A LAGRANGIAN APPROACH FOR THE COMPRESSIBLE NAVIER-STOKES EQUATIONS RAPHAE¨LDANCHIN 2 1 Abstract. HereweinvestigatetheCauchyproblemforthebarotropicNavier-Stokesequations 0 2 in Rn, in the critical Besov spaces setting. We improve recent results (see [4, 8, 9]) as regards the uniqueness condition: initial velocities in critical Besov spaces with (not too) negative in- n dices generate a unique local solution. Apart from (critical) regularity, the initial density just a has to be bounded away from 0 and to tend to some positive constant at infinity. Density- J dependent viscosity coefficients may be considered. Using Lagrangian coordinates is the key 0 to our statements as it enables us to solve the system by means of the basic contraction map- 3 ping theorem. As a consequence, conditions for uniqueness are the same as for existence, and ] Lipschitz continuity of the flow map (in Lagrangian coordinates) is established. P A . h t Introduction a m We address the well-posedness issue for the barotropic compressible Navier-Stokes equations [ with variable density in the whole space Rn (n 2): ≥ 1 ∂ ρ+div(ρu) = 0, t v 3 (0.1) ∂ (ρu)+div(ρu u) 2div(µ(ρ)D(u)) (λ(ρ)divu)+ (P(ρ)) = 0,  t 0 ⊗ − −∇ ∇  2  ρ|t=0 = ρ0, u|t=0 = u0. | | 6 Above ρ =ρ(t,x) R stands for the density, u = u(t,x) Rn, for the velocity field. The .  + 1 spacevariable x bel∈ongstothewhole Rn. Thenotation D(u) ∈designates thedeformation tensor 0 2 which is defined by 1 1 : D(u) := (Du+ u) with (Du) := ∂ ui and ( u) := ∂ uj. v 2 ∇ ij j ∇ ij i i X The pressure function P and the viscosity coefficients λ and µ are given suitably smooth r functions of the density. With no loss of generality, one may assume that P is defined over R a and vanishes at 0. As we focus on viscous fluids, we suppose that (0.2) α := min inf(λ(ρ)+2µ(ρ)),inf µ(ρ) > 0, ρ>0 ρ>0 (cid:16) (cid:17) whichensuresthesecondorderoperatorinthevelocity equation of (0.1) tobeuniformlyelliptic. We supplement System (0.1) with the condition at infinity that u tends to 0 and ρ, to some positive constant (that may be taken equal to 1 after suitable normalization). The exact meaning of those boundary conditions will be given by the functional framework in which we shall consider the system. In the present paper, we aim at solving (0.1) in critical functional spaces, that is in spaces which have the same invariance with respect to time and space dilation as the system itself (see e.g. [8] for more explanations about this nowadays classical approach). In this framework, it has been stated [8, 9] in the constant coefficients case that, for data (ρ ,u ) such that 0 0 a := (ρ 1) B˙n/p(Rn), u B˙n/p−1(Rn) 0 0− ∈ p,1 0 ∈ p,1 Date: January 31, 2012. 1 2 R.DANCHIN and that, for a small enough constant c, (0.3) a c, k 0kB˙n/p(Rn) ≤ p,1 we have for any p [1,2n): ∈ existence of a local solution (ρ,u) such that a := (ρ 1) ([0,T];B˙n/p), u • − ∈ Cb p,1 ∈ ([0,T];B˙n/p−1) and ∂ u, 2u L1(0,T;B˙n/p−1); Cb p,1 t ∇ ∈ p,1 uniqueness in the above space if in addition p n. • ≤ If p n then theviscosity coefficients may depend(smoothly) on ρ andthe smallness condition ≤ (0.3) may be replaced by the following positivity condition (see [4, 10]): (0.4) inf ρ (x) > 0. 