Ferruccio Colombini
email: colombini@dm.unipi.it

Daniele Del Santo
email: delsanto@units.it

Francesco Fanelli
email: francesco.fanelli@sissa.it

Guy Métivier
email: guy.metivier@math.u-bordeaux1.fr

TIME-DEPENDENT LOSS OF DERIVATIVES FOR HYPERBOLIC OPERATORS WITH NON-REGULAR COEFFICIENTS

In this paper we will study the Cauchy problem for strictly hyperbolic operators with low regularity coecients in any space dimension N ≥ 1. We will suppose the coecients to be log-Zygmund continuous in time and log-Lipschitz continuous in space. Paradierential calculus with parameters will be the main tool to get energy estimates in Sobolev spaces and these estimates will present a time-dependent loss of derivatives.

Introduction

This paper is devoted to the study of the Cauchy problem for a second order strictly hyperbolic operator dened in a strip [0, T ] × R N , for some T > 0 and N ≥ 1. Consider a second order operator of the form [START_REF] Bahouri | Fourier Analysis and Nonlinear Partial Dierential Equations[END_REF] Lu := ∂ 2 t u -N j,k=1

∂ j (a jk (t, x) ∂ k u)

(with a jk = a kj for all j, k) and assume that L is strictly hyperbolic with bounded coecients, i.e. there exist two constants 0 < λ 0 ≤ Λ 0 such that

λ 0 |ξ| 2 ≤ N j,k=1
a jk (t, x) ξ j ξ k ≤ Λ 0 |ξ| 2 for all (t, x) ∈ [0, T ] × R N and all ξ ∈ R N . It is well-known (see e.g. [START_REF] Hörmander | Linear partial dierential operators[END_REF] or [START_REF] Mizohata | The Theory of Partial Dierential Equations[END_REF]) that, if the coecients a jk are Lipschitz continuous with respect to t and only measurable in x, then the Cauchy problem for L is well-posed in H 1 L 2 . If the a jk 's are Lipschitz continuous with respect to t and C ∞ b (i.e. C ∞ and bounded with all their derivatives) with respect to the space variables, one can recover the well-posedness in H s+1 H s for all s ∈ R. Moreover, in the latter case, one gets, for all s ∈ R and for a constant C s depending only on it, the following energy estimate:

sup 0≤t≤T u(t, •) H s+1 + ∂ t u(t, •) H s ≤ (2) ≤ C s u(0, •) H s+1 + ∂ t u(0, •) H s + T 0 Lu(t, •) H s dt
for all u ∈ C([0, T ]; H s+1 (R N )) ∩ C 1 ([0, T ]; H s (R N )) such that Lu ∈ L 1 ([0, T ]; H s (R N )). Let us explicitly remark that previous inequality involves no loss of regularity for the function u: estimate (2) holds for every u ∈ C 2 ([0, T ]; H ∞ (R N )) and the Cauchy problem for L is well-posed in H ∞ with no loss of derivatives.

If the Lipschitz continuity (in time) hypothesis is not fullled, then [START_REF] Bony | Calcul symbolique et propagation des singularités pour les équations aux dérivées partielles non linéaires[END_REF] is no more true. Nevertheless, one can still try to recover H ∞ -well-posedness, with a loss of derivatives in the energy estimate.

The rst case to consider is the case of the coecients a jk depending only on t:

Lu = ∂ 2 t u - N j,k=1
a jk (t) ∂ j ∂ k u .

In [START_REF] Colombini | Sur les équations hyperboliques avec des coecients qui ne dépendent que du temps[END_REF], Colombini, De Giorgi and Spagnolo assumed the coecients to satisfy an integral log-Lipschitz condition:

(

T -ε

0 |a jk (t + ε) -a jk (t)| dt ≤ C ε log 1 + 1 ε ,
for some constant C > 0 and all ε ∈ ]0, T ]. To get the energy estimate, they rst smoothed coecients using a mollier kernel (ρ ε ). Then, by Fourier transform, they dened an approximated energy E ε (ξ, t) in phase space, where the problem becomes a family of ordinary dierential equations. At that point, the key idea was to perform a dierent approximation of the coecients in dierent zones of the phase space: in particular, they set ε = |ξ| -1 . Finally, they obtained an energy estimate with a xed loss of derivatives: there exists a constant δ > 0 such that, for all s ∈ R, the inequality

sup 0≤t≤T u(t, •) H s+1-δ + ∂ t u(t, •) H s-δ ≤ (4)
≤ C s u(0, •) H s+1 + ∂ t u(0, •) H s + T 0 Lu(t, •) H s dt
holds true for all u ∈ C 2 ([0, T ]; H ∞ (R N )), for some constant C s depending only on s. Let us remark that if the coecients a jk are not Lipschitz continuous then a loss of regularity cannot be avoided, as shown by Cicognani and Colombini in [START_REF] Cicognani | Modulus of continuity of the coecients and loss of derivatives in the strictly hyperbolic Cauchy problem[END_REF]. Besides, in this paper the authors prove that, if the regularity of the coecients a jk is measured by a modulus of continuity, any intermediate modulus of continuity between the Lipschitz and the log-Lipschitz ones entails necessarily a loss of regularity, which, however, can be made arbitrarily small. Recently Tarama (see paper [START_REF] Tarama | Energy estimate for wave equations with coecients in some Besov type class[END_REF]) analysed the problem when coecients satisfy an integral log-Zygmund condition: there exists a constant C > 0 such that, for all j, k and all ε ∈ ]0, T /2[, one has [START_REF] Coifman | Au delà des opérateurs pseudo-diérentiels[END_REF] T -ε

ε |a jk (t + ε) + a jk (t -ε) -2 a jk (t)| dt ≤ C ε log 1 + 1 ε .
On the one hand, this condition is somehow related to the pointwise condition (for a function a ∈ C 2 ([0, T ]) ) |a(t)| + |t a ′ (t)| + |t 2 a ′′ (t)| ≤ C, considered by Yamazaki in [START_REF] Yamazaki | On the L 2 (R n ) well-posedness of some singular or degenerate partial differential equations of hyperbolic type[END_REF]. On the other hand, it's obvious that, if the a jk 's satisfy [START_REF] Chemin | Fluides parfaits incompressibles[END_REF], then they satisfy also [START_REF] Coifman | Au delà des opérateurs pseudo-diérentiels[END_REF]: so, a more general class of functions is considered. Again, Fourier transform, smoothing out the cocients and linking the approximation parameter with the dual variable were fundamental tools in the analysis of Tarama.

The improvement with respect to paper [START_REF] Colombini | Sur les équations hyperboliques avec des coecients qui ne dépendent que du temps[END_REF], however, was obtained dening a new energy, which involved (by derivation in time) second derivatives of the approximated coecients. Finally, he got an estimate analogous to (4), which implies, in particular, well-posedness in the space H ∞ . In paper [START_REF] Colombini | Hyperbolic operators with non-Lipschitz coecients[END_REF], Colombini and Lerner considered instead the case in which coecients a jk depend both in time and in space variables. In particular, they assumed an isotropic punctual log-Lipschitz condition, i.e. there exists a constant C > 0 such that, for all ζ = (τ,

ξ) ∈ R × R N , ζ = 0, one has sup z=(t,x)∈R×R N |a jk (z + ζ) -a jk (z)| ≤ C |ζ| log 1 + 1 |ζ| .
Again, smoothing coecients with respect to the time variable is required; on the contrary, one cannot use the Fourier transform, due to the dependence of a jk on x. The authors bypassed this problem taking advantage of the Littlewood-Paley decomposition and paradierential calculus.

