. Proof. [of Thm. 7.1] The proof will be given for X ⊥ only. The simpler case of X 0 draws on the same ideas. We are using vector proxy notation, because the proof takes us beyond the calculus of differential forms. Note that the inner product chosen for the vector proxies does not affect the statement of the theorem.

A key tool is the so-called regular decomposition theorem that was discovered in [START_REF] Birman | L 2 -theory of the Maxwell operator in arbitrary domains[END_REF], consult [START_REF] Hiptmair | Finite elements in computational electromagnetism[END_REF]Sect. 2.4] for a comprehensive presentation including proofs. It asserts that there is C > 0 depending only on D such that for all u ∈ H(curl, D) there are functions Φ ∈ (H 1 (D)) 3 , ϕ ∈ H 1 (D), with

u = Φ + grad ϕ , Φ H 1 (D) + |ϕ| H 1 (D) ≤ C u H(curl,D) . (7.1)
Let (u n ) n∈N be a bounded sequence in X ⊥ that is div u n = 0 in D and div ∂ (γ t u n ) = 0 on ∂D , (7.2) ∃C > 0 :

u n L 2 (D) + curl u n L 2 (D) ≤ C . (7.3)
Write u n = Φ n + grad ϕ n for the regular decomposition according to (7.1). Thus, (Φ n ) n∈N is bounded in (H 1 (D)) 3 and, by Rellich's theorem, will possess a subsequence that converges in L 2 (D). We pick the corresponding sub-sequence of (u n ) n∈N without changing the notation. Further,

div u n = 0 ⇒ -∆ϕ n = div Φ n (bounded in L 2 (D)) , (7.4) div ∂ (γ t u) = 0 ⇒ -∆ ∂D (γϕ n ) = div ∂ (γ t Φ n ) (bounded in H -1 2 (∂D)) . (7.5)
We conclude that (γϕ n ) n∈N is bounded in H 1 (∂D) and, hence, has a convergente sub-sequence in H 1 2 (∂D) (for which we still use the same notation). The harmonic extensions ϕ n of γϕ n will converge in H 1 (D).

Finally, the solutions ϕ n ∈ H 1 (D) of the boundary value problems

-∆ ϕ n = div Φ n in D , ϕ n = 0 on ∂D , (7.6)
will possess a sub-sequence that converges in H 1 (D), as (-∆ Dir ) -1 L 2 (D) is compactly embedded in H 1 (D). Since ϕ n = ϕ n + ϕ n , this provides convergence of a subsequence of (Φ n + grad ϕ n ) n∈N in L 2 (D). Let curl s : D s ⊂ L 2 (Λ 1 (D)) → L 2 (Λ 1 (D)) be one of the self-adjoint realizations of curl discussed in the previous section. Recall that we pursued two constructions based on closed and co-closed traces, respectively.

Remark 10. Even if the domain D s of the self-adjoint curl s is known only up to the contribution of a Lagrangian subspace of L H , we can already single out special subspaces of D s :

(i) For the curl operators based on closed traces, see Sect. 2 (d, ∂D). We conclude using (6.13). (ii) For the curl operators based on co-closed traces introduced in Sect. 6.2, it follows that

d W 0 0 (d, D) ⊂ D s . (7.8)
This is immediate from the fact that

η ∈ W 0 0 (d, D) and ω = d η implies i * ω = d i * η = 0 , (7.9)
which means that ω belongs to D s , see (6.26). In the sequel, the kernel of curl s will be required. We recall that

N (curl s ) = D s ∩ N (curl max )
is a closed subspace of L 2 (Λ 1 (D)). Moreover, since d 2 = 0 and due to (7.7) and (7.

∃C = C(D) : ω ≤ C curl s ω ∀ω ∈ D s ∩ N (curl s ) ⊥ .
Proof. The indirect proof will be elaborated for the case of co-closed traces only. The same approach will work for closed traces.

We assume that there is a sequence (

ω n ) n∈N ⊂ D s ∩ N (curl s ) ⊥ such that ω n = 1 , curl ω n ≤ n -1 ∀n ∈ N . (7.12)
Since ω n ∈ N (curl s ) ⊥ , the inclusion (7.11) implies that d * ω n = 0. As a consequence of (7.12), (ω n ) n∈N is a bounded sequence in X ⊥ . Theorem 7.1 tells us that it will possess a subsequence that converges in L 2 (Λ 1 (D)), again we call it (ω n ) n∈N . Thanks to (7.12) it will converge in the graph norm on D s and the non-zero limit will belong to N (curl s ) ∩ N (curl s ) ⊥ = {0}. This contradicts ω n = 1. From Lemma 7.2 we conclude that the range space R(curl s ) is a closed subspace of L 2 (Λ 1 (D)), which means, R(curl s ) = N (curl s ) ⊥ .