0 x∈Rn Those results have been somewhat extended in [16] where it has been noticed that a may be 0 taken in a larger Besov space, with another Lebesgue exponent. The above results are based on maximal regularity estimates in Besov spaces for the evo- lutionary Lam´e system, and on the Schauder-Tychonoff fixed point theorem. In effect, owing to the hyperbolicity of the density equation, there is a loss of one derivative in the stability estimates thus precluding the use of the contraction mapping (or Banach fixed point) theorem. As a consequence, with this method it is found that the conditions for uniqueness are stronger than those for existence. Following our recent paper [13] dedicated to the incompressible density-dependent Navier- Stokes equation, and older works concerning the compressible Navier-Stokes equations (see [20, 21,22]),wehereaimatsolvingSystem(0.1)intheLagrangian coordinates. Themainmotivation is that the mass is constant along the flow hence, to some extent, only the (parabolic type) equation for the velocity has to be considered. After performing this change of coordinates, we shall see that solving (0.1) may be done by means of the Banach fixed point theorem. Hence, the condition for uniqueness is the same as that for the existence, and the flow map is Lipschitz continuous. In addition, in the case of fully nonhomogeneous fluids with variable viscosity coefficients, the analysis turns out to be simpler than in [4, 10] even for density-dependent viscosity coefficients and in the case where the density is not close to a constant. Indeed, our proof relies essentially on a priori estimates for a parabolic system (a suitable linearization of the momentum equation in Lagrangian coordinates) with rough constant depending only on the initial density hence time-independent. In contrast, in [4, 10] tracking the time-dependency of the coefficients was quite technical. We now come to the plan of the paper. In the next section, we introduce the compressible Navier-Stokes equations in Lagrangian coordinates and present our main results. Section 2 is devoted to the proof of our main existence and uniqueness result in the simpler case where the density is close to a constant and the coefficients, density independent. In Section 3, we treat the general fully nonhomogeneous case with nonconstant coefficients. A great deal of the analysis is contained in the study of the linearized momentum equation for (0.1) (see Subsection 3.1) which turns out to be a Lam´e type system with variable rough coefficients. This will enable us to define a self-map Φ on a suitably small ball of some Banach space E (T) and to p apply the contraction mapping theorem so as to solve the compressible Navier-Stokes equations in Lagrangian coordinates. In the Appendix we prove several technical results concerning the Lagrangian coordinates and Besov spaces. Notation: Throughout, the notation C stands for a generic constant (the meaning of which depends on the context), and we sometimes write X . Y instead of X CY. For X a Banach space, p [1,+ ] and T > 0, the notation Lp(0,T;X) or Lp(X) des≤ignates the set of ∈ ∞ T COMPRESSIBLE NAVIER-STOKES EQUATIONS 3 measurable functions f : [0,T] X with t f(t) in Lp(0,T). We agree that ([0,T];X) X → 