Hence, they considered the energy concerning each localized part ∆ ν u of the solution u, and then they performed a weighed summation to put all these pieces together. Also in this case, they had to consider a dierent approximation of the coecients in dierent zones of the phase space, which was obtained setting ε = 2 -ν (recall that 2 ν is the size of the frequencies in the ν-th ring, see subsection 3.1 below). In the end, they got the following statement: for all s ∈ ]0, 1/4], there exist positive constants β and C s and a time

T * ∈ ]0, T ] such that sup 0≤t≤T * u(t, •) H -s+1-βt + ∂ t u(t, •) H -s-βt ≤ (6) ≤ C s u(0, •) H -s+1 + ∂ t u(0, •) H -s + T * 0 Lu(t, •) H -s-βt dt for all u ∈ C 2 ([0, T ]; H ∞ (R N )).
Let us point out that the bound on s was due to this reasons: the product by a log-Lipschitz function is well-dened in H s if and only if |s| < 1. Note also that this fact gives us a bound on the lifespan of the solution: the regularity index -s + 1 -βT * has to be strictly positive, so one can expect only local in time existence of a solution. Moreover in the case the coecients a jk are C ∞ b in space, the authors proved inequality (6) for all s: so, they still got well-posedness in H ∞ , but with a loss of derivatives increasing in time.

The case of a complete strictly hyperbolic second order operator,

Lu = N j,k=0 ∂ y j (a jk ∂ y k u) + N j=0 b j ∂ y j u + ∂ y j (c j u) + d u (here we set y = (t, x) ∈ R t × R N
x ), was considered by Colombini and Métivier in [9]. They assumed the same isotropic log-Lipschitz condition of [START_REF] Colombini | Hyperbolic operators with non-Lipschitz coecients[END_REF] on the coecients of the second order part of L, while b j and c j were supposed to be α-Hölder continuous (for some α ∈ ]1/2, 1[ ) and d to be only bounded. The authors headed towards questions such as local existence and uniqueness, and also nite propagation speed for local solutions.

Recently, Colombini and Del Santo, in [START_REF] Colombini | A note on hyperbolic operators with log-Zygmund coecients[END_REF] (for a rst approach to the problem see also [START_REF] Santo | The Cauchy problem for a hyperbolic operator with Log-Zygmund coecients, Further Progress in Analysis[END_REF],

where smoothness in space were required), came back to the Cauchy problem for the operator (1), mixing up a Tarama-like hypothesis (concerning the dependence on the time variable) with the one of Colombini and Lerner (with respect to x). More precisely, they assumed a punctual log-Zygmund condition in time and a punctual log-Lipschitz condition in space, uniformly with respect to the other variable (see conditions [START_REF] Colombini | The Cauchy problem for wave equations with non-Lipschitz coecients; application to continuation of solutions of some nonlinear wave equations[END_REF] and (10) below). However, they had to restrict themselves to the case of space dimension N = 1: as a matter of fact, a Tarama-kind energy was somehow necessary to compensate the bad behaviour of the coecients with respect to t, but it was not clear how to dene it in higher space dimensions. Again, localizing energy by Littlewood-Paley decomposition and linking approximation parameter and dual variable lead to an estimate analogous to [START_REF] Colombini | Sur les équations hyperboliques avec des coecients qui ne dépendent que du temps[END_REF].

The aim of the present paper is to extend the result of Colombini and Del Santo to any dimension N ≥ 1. As just pointed out, the main diculty was to dene a suitable energy related to the solution. So, the rst step is to pass from functions a(t, x) with low regularity modulus of continuity, to more general symbols σ a (t, x, ξ) (obviously related to the initial function a) satisfying the same hypothesis in t and x, and then to consider paradierential operators associated to these symbols. Nevertheless, positivity hypothesis on a (required for dening a strictly hyperbolic problem) does not translate, in general, to positivity of the corresponding operator, which is fundamental in obtaining energy estimates. At this point, paradierential calculus depending on a parameter γ ≥ 1, dened and developed by Métivier in [START_REF] Métivier | Interactions de deux chocs pour un système de deux lois de conservation, en dimension deux d'espace[END_REF] (see also [START_REF] Métivier | Large viscous boundary layers for noncharacteristic nonlinear hyperbolic problems[END_REF]), comes into play and allows us to recover positivity of the (new) paradierential operator associated to a. Dening a localized energy and an approximation of the coecients depending on the dual variable are, once again, basic ingredients in closing estimates. Hence, in the end we will get an inequality similar to (6), for any s ∈ ]0, 1[.

The paper is organized as follows.

First of all, we will introduce the work hypothesis for our strictly hyperbolic problem, and we will state our main results.

Then, we will present the tools we need, all from Fourier Analysis. In particular, we will recall Littlewood-Paley decomposition and some results about (classical) paradierential calculus, as introduced rst by J.-M. Bony in the famous paper [START_REF] Bony | Calcul symbolique et propagation des singularités pour les équations aux dérivées partielles non linéaires[END_REF]. We will need also to dene a dierent class of Sobolev spaces, of logarithmic type, as done in [START_REF] Colombini | The Cauchy problem for wave equations with non-Lipschitz coecients; application to continuation of solutions of some nonlinear wave equations[END_REF]: they will come into play in our computations. Moreover, we will present also paradierential calculus depending on a parameter (which is basic in our analysis, as already pointed out), as introduced in [START_REF] Métivier | Interactions de deux chocs pour un système de deux lois de conservation, en dimension deux d'espace[END_REF] and [START_REF] Métivier | Large viscous boundary layers for noncharacteristic nonlinear hyperbolic problems[END_REF]. A complete treatement about functions with low regularity modulus of continuity will end this section. In particular, we will focus on log-Zymgund and log-Lipschitz conditions: taking advantage of paradierential calculus, we will state properties of functions satisfying such hypothesis and of the relative smoothed-intime (by a convolution kernel) ones. Hence, we will pass to consider more general symbols and the associated paradierential operators, for which we will develop also a symbolic calculus and we will state a fundamental positivity estimate.

This having been done, we will be then ready to tackle the proof of our main result: we will go back to the main ideas of paper [START_REF] Colombini | A note on hyperbolic operators with log-Zygmund coecients[END_REF]. First of all, taking advantage of a convolution kernel, we will smooth out the coecients, but with respect to the time variable only. As a matter of facts, low regularity in x will be compensated by considering paradierential operators associated to our coecients. Then, we will decompose the solution u to the Cauchy problem for (1) into dyadic blocks ∆ ν u, for which we will dene an approximate localized energy e ν : the dependence on the approximation parameter ε will be linked to the phase space localization, setting ε = 2 -ν . The piece of energy e ν will be of Tarama type, but this time multiplication by functions will be replaced by action of paradierential operators associated to them. A weighed summation of these pieces will dene the total energy E(t) associated to u. The rest of the proof is classical: we will derive E with respect to time and, using Gronwall Lemma, we will get a control for it in terms of initial energy E(0) and external force Lu only.

Basic denitions and main result

This section is devoted to the presentation of our work setting and of our main results.

Let us consider the operator over [0, T 0 ] × R N (for some T 0 > 0 and N ≥ 1)

(7) Lu = ∂ 2 t u - N i,j=1 ∂ i (a ij (t, x) ∂ j u) ,
and let us suppose L to be strictly hyperbolic with bounded coecients, i.e. there exist two positive constants 0 < λ 0 ≤ Λ 0 such that, for all (t, x) ∈ R t × R N x and all ξ ∈ R N , one has (8)

λ 0 |ξ| 2 ≤ N i,j=1 a ij (t, x) ξ i ξ j ≤ Λ 0 |ξ| 2 .
Moreover, let us suppose the coecients to be log-Zygmund-continuous in the time variable t, uniformly with respect to x, and log-Lipschitz-continuous in the space variables, uniformly with respect to t. This hypothesis reads as follow: there exists a constant K 0 such that, for all τ > 0 and all y ∈ R N \ {0}, one has

sup (t,x) |a ij (t + τ, x) + a ij (t -τ, x) -2a ij (t, x)| ≤ K 0 τ log 1 + 1 τ (9) sup (t,x) |a ij (t, x + y) -a ij (t, x)| ≤ K 0 |y| log 1 + 1 |y| . (10)
Now, let us state our main result, i.e. an energy estimate for the operator (7).

Theorem 2.1. Let us consider the operator L dened in [START_REF] Colombini | A note on hyperbolic operators with log-Zygmund coecients[END_REF], and let us suppose L to be strictly hyperbolic, i.e. relation ( 8) holds true. Moreover, let us suppose that coecients a ij satisfy also conditions ( 9) and [START_REF] Santo | The Cauchy problem for a hyperbolic operator with Log-Zygmund coecients, Further Progress in Analysis[END_REF].