(7.13) Thus, we are lead to consider the symmetric, bijective operator

C := curl s : D s ∩ N (curl s ) ⊥ ⊂ N (curl s ) ⊥ → N (curl s ) ⊥ . (7.14)
It is an isomorphism, when D s ∩ N (curl s ) ⊥ is equipped with the graph norm, and N (curl s ) ⊥ with the L 2 (Λ 1 (D))-norm. Its inverse C -1 is a bounded, self-adjoint operator.

Theorem 7.3. The operator curl s has a pure point spectrum with ∞ as sole accumulation point. It possesses a complete L 2 -orthonormal system of eigenfunctions.

Proof. The inverse operator

C -1 : N (curl s ) ⊥ → D s ∩ N (curl s ) ⊥ (7.15) is even compact as a mapping L 2 (Λ 1 (D)) → L 2 (Λ 1 (D))
. Indeed, due to (7.10) and (7.11) the range of C -1 satisfies

D s ∩ N (curl s ) ⊥ ⊂ X 0 in the closed case, (7.16)
D s ∩ N (curl s ) ⊥ ⊂ X ⊥ in the co-closed case. (7.17)
By Theorem 7.1, the compactness follows. Riesz-Schauder theory [40, Sect. X.5] tells us that, except for 0 its spectrum will be a pure (discrete) point spectrum with zero as accumulation point and it will possess a complete orthonormal system of eigenfunctions.

The formula, see [START_REF] Weidmann | Linear Operators in Hilbert spaces[END_REF]Thm. 5.10],

λ -1 -C -1 = λ -1 (C -λ)C -1 (7.18)
shows that for λ = 0,

• λ -1 -C -1 bijective ⇒ C -λ bijective , • N (λ -1 -C -1 ) = N (C -λ) .
Thus, σ(C) = (σ(C -1 ) \ {0}) -1 and the eigenfunctions are the same.

8. curl and curl curl.

8.1. Self-adjoint curl curl operators. In the context of electromagnetism we mainly encounter the self-adjoint operator curl curl. Now we explore its relationship with the curl operators discussed before. A metric on D and an associated Hodge operator ⋆ will be taken for granted.

Definition 8.1. A linear operator S :

D(S) ⊂ L 2 (Λ 1 (D)) → L 2 (Λ 1 (D)
) is a curl curl operator, if and only if S is a closed extension of the operator ⋆ d ⋆ d defined for smooth compactly supported 1-forms.

Two important extensions of the curl curl operator are the maximal and the minimal extensions:

Lemma 8.2. The domain of the minimal closed extension (curl curl) min of the curl curl operator is

D min = ω ∈ W 1 0 (d, D) : ⋆ d ω ∈ W 1 0 (d, D) (8.1)
or, equivalently, in terms of Euclidean vector proxies

D min = u ∈ L 2 (D) : curl u ∈ L 2 (D), curl curl u ∈ L 2 (D),
γ t (u) = 0, and γ t (curl (u)) = 0 on ∂D .

The adjoint of (curl curl) min is the maximal closed extension (curl curl) max . It is an extension of the curl curl operator with domain

D max = D 1 ⊕ D 2 , (8.2)
with

D 1 = ω ∈ W 1 0 (d, D) : ⋆ d ω ∈ W 1 (d, D) , (8.3)
D 2 = ω ∈ L 2 (Λ 1 (D)) : d ⋆ d ω = 0 . (8.4)
Proof. The domain D min of the minimal closure is straightforward. We recall the definition of the domain of the adjoint T * of an operator T :

D(T) ⊂ H → H D(T * ) = u ∈ H : ∃C u > 0 : (u, Tv) H C u v H ∀v ∈ D(T) . (8.5)
Let D max stand for the domain of the adjoint of the minimal curl curl operator. First we show that

D 1 ⊕ D 2 ⊂ D max . (8.6)
Let us consider ω ∈ D 1 and η ∈ D min . By integration by parts and the isometry properties of ⋆ we get

D ω ∧ d ⋆ d η = D d ⋆ d ω ∧ η ≤ d ⋆ d ω η . ( 8
⋆ d ⋆ d ω 1 = ϕ in D, i * ω 1 = 0 on ∂D. (8.13) Note that this ω 1 belongs to D 1 . Then ω 2 = ω -ω 1 satisfies D (ω -ω 1 ) ∧ d ⋆ d η = 0 ∀η ∈ D min =⇒ d ⋆ d ω 2 = 0 . (8.14)
It follows that ω 2 ∈ D 2 . Since ω = ω 1 + ω 2 , we have proven (8.11). Remark 11. The last lemma gives a nice example for (T 2 ) * = (T 2 ) * .