7→ k k C denotes the set of continuous functions from [0,T] to X. 1. Main results Before deriving the Lagrangian equations corresponding to (0.1), let us introduce more nota- tion. We agree that for a C1 function F : Rn Rn Rm then divF : Rn Rm with → × → (divF)j := ∂ F for 1 j m, i ij ≤ ≤ i X and that for A= (A ) and B = (B ) two n n matrices, we denote ij 1≤i,j≤n ij 1≤i,j≤n × A: B = TrAB = A B . ij ji i,j X The notation adj(A) designates the adjugate matrix that is the transposed cofactor matrix. Of course if A is invertible then we have adj(A) = (detA)A−1. Finally, given some matrix A, we define the “twisted” deformation tensor and divergence operator (acting on vector fields z) by the formulae 1 D (z) := Dz A+TA z and div z := TA : z. A A 2 · ·∇ ∇ Let X be the flow associated t(cid:0)o the vector-field(cid:1)u, that is the solution to t (1.1) X(t,y) = y+ u(τ,X(τ,y))dτ. Z0 Denoting ρ¯(t,y) := ρ(t,X(t,y)) and u¯(t,y) = u(t,X(t,y)) with (ρ,u) a solution of (0.1), and using the chain rule and Lemma 1 from the Appendix, we gather that (ρ¯,u¯) satisfies ∂ (Jρ¯) = 0 t (1.2)  Jρ ∂ u¯ div adj(DX) 2µ(ρ¯)D (u¯)+λ(ρ¯)div u¯Id+P(ρ¯)Id = 0 0 t A A  − (cid:16) (cid:17) with J := detDX and A := (D X)−(cid:0)1. Note that one may forget any refer(cid:1)ence to the initial  y Eulerian vector-field u by defining directly the “flow” X of u¯ by the formula t (1.3) X(t,y) = y+ u¯(τ,y)dτ. Z0 We want to solve the above system in critical homogeneous Besov spaces. Let us recall that, for 1 p and s n/p, a tempered distribution u over Rn belongs to the homogeneous ≤ ≤ ∞ ≤ Besov space B˙s (Rn) if p,1 u = ∆˙ u in ′(Rn) j S j∈Z X and (1.4) kukB˙ps,1(Rn) := 2jsk∆˙ jukLp(Rn) < ∞. j∈Z X Here (∆˙j)j∈Z denotes a homogeneous dyadic resolution of unity in Fourier variables –the so- called Littlewood-Paley decomposition (see e.g. [1], Chap. 2 for more details on the Littlewood- Paley decomposition and Besov spaces). Loosely speaking, a function belongs to B˙s (Rn) if it has s derivatives in Lp(Rn). In the p,1 present paper, we shall mainly use the following classical properties: 4 R.DANCHIN the Besov space B˙n/p(Rn) is a Banach algebra embedded in the set of continuous func- • p,1 tions going to 0 at infinity, whenever 1 p < ; ≤ ∞ the usual product maps B˙n/p−1(Rn) B˙n/p(Rn) in B˙n/p−1(Rn) whenever 1 p < 2n; • p,1 × p,1 p,1 ≤ Let F : I R be a smooth function (with I an open interval of R containing 0) • → vanishing at 0. Then for any s> 0, 1 p and interval J compactly supported in ≤ ≤ ∞ I there exists a constant C such that (1.5) F(a) C a k kB˙ps,1(Rn) ≤ k kB˙ps,1(Rn) for any a B˙s (Rn) with values in J. In addition, if a and a are two such functions ∈ p,1 1 2 and s = n/p then we have (1.6) F(a ) F(a ) C a a . k 2 − 1 kB˙n/p(Rn) ≤ k 2− 1kB˙n/p(Rn) p,1 p,1 Fromnowon,weshallomit Rn inthenotationforBesovspaces. Weshallobtaintheexistence and uniqueness of a local-in-time solution (ρ¯,u¯) for (1.2), with a¯ := ρ¯ 1 in ([0,T];B˙n/p) and − C p,1 u¯ in the space E (T):= v ([0,T];B˙n/p−1), ∂ v, 2v L1(0,T;B˙n/p−1) p ∈ C p,1 t ∇ ∈ p,1 · That space will be endowe(cid:8)d with the norm (cid:9) kvkEp(T) := kvkL∞(B˙n/p−1)+k∂tv,∇2vkL1(B˙n/p−1). T p,1 T p,1 Let us now state our main result. Theorem 1. Let 1 < p < 2n and n 2. Let u be a vector-field in B˙n/p−1. Assume that the ≥ 0 p,1 initial density ρ satisfies a := (ρ 1) B˙n/p and 0 0 0− ∈ p,1 (1.7) infρ (x) > 0. 