Then, for all xed θ ∈ ]0, 1[ , there exist a β * > 0, a time T > 0 and a constant C > 0 such that the following estimate,

sup 0≤t≤T u(t, •) H -θ+1-β * t + ∂ t u(t, •) H -θ-β * t ≤ (11) ≤ C u(0, •) H -θ+1 + ∂ t u(0, •) H -θ + T 0 Lu(t, •) H -θ-β * t dt , holds true for all u ∈ C 2 ([0, T ]; H ∞ (R N )).
So, it's possible to control the Sobolev norms of solutions to [START_REF] Colombini | A note on hyperbolic operators with log-Zygmund coecients[END_REF] in terms of those of initial data and of the external force only: the price to pay is a loss of derivatives, increasing (linearly) in time.

Tools

In this section we will introduce the main tools, from Fourier Analysis, we will need to prove our statement.

First of all, we will recall classical Littlewood-Paley decomposition and some basic results on dyadic analysis. We will also dene a dierent class of Sobolev spaces, of logarithmic type. Then, we will need to introduce a paradierential calculus depending on some parameter γ ≥ 1: the main ideas are the same of the classic version, but the introduction of the parameter allows us to perform a more rened analysis. This will be a basic tool to get our result. After this, we will consider functions with low regularity modulus of continuity. In particular, we will focus on log-Zygmund and log-Lipschitz functions: dyadic decomposition allows us to get some of their properties. Moreover, we will analyse also the convolution of a log-Zygmund function by a smoothing kernel.

Finally, taking advantage of paradierential calculus with parameters, we will consider general symbols having such a low regularity in time and space variables. Under suitable hypothesis on such a symbol, we will also get positivity estimates for the associated operator.

Littlewood-Paley decomposition

Let us rst dene the so called Littlewood-Paley decomposition, based on a non-homogeneous dyadic partition of unity with respect to the Fourier variable. We refer to [START_REF] Bahouri | Fourier Analysis and Nonlinear Partial Dierential Equations[END_REF], [START_REF] Bony | Calcul symbolique et propagation des singularités pour les équations aux dérivées partielles non linéaires[END_REF] and [START_REF] Métivier | Para-dierential calculus and applications to the Cauchy problem for nonlinear systems[END_REF] for the details.

So, x a smooth radial function χ supported in the ball B(0, 2), equal to 1 in a neighborhood of B(0, 1) and such that r → χ(r e) is nonincreasing over R + for all unitary vectors e ∈ R N . Set also ϕ (ξ) = χ (ξ) -χ (2ξ) .

The dyadic blocks (∆ j ) j∈Z are dened by1

∆ j := 0 if j ≤ -1, ∆ 0 := χ(D) and ∆ j := ϕ(2 -j D) if j ≥ 1.
We also introduce the following low frequency cut-o:

S j u := χ(2 -j D) = k≤j ∆ k for j ≥ 0.
The following classical properties will be used freely throughout the paper:

• for any u ∈ S ′ , the equality u = j ∆ j u holds true in S ′ ;

• for all u and v in S ′ , the sequence (S j-3 u ∆ j v) j∈N is spectrally supported in dyadic annuli.

Let us also mention a fundamental result, which explains, by the so-called Bernstein's inequalities, the way derivatives act on spectrally localized functions. Lemma 3.1. Let 0 < r < R. A constant C exists so that, for any nonnegative integer k, any couple (p, q) in [1, +∞] 2 with p ≤ q and any function u ∈ L p , we have, for all λ > 0,

supp u ⊂ B(0, λR) =⇒ ∇ k u L q ≤ C k+1 λ k+N 1 p -1 q u L p ; supp u ⊂ {ξ ∈ R N | rλ ≤ |ξ| ≤ Rλ} =⇒ C -k-1 λ k u L p ≤ ∇ k u L p ≤ C k+1 λ k u L p .
Let us recall the characterization of (classical) Sobolev spaces via dyadic decomposition: for all s ∈ R there exists a constant C s > 0 such that

(12) 1 C s +∞ ν=0 2 2 ν s u ν 2 L 2 ≤ u 2 H s ≤ C s +∞ ν=0 2 2 ν s u ν 2 L 2 ,
where we have set u ν := ∆ ν u.

So, the H s norm of a tempered distribution is the same as the ℓ 2 norm of the sequence (2 sν ∆ ν u L 2 ) ν∈N . Now, one may ask what we get if, in the sequence, we put weights dierent to the exponential term 2 sν . Before answering this question, we introduce some denitions. For the details of the presentiation, we refer also to [START_REF] Colombini | The Cauchy problem for wave equations with non-Lipschitz coecients; application to continuation of solutions of some nonlinear wave equations[END_REF].

Let us set Π(D) := log(2 + |D|), i.e. its symbol is π(ξ) := log(2 + |ξ|). Denition 3.2. For all α ∈ R, we dene the space H s+α log as the space Π -α H s , i.e.

f ∈ H s+α log ⇔ Π α f ∈ H s ⇔ π α (ξ) 1 + |ξ| 2 s/2 f (ξ) ∈ L 2 .
From the denition, it's obvious that the following inclusions hold for s 1 > s 2 , α 1 > α 2 > 0:

H s 1 +α 1 log ֒→ H s 1 +α 2 log ֒→ H s 1 ֒→ H s 1 -α 2 log ֒→ H s 1 -α 1 log ֒→ H s 2 .
We have the following dyadic characterization of these spaces (see [START_REF] Métivier | Para-dierential calculus and applications to the Cauchy problem for nonlinear systems[END_REF]Prop. 4.1.11]). Proposition 3.3. Let s, α ∈ R. A tempered distribution u belongs to the space H s+α log if and only if:

(i) for all k ∈ N, ∆ k u ∈ L 2 (R N ); (ii) set δ k := 2 ks (1 + k) α ∆ k u L 2 for all k ∈ N, the sequence (δ k ) k belongs to ℓ 2 (N). Moreover, u H s+α log ∼ (δ k ) k ℓ 2 .
Hence, this proposition generalizes property [START_REF] Métivier | Interactions de deux chocs pour un système de deux lois de conservation, en dimension deux d'espace[END_REF]. This new class of Sobolev spaces, which are in a certain sense of logarithmic type, will come into play in our analysis. As a matter of fact, operators associated to log-Zygmund or log-Lipschitz symbols give a logarithmic loss of derivatives. We will clarify in a while what we have just said; rst of all, we need to introduce a new version of paradierential calculus, depending on a parameter γ ≥ 1.

Paradierential calculus with parameters

Let us present here the paradierential calculus depending on some parameter γ. One can nd a complete and detailed treatement in [START_REF] Métivier | Large viscous boundary layers for noncharacteristic nonlinear hyperbolic problems[END_REF].

Fix γ ≥ 1 and take a cut-o function ψ ∈ C ∞ (R N ×R N ) which veries the following properties:

• there exist 0 < ε 1 < ε 2 < 1 such that ψ(η, ξ) = 1 for |η| ≤ ε 1 (γ + |ξ|) 0 for |η| ≥ ε 2 (γ + |ξ|) ;
• for all (β, α) ∈ N N × N N , there exists a constant C β,α such that

∂ β η ∂ α ξ ψ(η, ξ) ≤ C β,α (γ + |ξ|) -|α|-|β| .
For example, if γ = 1, one can take

ψ(η, ξ) ≡ ψ -3 (η, ξ) := +∞ k=0 χ k-3 (η) ϕ k (ξ) ,
where χ and ϕ are the localization (in phase space) functions associated to a Littlewood-Paley decomposition, see [START_REF] Métivier | Para-dierential calculus and applications to the Cauchy problem for nonlinear systems[END_REF]Ex. 5.1.5]. Similarly, if γ > 1 it is possible to nd a suitable integer µ ≥ 0 such that ( 13)

ψ(η, ξ) ≡ ψ µ (η, ξ) := χ µ (η) χ µ+2 (ξ) + +∞ k=µ+3 χ k-3 (η) ϕ k (ξ)
is a function with the properties just described. Dene now

G ψ (x, ξ) := F -1 η ψ (x, ξ) ,
the inverse Fourier transform of ψ with respect to the variable η.