Indeed, the minimal extension of the formal curl curl boils down to the squared minimal curl operator curl min with domain W 1 0 (d, D)

(curl curl) min = curl min curl min
The adjoint of curl min is the curl max operator with domain W 1 (d, D), but (curl curl) max = curl max curl max .

To identify self-adjoint curl curl operators we could also rely on the toolkit of symplectic algebra, using the metric-dependent symplectic pairing As before, complete Lagrangian subspaces will give us self-adjoint extensions of (curl curl) min that are restrictions of (curl curl) max . However, we will not pursue this further.

since curl max and curl min are adjoint and S is self-adjoint. Due to lemma 8.3, we have D(curl max ) = D(S) and therefore S = curl max (8.22) since S and curl max are both curl operators. Clearly, this is not possible since curl max is not self-adjoint. Remark 12. We remark that the same arguments apply to the operator (curl curl) Neu . 8.3. curl curl = curl curl * is possible. Finally, we would like to show that not all the self-adjoint curl curl operators are of the form R R * with R a curl operator.

Following an idea of Everitt and Markus -a similar construction or the Laplacian is introduced in [START_REF]Complex symplectic spaces and boundary value problems[END_REF]-we consider the self-adjoint curl curl operator Hence, this u satisfies u ∈ D 2 but u / ∈ H(curl, D). Remark 13. In the same way, we show that there exists no curl operators R 1 and R 2 satisfying

T 0 = R 1 R 2 . ( 8

  ) which are Hilbert spaces with the graph norm. The completion of the subset of smooth k-forms with compact support in W k (d, M ) is denoted by W k 0 (d, M ). By construction, the Hodge star operator satisfies ⋆⋆ = (-1) (m-k)k Id . (2.13) Now, let us assume ∂M = ∅. Based on the inner product (2.11) we can introduce the adjoint d * : W k+1 (d, M ) → W k (d, M ) of the exterior derivative operator by

Fig. 2 . 1 .

 21 Fig. 2.1. Differential operators and their relationship with exterior derivatives

1 2

 1 ,0 (d, D) as readers accustomed to the conventions used with Sobolev spaces might expect).

3 2

 3 ,0 (∂D). The Hodge decomposition of Theorem 5.2 offers a tool for the evaluation of the symplectic pairing [•, •] ∂D . Below, tag the three components of the Hodge decomposition of Theorem 5.2 by subscripts 0, ⊥ and H: for ω, η ∈ W -1 2 ,1 (d, ∂D)

.14) 5 . 3 .

 53 The symplectic space H 1 (∂D). Let us recall that the space of harmonic 1-forms on ∂D (a 2 dimensional compact C ∞ -manifold without boundary) is a finite dimensional linear space with dim(H 1 (∂D)) = 2g, (5.15) with g the genus of the boundary, that is, the first Betti number of D. The reader can refer to Theorem 5.1, Proposition 5.3.1 of [4] and Theorem 7.4.3 of [29].

. 18 )

 18 Proof. For ω co-closed, d ω 0 in (5.11) satisfies d(d ω 0 ) = 0, d ⋆(d ω 0 ) = 0, (d ω 0 ) H = 0 (6.19)

  6.1, in particular Thm. 6.4, we find d W 0 (d, D) ⊂ D s . (7.7) Indeed, for ω ∈ d W 0 (d, D) there exists η ∈ W 0 (d, D) with ω = d η. Due to the trace theorem, i * η belongs to W -1 2 (d, ∂D). Consequently, it follows from the commutative relation (2.6) that i * ω = d i * η belongs to d W -1

Lemma 7 . 2 .