Lemma 3.4. For all (β, α)

∈ N N × N N , ∂ β x ∂ α ξ G ψ (•, ξ) L 1 (R N x ) ≤ C β,α (γ + |ξ|) -|α|+|β| , (14)
| • | log 2 + 1 | • | ∂ β x ∂ α ξ G ψ (•, ξ) L 1 (R N x ) ≤ C β,α (γ + |ξ|) -|α|+|β|-1 log(1 + γ + |ξ|) . (15)
Proof. See [START_REF] Métivier | Para-dierential calculus and applications to the Cauchy problem for nonlinear systems[END_REF]Lemma 5.1.7].

Thanks to G, we can smooth out a symbol a in the x variable and we can dene the paradierential operator associated to a as the pseudodierential operator associated to this smooth function. We set the classical symbol associated to a to be

σ a (x, ξ) := ( ψ(D x , ξ) a ) (x, ξ) = G ψ (•, ξ) * x a(•, ξ) (x) ,
and then the paradierential operator associated to a:

T a := σ a (x, D x ) ,
where we have omitted ψ because the denition is independent of it, up to lower order terms. Remark 3.5. Let us note that if a = a(x) ∈ L ∞ and if we take the cut-o function ψ -3 , we get that T a is actually the usual paraproduct operator. If we take ψ µ as dened in [START_REF] Métivier | Para-dierential calculus and applications to the Cauchy problem for nonlinear systems[END_REF], instead, we get a paraproduct operator which starts from high enough frequencies, which will be indicated with T µ a (see [START_REF] Colombini | The Cauchy problem for wave equations with non-Lipschitz coecients; application to continuation of solutions of some nonlinear wave equations[END_REF]Par. 3.3]). Let us point out that we can also dene a γ-dyadic decomposition. First of all, we set

Λ(ξ, γ) := γ 2 + |ξ| 2 1/2 .
Then, taken the usual smooth function χ associated to a Littlewood-Paley decomposition, we dene

χ ν (ξ, γ) := χ 2 -ν Λ(ξ, γ) , S γ ν := χ ν (D x , γ) , ∆ γ ν := S γ ν+1 -S γ ν .
The usual properties of the support of the localization functions still hold, and for all xed γ ≥ 1 and all tempered distributions u, we have

u = +∞ ν=0 ∆ γ ν u in S ′ .
Moreover, with natural modications in denitions, we can introduce the space H s+α log γ as the set of tempered distributions for which

u 2 H s+α log γ := R N ξ Λ 2s (ξ, γ) log 2α (1 + γ + |ξ|) | u(ξ)| 2 dξ < +∞ .
For the details see [START_REF] Métivier | Large viscous boundary layers for noncharacteristic nonlinear hyperbolic problems[END_REF]Par. 2.1.1]. What is important to retain is that, once we x γ ≥ 1 (for example, to obtain positivity of paradierential operators involved in our computations), all the previous construction is equivalent to the classical one; in particular, the space H s+α log γ coincides with H s+α log , the respective norms are equivalent and the characterization given by Proposition 3.3 still holds true.

3.3

On log-Lipschitz and log-Zygmund functions Let us now give the rigorous denitions of the modulus of continuity of funtions we are dealing with, and state some of their properties. Denition 3.6. A function f ∈ L ∞ (R N ) is said to be log-Lipschitz, and we write f ∈ LL(R N ), if the quantity

|f | LL := sup x,y∈R N , |y|<1   |f (x + y) -f (x)| |y| log 1 + 1 |y|   < +∞ .
We dene

f LL := f L ∞ + |f | LL .
Let us dene also the space of log-Zygmund functions. We will give the general denition in R N , even if one dimensional case will be the only relevant one for our purposes. Denition 3.7. A function g ∈ L ∞ (R N ) is said to be log-Zygmund, and we write g ∈ LZ(R N ), if the quantity

|g| LZ := sup x,y∈R N , |y|<1   |g(x + y) + g(x -y) -2 g(x)| |y| log 1 + 1 |y|   < +∞ .
We dene g LZ := g L ∞ + |g| LZ .

Remark 3.8. Let us immediately point out that, by monotonicity of logarithmic function, we can replace the factor log (1 + 1/|y|) in previous denitions with log (1 + γ + 1/|y|), for all parameters γ ≥ 1. As paradierential calculus with parameters will play a fundamental role in our computations, it's convenient to perform such a change, and so does also in hypothesis ( 9) and ( 10) of section 2.

Let us give a characterization of the space LZ. Recall that the space of Zygmund functions is actually B 1 ∞,∞ : following the same proof of this case (see e.g. [START_REF] Chemin | Fluides parfaits incompressibles[END_REF]) one can prove next proposition.

Proposition 3.9. The space LZ(R N ) coincides with the logarithmic Besov space B 1-log ∞,∞ , i.e. the space of tempered distributions u such that ( 16)

sup ν≥0 2 ν (ν + 1) -1 ∆ ν u L ∞ < +∞ .
Proof.

(i) Let us rst consider a u ∈ B 1-log ∞,∞ and take x and y ∈ R N , with |y| < 1. For all xed n ∈ N we can write:

u(x + y) + u(x -y) -2u(x) = k<n (∆ k u(x + y) + ∆ k u(x -y) -2∆ k u(x)) + + k≥n (∆ k u(x + y) + ∆ k u(x -y) -2∆ k u(x)) .
First, we take advantage of the Taylor's formula up to second order to handle the former terms; then, we use property [START_REF] Tarama | Energy estimate for wave equations with coecients in some Besov type class[END_REF]. Hence we get

|u(x + y) + u(x -y) -2u(x)| ≤ C |y| 2 k<n ∇ 2 ∆ k u L ∞ + 4 k≥n ∆ k u L ∞ ≤ C   |y| 2 k<n 2 k (k + 1) + k≥n 2 -k (k + 1)   ≤ C (n + 1) |y| 2 2 n + 2 -n .
Now, as |y| < 1, the choice n = 1 + [log 2 (1/|y|)] (where with [σ] we mean the greatest positive integer less than or equal to σ) completes the proof of the rst part.

(ii) Now, given a log-Zygmund function u, we want to estimate the L ∞ norm of its localized part ∆ k u.

Let us recall that applying the operator ∆ k is the same of the convolution with the inverse Fourier transform of the function ϕ(2 -k •), which we call h k (x) = 2 kN h(2 k •), where we set

h = F -1 ξ (ϕ).
As ϕ is an even function, so does h; moreover we have

h(z) dz = F -1 ξ (ϕ)(z) dz = ϕ(ξ) |ξ=0 = 0 .
Therefore, we can write:

∆ k u(x) = 2 kN -1 h(2 k y) (u(x + y) + u(x -y) -2u(x)) dy ,
and noting that σ → σ log (1 + γ + 1/σ) is increasing completes the proof of the second part.

From denitions 3.6 and 3.7, it's obvious that LL(R N ) ֒→ LZ(R N ): Proposition 3.3 of [8] explains this property in terms of dyadic decomposition. Proposition 3.10. There exists a constant C such that, for all a ∈ LL(R N ) and all integers k > 0, we have [START_REF] Yamazaki | On the L 2 (R n ) well-posedness of some singular or degenerate partial differential equations of hyperbolic type[END_REF] Using dyadic characterization of the space LZ and following the same ideas of the proof of Proposition 3.9, we can prove the following property. For our purposes, it's enough to consider a log-Zygmund function a depending only on the time variable t, but the same reasonning holds also in higher dimensions. Lemma 3.12. For all a ∈ LZ(R), there exists a constant C, depending only on the LZ norm of a, such that, for all γ ≥ 1 and all 0 < |τ | < 1 one has (20)

∆ k a L ∞ ≤ C (k + 1) 2 -k a LL . Moreover, for all k ∈ N we have a -S k a L ∞ ≤ C (k + 1) 2 -k a LL (18) S k a C 0,1 ≤ C (k + 1) a LL . (19)
sup t∈R |a(t + τ ) -a(t)| ≤ C |τ | log 2 1 + γ + 1 |τ | .
Proof. As done in proving Proposition 3.9, for all n ∈ N we can write

a(t + τ ) -a(t) = k<n (∆ k a(t + τ ) -∆ k a(t)) + k≥n (∆ k a(t + τ ) -∆ k a(t)) ,
where, obviously, the localization in frequencies is done with respect to the time variable. For the former terms we use the mean value theorem, while for the latter ones we use characterization [START_REF] Yamazaki | On the L 2 (R n ) well-posedness of some singular or degenerate partial differential equations of hyperbolic type[END_REF]; hence, we get

|a(t + τ ) -a(t)| ≤ k<n d dt ∆ k a L ∞ |τ | + 2 k≥n ∆ k a L ∞ ≤ C   n 2 |τ | + k≥n 2 -k k   .
The series in the right-hand side of the previous inequality can be bounded, up to a multiplicative constant, by 2 -n n; therefore

|a(t + τ ) -a(t)| ≤ C n n |τ | + 2 -n ,
and the choice n = 1 + [log 2 (1/|τ |)] completes the proof. Now, given a log-Zygmund function a(t), we can regularize it by convolution. So, take an even function ρ ∈ C ∞ 0 (R t ), 0 ≤ ρ ≤ 1, whose support is contained in the interval [-1, 1] and such that ρ(t)dt = 1, and dene the mollier kernel

ρ ε (t) := 1 ε ρ t ε ∀ ε ∈ ]0, 1] .
We smooth the function a setting, for all ε ∈ ]0, 1],

(21)

a ε (t) := (ρ ε * a) (t) = Rs ρ ε (t -s) a(s) ds .
The following proposition holds.

Proposition 3.13. Let a be a log-Zygmund function. For all γ ≥ 1, there exist constants C γ such that

|a ε (t) -a(t)| ≤ C γ a LZ ε log 1 + γ + 1 ε (22) |∂ t a ε (t)| ≤ C γ a LZ log 2 1 + γ + 1 ε (23)
∂ 2 t a ε (t) ≤ C γ a LZ 1 ε log 1 + γ + 1 ε . (24)
Proof. For rst and third inequalities, the proof is the same as in [START_REF] Colombini | A note on hyperbolic operators with log-Zygmund coecients[END_REF]. We have to pay attention only to (23). As ρ ′ has null integral, the relation

∂ t a ε (t) = 1 ε 2 |s|≤ε ρ ′ s ε (a(t -s) -a(t)) ds
holds, and hence, taking advantage of (20), it implies

|∂ t a ε (t)| ≤ C ε 2 |s|≤ε ρ ′ s ε |s| log 2 1 + γ + 1 |s| ds .
Observing that the function σ → σ log 2 (1 + γ + 1/σ) is increasing in the interval [0, 1], and so does in [0, ε], allows us to complete the proof.

Low regularity symbols and calculus

For the analysis of our strictly hyperbolic problem, it's important to pass from LZ t -LL x functions to more general symbols in variables (t, x, ξ) which have this same regularity in t and x.

We want to investigate properties of such these symbols and of the associated operators. For reasons which will appear clear in the sequel, we will have to take advantage not of the classical paradierential calculus, but of the calculus with parameters. Therefore, we will allow also the symbols to depend on a parameter γ ≥ 1.

So, let us take a symbol a(t, x, ξ, γ) of order m ≥ 0, such that a is log-Zygmund in t and log-Lipschitz in x, uniformly with respect to the other variables. Now we smooth out a with respect to time, as done in (21). Next lemma provides us some estimates on classical symbols associated to a ε and its time derivatives. Lemma 3.14. The classical symbols associated to a ε and its time derivatives satisfy:

∂ α ξ σ aε ≤ C α (γ + |ξ|) m-|α| ∂ β x ∂ α ξ σ aε ≤ C β,α (γ + |ξ|) m-|α|+|β|-1 log (1 + γ + |ξ|) ∂ α ξ σ ∂taε ≤ C α (γ + |ξ|) m-|α| log 2 1 + γ + 1 ε ∂ β x ∂ α ξ σ ∂taε ≤ C β,α (γ + |ξ|) m-|α|+|β|-1 log (1 + γ + |ξ|) 1 ε ∂ α ξ σ ∂ 2 t aε ≤ C α (γ + |ξ|) m-|α| log 1 + γ + 1 ε 1 ε ∂ β x ∂ α ξ σ ∂ 2 t aε ≤ C β,α (γ + |ξ|) m-|α|+|β|-1 log (1 + γ + |ξ|) 1 ε 2 .
Proof. The rst inequality is a quite easy computation.

For the second one, we have to observe that

∂ i G(x -y, ξ)dx = ∂ i G(z, ξ)dz = F -1 η (η i ψ(η, ξ)) dz = (η i ψ(η, ξ)) |η=0 = 0 .

So, we have

∂ i σ aε = ∂ i G(y, ξ) (a ε (t, x -y, ξ) -a ε (t, x, ξ)) dy ,
and from this, remembering lemma 3.4, we get the nal control.

The third estimate immediately follows from the hypothesis on a and from (23). Moreover, in the case of space derivatives, we can take advantage once again of the fact that ∂ i G has null integral:

∂ i σ ∂taε = ∂ i G(x -y, ξ) ∂ t a ε (t, y, ξ) dy = Rs 1 ε 2 ρ ′ t -s ε R N y ∂ i G(y, ξ) (a(s, x -y, ξ) -a(s, x, ξ)) dy ds .
Hence, the estimate follows from the log-Lipschitz continuity hypothesis and from inequality [START_REF] Mizohata | The Theory of Partial Dierential Equations[END_REF]. About the ∂ 2 t a ε term, the rst estimate comes from (24), while for the second one we argue as before:

∂ i σ ∂ 2 t aε = ∂ i G(x -y, ξ) ∂ 2 t a ε (t, y, ξ) dy = R N y ∂ i G(x -y, ξ) 1 ε 3 Rs ρ ′′ t -s ε (a(s, y, ξ) -a(s, x, ξ)) ds dy = 1 ε 3 Rs ρ ′′ t -s ε R N y ∂ i G(y, ξ) (a(s, x -y, ξ) -a(s, x, ξ)) dy ds ,
and the thesis follows again from log-Lipschitz continuity and from [START_REF] Mizohata | The Theory of Partial Dierential Equations[END_REF].

Note that rst and second inequalities are satised also by the symbol a (not smoothed in time). Now let us quote some basic facts on symbolic calculus, which follow from previous lemma. Before doing this, we recall a denition. Denition 3.15. We say that an operator P is of order m + δ log if, for every (s, α) ∈ R 2 , P maps H s+α log into H (s-m)+(α-δ) log continuously. (i) Let a be a symbol of order m which is LL in the x variable. Then T a maps H s+α log γ into H s-m+α log γ .

(ii) Let us take two symbols a, b of order m and m ′ respectively. Suppose that a, b are LL in the x variable. The composition of the associated operators can be approximated by the symbol associated to the product of these symbols, up to a remainder term:

T a • T b = T ab + R . The remainder operator R maps H s+α log γ into H s-m-m ′ +1+(α+1) log γ .
(iii) Let a be a symbol of order m which is LL in the x variable. The adjoint (over L 2 ) operator of T a is, up to a remainder operator, T a . The remainder operator maps H s+α log

γ into H s-m+1+(α+1) log γ .
Let us end this subsection stating a basic positivity estimate. In this situation, paradierential calculus with parameters comes into play. Proposition 3.17. Let a(t, x, ξ, γ) be a symbol of order m, which is LL in the x variable and such that Re a(t, x, ξ, γ) ≥ λ 0 (γ + |ξ|) m .