 72 8), one has d W 0 (d, D) ⊂ N (curl s ) in the closed case, (7.10) d W 0 0 (d, D) ⊂ N (curl s ) in the co-closed case. (7.11) The operator curl s is bounded from below on D s ∩ N (curl s ) ⊥ :

. 7 )

 7 This involves D 1 ⊂ D max . Now we consider ω ∈ D 2 . The relation d ⋆ d ω = 0 has to be understood as D d ⋆ d ω ∧ η = 0 ∀η ∈ Λ 1 (D) smooth, compactly supported . (8.8)As the smooth compactly supported 1-forms are dense in D min with respect to the topology induced by the normω + curl(ω) + curl(curl(ω)) ,(8.9)it follows thatD ω ∧ d ⋆ d η = 0 ∀η ∈ D min ,(8.10)and, finally, D 2 ⊂ D max . This confirms(8.6).Next, we proveD max ⊂ D 1 ⊕ D 2 . (8.11) Pick, ω ∈ D max . There exists ϕ ∈ L 2 (Λ 1 (D)) such that D ω ∧ d ⋆ d η; = D ϕ ∧ ⋆η ∀η ∈ D min . (8.12) Since d * ϕ = 0 (pick η = d ν in (8.12)), and D ϕ ∧ ⋆η H = 0 for η H ∈ H 1 (D), there exists ω 1 ∈ W 1 (d, D) satisfying

  [ω, η] = D d ⋆ d ω ∧ η -D ω ∧ d ⋆ d η . (8.15)

T 0 :

 0 D(T 0 ) ⊂ L 2 (D) -→ L 2 (D), u -→ curl curl u (8.23)with domainD(T 0 ) = D min ⊕ D 2 ,(8.24)where D min and D 2 are defined in (8.1) and (8.4). Proposition 8.5. There exists no curl operator R such thatT 0 = R R * .(8.25)Proof. Suppose that there exists a curl operator R satisfying(8.25). By definition of the composition of operators one hasD(T 0 ) = u ∈ D(R * ) : Ru ∈ D(R) .Hence, this impliesD 2 ⊂ D(T 0 ) ⊂ D(R * ) ⊂ W 1 (d, D) . This is not possible since D 2 is not a subspace W 1 (d, D).This can be illustrated by means of vector proxies and in the case of the unit sphere D. Consider the function u(r, θ, z) = +∞ n=1 r n sin nθ e z , given the cylindrical coordinates. The curl and curl curl of u are curl u = +∞ n=1 n r n-1 cos nθ e r -+∞ n=1 n r n-1 sin nθ e θ , curl curl u = 0 . Direct computation leads to u 2 < +∞ and curl u = +∞.

. 26 )

 26 Appendix. Frequently used notations: D bounded (open) Lipschitz domain in affine space R 3 D ′ (compactified) complement D ′ := R 3 \ D ∂D boundary of D n exterior unit normal vector field on ∂D u, v, . . . vector fields on a three-dimensional domain ω, η, . . . differential forms v, u elements of a factor space/trace space of vector proxies • Euclidean inner product in R 3 × cross product of vectors ∈ R 3 T, S, . . . (unbounded) linear operators on a Hilbert space T * adjoint operator T minThe minimal closure of T T maxThe maximal closure of T D(T) domain of definition of the linear operator T N (T) kernel (null space) of linear operator T R(T) range space of an operator TC ∞ (D) space of infinite differentiable functions on D C ∞ (D) space of smooth vector fields (C ∞ (D)) 3 C ∞ 0 (D) functions in C ∞ (D) with compact support in D C ∞ 0 (D) vector fields in (C ∞ 0 (D)) 3 L 2 (D)real Hilbert space of square integrable functions on D L 2 (D) square integrable vector fields in (L 2 (D)) 3 H(curl, D) real Hilbert space {v ∈ L 2 (D) : curl v ∈ L 2 (D)} with graph norm H 0 (curl, D) closure of C ∞ 0 (D) in H(curl, D) γ t tangential boundary trace of a vector field γ n normal component trace of a vector field grad ∂ surface gradient curl ∂ scalar valued surface rotation div ∂ surface divergence d exterior derivative of differential forms Λ k (M ) differential k-forms on manifold M ∧ exterior product of differential forms ⋆ g Hodge operator induced by metric g (•, •) k,M inner product on Λ k (M ) induced by a Hodge operator ⋆ L 2 (Λ k (M )) Hilbert space of square integrable k-forms on M • norm of L 2 (Λ k (M )) ("L 2 -norm"): ω 2 := (ω, ω) k,M W k (d, D) Sobolev space of square integrable k-forms with square integrable exterior derivative W k 0 (d, D) completion of compactly supported k-forms in W k (d, D) i * natural trace operator for differential forms [•, •] generic symplectic pairing

  Note, that curl max is no longer symmetric, and neither operator is self-adjoint. This motivates the search for self-adjoint extensions curl The classical route in the study of self-adjoint extensions of symmetric operators is via the famous Stone-von Neumann extension theory, see[START_REF] Weidmann | Linear Operators in Hilbert spaces[END_REF] Ch. 6]. It suggests that, after complexification, we examine the deficiency spaces

		.1)
	Its adjoint is the maximal curl operator, see [34, Sect. 0],
	curl max := curl * min : H(curl, D) → L 2 (D) .	(2.2)

s : D(curl s ) ⊂ L 2 (D) → L 2 (D) of curl min . If they exist, they will satisfy, c.f. [16, Example 1.13],

curl min ⊂ curl s ⊂ curl max .