Then, there exists a constant λ 1 , depending only on |a| LL and on λ 0 (so not on γ) such that, for γ large enough, one has Re (T a u, u)

L 2 ≥ λ 1 u 2 H m/2 γ .
Proof. The result is an immediate consequence of Theorem 2.19 of [START_REF] Métivier | Large viscous boundary layers for noncharacteristic nonlinear hyperbolic problems[END_REF], together with Proposition 3.16 about remainder for composition and adjoint operators.

Remark 3.18. Let us note the following fact, which comes again from Theorem 2.19 of [START_REF] Métivier | Large viscous boundary layers for noncharacteristic nonlinear hyperbolic problems[END_REF]. If the positive symbol a has low regularity in time and we smooth it by convolution with respect to this variable, we obtain a family (a ε ) ε of positive symbols, with same constant λ 0 . Now, all the paradierential operators associated to these symbols will be positive operators, uniformly in ε: i.e. the constant λ 1 of previous inequality can be choosen independently of ε.

Let us observe that previous proposition generalizes Corollary 3.12 of [START_REF] Colombini | The Cauchy problem for wave equations with non-Lipschitz coecients; application to continuation of solutions of some nonlinear wave equations[END_REF] (stated for the paraproduct by a positive LL function) to the more general case of a paradierential operator with a strictly positive symbol of order m.

Finally, thanks to Theorem 2.18 of [START_REF] Métivier | Large viscous boundary layers for noncharacteristic nonlinear hyperbolic problems[END_REF] about the remainder operator for the adjoint, we have the following corollary, which turns out to be fundamental in our energy estimates. Corollary 3.19. Let a be a positive symbol of order 1 and suppose that a is LL in the x variable.

Then there exists γ ≥ 1, depending only on the symbol a, such that

T a u L 2 ∼ ∇u L 2 for all u ∈ H 1 (R N ).
4 Proof of the energy estimate for L Finally, we are able to tackle the proof of Theorem 2.1. We argue in a stadard way: rst of all, we dene an energy associated to a solution of equation [START_REF] Colombini | A note on hyperbolic operators with log-Zygmund coecients[END_REF], and then we prove estimates on its time derivative in terms of the energy itself. In the end, we will close the estimates thanks to Gronwall's Lemma.

The key idea to the proof is to split the total energy into localized components e ν , each one of them associated to the dyadic block ∆ ν u, and then to put all these pieces together (see also [START_REF] Colombini | Hyperbolic operators with non-Lipschitz coecients[END_REF] and [START_REF] Colombini | A note on hyperbolic operators with log-Zygmund coecients[END_REF]). Let us see the proof into details.

Approximate and total energy

Let us rst regularize coecients a ij in the time variable by convolution, as done in (21), and let us dene the 0-th order symbol

α ε (t, x, ξ) := γ 2 + |ξ| 2 -1/2   γ 2 + i,j a ij,ε (t, x) ξ i ξ j   1/2 .
We take ε = 2 -ν (see also [START_REF] Colombini | Hyperbolic operators with non-Lipschitz coecients[END_REF] and [START_REF] Colombini | A note on hyperbolic operators with log-Zygmund coecients[END_REF]), and (for notation convenience) we will miss out the ε.

Before going on, let us x a real number γ ≥ 1, which will depend only on λ 0 and on the sup i,j a ij LLx , such that (see Corollary 3.19) (25)

T α -1/2 w L 2 ≥ λ 0 2 w L 2
and

T α 1/2 (γ 2 +|ξ| 2 ) 1/2 w L 2 ≥ λ 0 2 ∇w L 2
for all w ∈ H ∞ . Let us remark that the choice of γ is equivalent to the choice of the parameter µ in (13) (see Remark 3.5) and from now on, we will consider paraproducts starting from this µ, according to denition [START_REF] Métivier | Para-dierential calculus and applications to the Cauchy problem for nonlinear systems[END_REF], even if we will omit it in the notations. Consider in (7) a function u ∈ C 2 ([0, T 0 ]; H ∞ ). We want to get energy estimate for u. We rewrite the equation using paraproduct operators by the coecients a ij :

∂ 2 t u = i,j ∂ i (a ij ∂ j u) + Lu = i,j ∂ i T a ij ∂ j u + Lu, where Lu = Lu + i,j ∂ i (a ij -T a ij )∂ j u . Let us apply operator ∆ ν : we get (26)
∂ 2 t u ν = i,j ∂ i T a ij ∂ j u ν + i,j ∂ i ∆ ν , T a ij ∂ j u + ( Lu) ν ,
where u ν = ∆ ν u, ( Lu) ν = ∆ ν ( Lu) and ∆ ν , T a ij is the commutator between ∆ ν and the paramultiplication by a ij . Now, we set

v ν (t, x) := T α -1/2 ∂ t u ν -T ∂t(α -1/2 ) u ν w ν (t, x) := T α 1/2 (γ 2 +|ξ| 2 ) 1/2 u ν z ν (t, x) := u ν
and we dene the approximate energy associated to the ν-th component of u (as done in [START_REF] Colombini | A note on hyperbolic operators with log-Zygmund coecients[END_REF]):

(27)

e ν (t) := v ν (t) 2 L 2 + w ν (t) 2 L 2 + z ν (t) 2 L 2 .
Remark 4.1. Let us note that, thanks to hypothesis (8) and our choice of the frequence µ from which dening the paraproduct, we have that w ν (t) 2

L 2 ∼ ∇u ν 2 L 2 ∼ 2 2ν u ν 2 L 2 . Now, we x a θ ∈ ]0, 1[ ,
as required in hypothesis, and we take a β > 0 to be chosen later; we can dene the total energy associated to the solution u to be the quantity (28)

E(t) := ν≥0 e -2β(ν+1)t 2 -2νθ e ν (t) .
It's not dicult to prove (see also inequality (32) below) that there exist constants C θ and C ′ θ , depending only on the xed θ, for which one has:

(E(0)) 1/2 ≤ C θ ( ∂ t u(0) H -θ + u(0) H -θ+1 ) (29) (E(t)) 1/2 ≥ C ′ θ ( ∂ t u(t) H -θ-β * t + u(t) H -θ+1-β * t ) , (30)
where we have set β * = β (log 2) -1 .

4.2

Time derivative of the approximate energy

We want to nd an estimate on time derivative of the energy in order to get a control on it by Gronwall Lemma. Let us start analysing each term of (27).

z ν term

For the third term we have:

(31)

d dt z ν (t) 2 L 2 = 2 Re (u ν , ∂ t u ν ) L 2 .
Now, we have to control the term ∂ t u ν : using positivity of operator T α -1/2 , we have (32)

∂ t u ν L 2 ≤ C T α -1/2 ∂ t u ν L 2 ≤ C v ν L 2 + T ∂t(α -1/2 ) u ν L 2 ≤ C (e ν ) 1/2 .
So, we get the estimate:

(33)

d dt z ν (t) 2 L 2 ≤ C e ν (t) . 4.2.2 v ν term Straightforward computations show that ∂ t v ν (t, x) = T α -1/2 ∂ 2 t u ν -T ∂ 2 t (α -1/2 ) u ν .
Therefore, keeping in mind relation (26), we get:

d dt v ν (t) 2 L 2 = -2 Re v ν , T ∂ 2 t (α -1/2 ) u ν L 2 + 2 i,j Re v ν , T α -1/2 ∂ i T a ij ∂ j u ν L 2 + (34)
+ 2 i,j Re v ν , T α -1/2 ∂ i ∆ ν , T a ij ∂ j u L 2 + +2 Re v ν , T α -1/2 Lu ν L 2
.

Obviously, we have (35)

2 Re v ν , T α -1/2 Lu ν L 2 ≤ C (e ν ) 1/2 Lu ν L 2 ,
while from Lemma 3.14 we immediately get

2 Re v ν , T ∂ 2 t (α -1/2 ) u ν L 2 ≤ C v ν L 2 log 1 + γ + 1 ε 1 ε u ν L 2 (36) ≤ C (ν + 1) e ν ,
where we have used the fact that ε = 2 -ν . The other two terms of (34) will be treated later.

w ν term

We now derive w ν with respect to the time variable: thanks to a broad use of symbolic calculus, we get the following sequence of equalities:

d dt w ν 2 L 2 = 2 Re T ∂t(α 1/2 )(γ 2 +|ξ| 2 ) 1/2 u ν , w ν L 2 + 2 Re T α 1/2 (γ 2 +|ξ| 2 ) 1/2 ∂ t u ν , w ν L 2 (37) = 2 Re T α(γ 2 +|ξ| 2 ) 1/2 T -∂t(α -1/2 ) u ν , w ν L 2 + 2 Re (R 1 u ν , w ν ) L 2 + + 2 Re T α(γ 2 +|ξ| 2 ) 1/2 T α -1/2 ∂ t u ν , w ν L 2 + 2 Re (R 2 ∂ t u ν , w ν ) L 2 = 2 Re v ν , T α(γ 2 +|ξ| 2 ) 1/2 w ν L 2 + 2 Re (v ν , R 3 w ν ) L 2 + + 2 Re (R 1 u ν , w ν ) L 2 + 2 Re (R 2 ∂ t u ν , w ν ) L 2 = 2 Re v ν , T α -1/2 T α 3/2 (γ 2 +|ξ| 2 ) 1/2 w ν L 2 + 2 Re (v ν , R 4 w ν ) L 2 + + 2 Re (v ν , R 3 w ν ) L 2 + 2 Re (R 1 u ν , w ν ) L 2 + 2 Re (R 2 ∂ t u ν , w ν ) L 2 = 2 Re v ν , T α -1/2 T α 2 (γ 2 +|ξ| 2 ) u ν L 2 + + 2 Re (v ν , T α -1/2 R 5 u ν ) L 2 + 2 Re (v ν , R 4 w ν ) L 2 + + 2 Re (v ν , R 3 w ν ) L 2 + 2 Re (R 1 u ν , w ν ) L 2 + 2 Re (R 2 ∂ t u ν , w ν ) L 2 .
The important fact is that remainder terms are not bad and can be controlled in terms of approximate energy. As a matter of facts, taking advantage of Proposition 3.16 and Lemma 3.14, we get the following estimates.

• R 1 has principal symbol equal to

∂ ξ α(γ 2 + |ξ| 2 ) 1/2 ∂ x ∂ t (α -1/2 ), so |2 Re (R 1 u ν , w ν ) L 2 | ≤ C (ν + 1) e ν . (38)
• The principal symbol of R 2 is instead ∂ ξ α(γ 2 + |ξ| 2 ) 1/2 ∂ x (α -1/2 ), so, remembering also the control on ∂ t u ν L 2 , we have:

(39) |2 Re (R 2 ∂ t u ν , w ν ) L 2 | ≤ C ν (e ν ) 1/2 w ν L 2 ≤ C (ν + 1) e ν .
• Symbolic calculus tells us that the principal part of R 3 is given by

∂ ξ ∂ x α(γ 2 + |ξ| 2 ) 1/2 , therefore (40)
|2 Re (v ν , R 3 w ν ) L 2 | ≤ C v ν L 2 ν w ν L 2 ≤ C (ν + 1) e ν . • Now, R 4 has ∂ ξ α -1/2 ∂ x α 3/2 (γ 2 + |ξ| 2 ) 1/2 as principal symbol, so (41)
|2 Re (v ν , R 4 w ν ) L 2 | ≤ C v ν L 2 ν w ν L 2 ≤ C (ν + 1) e ν .
• Finally, R 5 is given, at the higher order, by the product of symbols ∂ ξ α 3/2 (γ 2 + |ξ| 2 ) 1/2 and ∂ x α 1/2 (γ 2 + |ξ| 2 ) 1/2 , and so we get (42)

2 Re (v ν , T α -1/2 R 5 u ν ) L 2 ≤ C v ν L 2 2 ν ν u ν L 2 ≤ C (ν + 1) e ν .

4.2.4

Principal part of the operator L Now, thanks to previous computations, it's natural to pair up the second term of (34) with the rst one of the last equality of (37). As α is a symbol of order 0, we have

2 Re   v ν , T α -1/2 i,j ∂ i T a ij ∂ j u ν   L 2 + 2 Re v ν , T α -1/2 T α 2 (γ 2 +|ξ| 2 ) u ν L 2 ≤ C v ν L 2 ζ ν L 2 ,
where we have set (43)

ζ ν := T α 2 (γ 2 +|ξ| 2 ) u ν + i,j ∂ i T a ij ∂ j u ν = ij T a ij,ε ξ i ξ j +γ 2 u ν + ∂ i T a ij ∂ j u ν .
We remark that

∂ i T a ij ∂ j u ν = T ∂ i a ij ∂ j u ν -T a ij ξ i ξ j u ν ,
where, with a little abuse of notations, we have written the derivative ∂ i a ij meaning that we are taking the derivative of the classical symbol associated to a ij . First of all, we have that

T ∂ i a ij ∂ j u ν L 2 ≤ S µ ∂ i a ij L ∞ S µ+2 ∂ j u ν L 2 + k≥µ+3 ∇S k-3 a ij L ∞ ∆ k ∇u ν L 2 (44) ≤ C (µ + 1) sup i,j a ij LLx ∇u ν L 2 + + k≥µ+3 , k∼ν (k + 1) sup i,j a ij LLx ∇∆ k u ν L 2 ≤ C µ (ν + 1) sup i,j a ij LLx (e ν ) 1/2 ,
where µ is the parameter xed in ( 13) and we have also used (19). Next, we have to control the term

T a ij,ε ξ i ξ j +γ 2 u ν -T a ij ξ i ξ j u ν = T (a ij,ε -a ij )ξ i ξ j u ν + T γ 2 u ν .
It's easy to see that

T (a ij,ε -a ij )ξ i ξ j u ν L 2 ≤ C ε log 1 + 1 ε 2 ν ∇u ν L 2 ,
and so, keeping in mind that ε = 2 -ν , (45)

T (a ij,ε -a ij )ξ i ξ j +γ 2 u ν L 2 ≤ C γ (ν + 1) (e ν ) 1/2 .
Therefore, from (44) and (45) we nally get

(46) 2 Re   v ν , T α -1/2 i,j ∂ i T a ij ∂ j u ν   L 2 + 2 Re v ν , T α -1/2 T α 2 (γ 2 +|ξ| 2 ) u ν L 2 ≤ C (ν + 1) e ν ,
where the constant C depends on the log-Lipschitz norm of coecients a ij of the operator L and on the xed parameters µ and γ.

To sum up, from inequalities (33), ( 35), ( 36) and ( 46) and from estimates of remainder terms (38)-(42), we can conclude that

d dt e ν (t) ≤ C 1 (ν + 1) e ν (t) + C 2 (e ν (t)) 1/2 Lu ν (t) L 2 + (47) + 2 i,j Re v ν , T α -1/2 ∂ i ∆ ν , T a ij ∂ j u L 2 .

4.3

Commutator term

We want to estimate the quantity

i,j Re v ν , T α -1/2 ∂ i ∆ ν , T a ij ∂ j u L 2 .
We start remarking that

[∆ ν , T a ij ]w = [∆ ν , S µ a]S µ+2 w + +∞ k=µ+3 [∆ ν , S k-3 a ij ] ∆ k w,
where µ is xed, as usual (see Remark 3.5). In fact ∆ ν and ∆ k commute so that

∆ ν (S µ a ij S µ+2 w) -S µ a ij (S µ+2 ∆ ν w) = ∆ ν (S µ a ij S µ+2 w) -S µ a ij ∆ ν (S µ+2 w),
and similarly

∆ ν (S k-3 a ij ∆ k w) -S k-3 a ij ∆ k (∆ ν w) = ∆ ν (S k-3 a ij ∆ k w) -S k-3 a ij ∆ ν (∆ k w).
Consequently, taking into account also that S k+2 and ∆ k commute with ∂ j , we have

∂ i [∆ ν , T a ij ] ∂ j u = ∂ i ([∆ ν , S µ a ij ] ∂ j (S µ+2 u)) + ∂   +∞ k=µ+3 [∆ ν , S k-3 a ij ] ∂ j (∆ k u)   .
Let's consider rst the term