(2.3) Remark 1.

  1 2 ,1 (d, ∂D). Proposition 5.3. The linear space d W -1 2 ,0 (d, ∂D) is a Lagrangian subspace of W -1 2 ,1 (d, ∂D) (w.r.t. the symplectic pairing [•, •] ∂D ) Proof. We have to show that [d ω, d η] ∂D = 0 ∀ω, η ∈ W -1 2 ,0 (d, ∂D) . (5.8) By density, we need merely consider ω, η in W 0 (d, ∂D). In this case, it is immediate from Stokes' Theorem (∂D has no boundary)

  .20) Using Theorem 5.2 for ω = d ω 0 + ⋆ d ω ⊥ + ω H and Lemma 6.5 for η= ⋆ d η ⊥ + η H , (5.13) gives [ω, η] ∂D = [d ω 0 , ⋆ d η ⊥ ] ∂D + [ω H , η H ] ∂D . (6.21) Choosing η = ⋆ω H = η H (here we use the stability of H 1 (∂D) with respect to the Hodge operator) this implies [ω, ⋆ω H

  [•, •] M symplectic pairing of 1-forms on 2-manifold M [•] Γ jump of trace of a function across 2-manifold Γ L ♯ symplectic orthogonal of subspace L of a symplectic space • (relative) homology class of a cycle H i (A; R) i th homology group of A with coefficients in R H i (A; R) i th co-homology space of A with coefficients in R H i (A, B; R) i th relative homology group of A relative to B with coefficients in R H i (A, B; R) relative co-homology space of A relative to B with coefficients in R H 1 (∂D) co-homology space of harmonic 1-forms on ∂D H 1 (∂D) first co-homology factor space of non-exact closed 1-forms on ∂D H 1 (∂D) first homology factor space of non-bounding cycles on ∂D • selects (relative) homology class of a cycle W -1 2 ,1 (d, ∂D) trace space of W 1 (d, D) (curl ∂ , ∂D) tangential traces of vector fields in H(curl, D) Z -1 2 (∂D) closed 1-forms in W -1 2 ,1 (d, ∂D) ω 0 , ω ⊥ components of the Hodge decomposition of ω ∈ W -1 2 ,1 (d, ∂D)

	H	-1 2 t

Throughout the paper • denotes the operation of taking the (relative) homology class of a cycle

There are two classical self-adjoint curl curl operators that play a central role in electromagnetic boundary value problems. Their domains are

)

Both can be written as the product of a curl operator and its adjoint:

Less familiar self-adjoint curl curl operators will emerge from taking the square of a self-adjoint curl operator as introduced in Section 6.

8.2. Square roots of curl curl operators. It is natural to ask whether any self-adjoint curl curl operator can be obtained as the square of a self-adjoint curl. We start with reviewing the abstract theory of square roots of operators, see [START_REF] Weidmann | Linear Operators in Hilbert spaces[END_REF]Sect. 7.3].

Let S be a positive (unbounded) self-adjoint operator on the Hilbert space H. We recall from [START_REF] Weidmann | Linear Operators in Hilbert spaces[END_REF]Thm. 7.20] that there exists a unique self-adjoint positive (unbounded) operator R saytisfying [START_REF] Rudin | Functional Analysis[END_REF]Theorem 13.31], u is equal to zero.

Hence, the spaces D(R 1 ) and D(R 2 ) share the dense subspace

. Moreover, their scalar products coincide on this subset:

We conclude using Cauchy sequences. Surprisingly, the simple self-adjoint operator (curl curl) Dir does not have a square root that is a self-adjoint curl:

Lemma 8.4. The curl curl operator curl max curl min does not have a square root that is a self-adjoint curl.

Proof. Let us suppose that T = curl max curl min admits a curl self-adjoint square root S which implies that curl max curl * max = curl max curl min = T = S 2 = S S * . (8.21)