∂ i ([∆ ν , S µ a ij ] ∂ j (S µ+2 u)) .
Looking at the support of the Fourier transform of [∆ ν , S µ a ij ] ∂ j (S µ+2 u), we have that it is contained in {|ξ| ≤ 2 µ+4 } and moreover [∆ ν , S µ a ij ] ∂ j (S µ+2 u) is identically 0 if ν ≥ µ + 5. From Bernstein's inequality and [START_REF] Coifman | Au delà des opérateurs pseudo-diérentiels[END_REF]Th. 35] we have that

∂ i ([∆ ν , S µ a ij ] ∂ j (S µ+2 u)) L 2 ≤ C µ sup i,j a ij LLx S µ+2 u L 2 ,
hence, putting all these facts together, we have

+∞ ν=0 e -2β(ν+1)t 2 -2νθ ij 2 Re v ν , T α -1/2 ∂ i ([∆ ν , S µ a ij ]∂ j (S µ+2 u) ) L 2 ≤ ( 48
) ≤ C µ sup i,j a ij LLx µ+4 ν=0 e -2β(ν+1)t 2 -2νθ v ν L 2 µ+2 h=0 u h L 2 ≤ C µ sup i,j a ij LLx e β(µ+5)T 2 (µ+4)θ µ+4 ν=0 e -β(ν+1)t 2 -νθ v ν L 2 • • µ+4 h=0 e -β(h+1)t 2 -hθ u h L 2 ≤ C µ sup i,j a ij LLx e β(µ+5)T 2 (µ+4)θ µ+4 ν=0
e -2β(ν+1)t 2 -2νθ e ν (t) .

Next, let's consider

∂ i   +∞ k=µ+3 [∆ ν , S k-3 a ij ] ∂ j (∆ k u)   .
Looking at the support of the Fourier transform, it is possible to see that

[∆ ν , S k-3 a ij ] ∂ j (∆ k u)
is identically 0 if |k -ν| ≥ 3. Consequently the sum over k is reduced to at most 5 terms:

∂ i ([∆ ν , S ν-5 a ij ] ∂ j (∆ ν-2 u)) + • • • + ∂ i ([∆ ν , S ν-1 a ij ] ∂ j (∆ ν+2 u))
, each of them having the support of the Fourier transform contained in {|ξ| ≤ 2 ν+1 }. Let's consider one of these terms, e.g. ∂ i ([∆ ν , S ν-3 a ij ] ∂ j (∆ ν u)), the computation for the other ones being similar. We have, from

Bernstein's inequality,

∂ i ([∆ ν , S ν-3 a ij ] ∂ j (∆ ν u)) L 2 ≤ C 2 ν [∆ ν , S ν-3 a ij ] ∂ j (∆ ν u) L 2 .
On the other hand, using [START_REF] Coifman | Au delà des opérateurs pseudo-diérentiels[END_REF]Th. 35] again, we have:

[∆ ν , S ν-3 a ij ]∂ j (∆ ν u) L 2 ≤ C ∇S ν-3 a ij L ∞ ∆ ν u L 2 ,
where C does not depend on ν. Consequently, using also (19), we deduce

∂ i ([∆ ν , S ν-3 a ij ] ∂ j (∆ ν u)) L 2 ≤ C 2 ν (ν + 1) sup i,j a ij LLx ∆ ν u L 2 .
From this last inequality and similar ones for the other terms, it is easy to obtain that i,j (ν + 1) e -2β(ν+1)t 2 -2νθ e ν (t) Let us point out that condition (52) gives us a condition on the lifespan T of a solution to the Cauchy problem for [START_REF] Colombini | A note on hyperbolic operators with log-Zygmund coecients[END_REF]. It depends on θ ∈ ]0, 1[ and on β * > 0, hence on constants C 1 . . . C 4 . Going after the guideline of the proof, one can see that, in the end, the time T depends only on the index θ, on the parameter µ dened by conditions (25), on constants λ 0 and Λ 0 dened by [START_REF] Colombini | Hyperbolic operators with non-Lipschitz coecients[END_REF] and on the quantities sup i,j a ij LZt and sup i,j a ij LLx .

Re

Remark 3 . 11 .

 311 Note that, again from Proposition 3.3 of[START_REF] Colombini | Hyperbolic operators with non-Lipschitz coecients[END_REF], property (19) is a characterization of the space LL.

[

  v ν , T α -1/2 ∂ i   +∞ k=µ+3 [∆ ν , S k-3 a ij ] ∂ j (∆ k u) ∆ ν , S k-3 a ij ] ∂ j (∆ k u) 1) e -2β(ν+1)t 2 -2νθ e ν (t) .

  Collecting the informations from (48) and (49), we obtain+∞ ν=0 e -2β(ν+1)t 2 -2νθ ij 2 Re v ν , T α -1/2 ∂ i ∆ ν , T a ij ∂ j u L 2 ≤ 1) e -2β(ν+1)t

∂∂∂∂H∂

  1) e -2β(ν+1)t 2 -2νθ e ν (t) i (a ij -T a ij )∂ j u i (a ij -T a ij )∂ j u i (a ij -T a ij )∂ j u H -θ-β * t-i (a ij -T a ij )∂ j u with C uniformly bounded for s in a compact set of ]0, 1[. Consequently, i (a ij -T a ij )∂ j u

  2 -2νθ e ν (t) , where C 3 depends on µ, sup i,j a ij LLx , on θ and on the product β T. ≤ (C 1 + C 3 -2β) ∂ i (a ij -T a ij )∂ j u -2νθ (e ν (t)) 1/2 (Lu(t)) ν L 2 . ∂ i (a ij -T a ij )∂ j u

	4.4	Final estimate
	From (47) and (50) we get
		d dt	ν=0 E(t) +∞	(ν + 1) e -2β(ν+1)t 2 -2νθ e ν (t) +
					+∞
				+ C 2	ν=0	e -2β(ν+1)t 2 -2νθ (e ν (t)) 1/2	Lu(t)	ν L 2
						+∞
			≤ (C 1 + C 3 -2β)	(ν + 1) e -2β(ν+1)t 2 -2νθ e ν (t) +
			+∞			ν=0		
			+ C 2	e -2β(ν+1)t 2 -2νθ (e ν (t)) 1/2			+
			ν=0			i,j	ν L 2
				+∞
	+ C 2 e -2β(ν+1)t 2 Now, applying Hölder inequality for series implies ν=0 +∞ 	
			e -2β(ν+1)t 2 -2νθ (e ν (t)) 1/2		
		ν=0			i,j	ν L 2

  ∂ i (a ij -T a ij )∂ j u -2β(ν+1)t 2 -2νθ e ν (t) , with C 4 uniformly bounded for β * t + θ in a compact set of ]0, 1[ . So, if we take β > 0 andT ∈ ]0, T 0 ] such that (recall that β * = β(log 2) -1 ) * T = δ < 1 -θ , we have 0 < θ ≤ θ + β * t ≤ θ + δ < 1. Therefore we obtain d dt E(t) ≤ (C 1 + C 4 C 2 + C 3 -2β) -2νθ (e ν (t)) 1/2 (Lu(t)) ν L 2 .Now, taking β large enough such that C 1 + C 4 C 2 + C 3 -2β ≤ 0, which corresponds to take T > 0 small enough, we nally arrive to the estimate

	and nally
	+∞		
	e -2β(ν+1)t 2 -2νθ (e ν (t)) 1/2			≤
	ν=0	i,j	ν L 2
			+∞
	≤ C 4 (ν + 1)e (52) ν=0
		+∞
			(ν + 1) e -2β(ν+1)t 2 -2νθ e ν (t) +
		ν=0
		+∞
	+ C 2	e -2β(ν+1)t 2
		ν=0
				1/2
				,

β d dt E(t) ≤ C 2 (E(t)) 1/2 Lu(t) H -θ-β * t ;

applying Gronwall's Lemma and keeping in mind (

29

) and (30) give us estimate

[START_REF] Hörmander | Linear partial dierential operators[END_REF]

.

Remark 4.2.

Throughout we agree that f (D) stands for the pseudo-dierential operator u → F -1 (f Fu).