0 x Then System (1.2) has a unique local solution (ρ¯,u¯) with (a¯,u¯) ([0,T];B˙n/p) E (T). ∈ C p,1 × p Moreover, the flow map (a ,u ) (a¯,u¯) is Lipschitz continuous from B˙n/p B˙n/p−1 to 0 0 7−→ p,1 × p,1 ([0,T];B˙n/p) E (T). C p,1 × p In Eulerian coordinates, this result recasts in: Theorem 2. Under the above assumptions, System (0.1) has a unique local solution (ρ,u) with u E (T), ρ bounded away from 0 and (ρ 1) ([0,T];B˙n/p). ∈ p − ∈ C p,1 Let us make a few comments concerning the above assumptions. We expect the Lagrangian method to improve the uniqueness conditions given in e.g. [8] • for the full Navier-Stokes equations. We here consider the barotropic case for simplicity. The condition 1 p < 2n is a consequence of the product laws in Besov spaces. It • ≤ implies that the regularity exponent for the velocity has to be greater than 1/2 (to − be compared with 1 for the homogeneous incompressible Navier-Stokes equations). It − would be interesting to see whether introducing a modified velocity as in B. Haspot’s works [15, 16] allows to consider different Lebesgue exponents for the Besov spaces pertaining to the density and the velocity so as to go beyond p = 2n for the velocity. The regularity condition over the density is stronger than that for density-dependent • incompressible fluids (see [13]). In particular, in contrast with incompressible fluids, it is not clear that combining Lagrangian coordinates and critical regularity approach allows to consider discontinuous densities. COMPRESSIBLE NAVIER-STOKES EQUATIONS 5 Owing to the fact that the density satisfies a transport equation, we do not expect • Lipschitz continuity of the flow map in high norm for the Eulerian formulation to be true. It is worth comparing our results with those of P. Germain in [14], and D. Hoff in [17] • concerning the weak-strong uniqueness problem. In both papers, the idea is to show that a finite energy weak solution coincides with a strong one under some additional assumptions. The weak solution turns out to have less regularity as in Theorem 2. At the same time, the assumptions on the strong solution (ρ,u) are much stronger. In both papers, u has to bein L1(0,T;L∞), andtosatisfy additional conditions: roughly 2u ∇ ∇ or ∂ u have to be in L2(0,T;Ld) in Germain’s work, while √tD2u Lr(0,T;L4) with t ∈ r = 4/3 if n = 2, and r = 8/5 if n = 3 in Hoff’s paper. Some regularity conditions are required on the density but they are, to some extent, weaker than ours. 2. The simple case of almost homogeneous compressible fluids As a warm up and for the reader convenience, we here explain how local well-posedness may be proved for the system in Lagrangian coordinates in the simple case where: (1) The viscosity coefficients are constant, (2) The density is very close to one. Let µ′ := λ +µ. Keeping in mind the above two conditions and using the fact that the first equation of (1.2) implies that (2.1) J(t, )ρ¯(t, ) ρ , 0 · · ≡ with J := detDX and | | t (2.2) X(t,y) := y+ u¯(τ,y)dτ, Z0 we rewrite the equation for the Lagrangian velocity as (recall that A:= (DX)−1): (2.3) ∂ u¯ µ∆u¯ µ′ divu¯ = (1 ρ J)∂ u¯+2µdiv adj(DX)D (u¯) D(u¯) t 0 t A − − ∇ − − +λdiv adj(DX)divAu¯(cid:0) divu¯Id div adj(D(cid:1)X)P(J−1ρ0) . − − The left-hand side of the above equa(cid:0)tion is the linear Lam´e sys(cid:1)tem wit(cid:0)h constant coefficie(cid:1)nts, the solvability of which may beeasily deduced from that of the heat equation in the whole space (see e.g. [1], Chap. 2). We get: Proposition 1. Let the viscosity coefficients (µ,µ′) R2 satisfy µ > 0 and µ+µ′ > 0. Let ∈ p [1, ] and s R. Let u B˙s and f L1(0,T;B˙s ). Then the Lam´e system ∈ ∞ ∈ 0 ∈ p,1 ∈ p,1 ∂ u µ∆u µ′ divu= f in (0,T) Rn t (2.4) − − ∇ × ( u|t=t0 = u0 on Rn has a unique solution u in ([0,T);B˙s ) such that ∂ u, 2u L1(0,T;B˙s ) and the following C p,1 t ∇ ∈ p,1 estimate is valid: (2.5) u +min(µ,µ+µ′) 2u C( f + u ) k kL∞T (B˙ps,1) k∇ kL1T(B˙ps,1) ≤ k kL1T(B˙ps,1) k 0kB˙ps,1 where C is an absolute constant with no dependence on µ,µ′ and T. In the rest of this section, we drop the bars on the Lagrangian velocity field. Granted with the above proposition, we define a map Φ : v u on E (T) where u stands for the solution to p 7→ (2.6) ∂ u µ∆u µ′ divu = I (v,v)+2µdivI (v,v)+λdivI (v,v) divI (v) t 1 2 3 4 − − ∇ − 6 R.DANCHIN with I (v,w) = (1 ρ J )∂ w, I (v,w) = adj(DX )D (w) D(w), 1 − 0 v t 2 v Av − I (v,w) = div wadj(DX ) divwId, I (v) = adj(DX )P(J−1ρ ). 3 Av v − 4 v v 0 Note that any fixed point of Φ is a solution in E (T) to (2.6). We claim that the existence of p such points is a consequence of the standard Banach fixed point theorem in a suitable closed ball of E (T). p First step: estimates for I , I , I and I . Throughout we assume that for a small enough 1 2 3 4 constant c, T (2.7) Dv dt c. k kB˙n/p ≤ Z0 p,1 In order to bound I (v,w), we decompose it into 1 I (v,w) = (1 J )∂ w a (1+(J 1))∂ w with a := ρ 1. 1 v t 0 v t 0 0 − − − − Taking advantage of the product law B˙n/p−1 B˙n/p B˙n/p−1 (if 1 p < 2n) and of the fact p,1 × p,1 → p,1 ≤ that B˙n/p is an algebra (if 1 p < ), of (A.11), (2.7) and of (1.5), we readily get p,1 ≤ ∞ (2.8) kI1(v,w)kL1(B˙n/p−1) ≤ C ka0kM(B˙n/p−1)+kDvkL1(B˙n/p) k∂twkL1(B˙n/p−1). T p,1 p,1 T p,1 T p,1 Above we introduced the multiplier(cid:0)norm (B˙s ) for B˙s , that i(cid:1)s defined by M p,1 p,1 (2.9) f := sup ψf k kM(B˙ps,1) k kB˙ps,1 where the supremum is taken over those functions ψ in B˙s with norm 1. p,1 Next, thanks to product laws, to (A.12), (A.13) and to (2.7), we have (2.10) I (v,w) + I (v,w) C Dv Dw . k 2 kL1(B˙n/p) k 3 kL1(B˙n/p) ≤ k kL1(B˙n/p)k kL1(B˙n/p) T p,1 T p,1 T p,1 T p,1 As regards the pressure term (that is I (v)), we use the fact that under assumption (2.7), we 4 have, by virtue of the composition inequality (1.5) and of flow estimates (see (A.9) and (A.11)), (2.11) I (v) C 1+ Dv 1+ a . k 4 kL∞(B˙n/p) ≤ k kL1(B˙n/p) k 0kB˙n/p T p,1 T p,1 p,1 (cid:0) (cid:1)(cid:0) (cid:1) Second step: Φ maps a suitable closed ball in itself. At this stage, one may assert that if v E (T) satisfies (2.7) then the right-hand side of (2.6) belongs to L1(0,T;B˙n/p−1). Hence ∈ p p,1 Proposition 1 implies that Φ(v) is well definedand maps E (T) to itself. However it is not clear p that it is contractive over the whole set E (T). So we introduce u the “free solution” to p L ∂ u µ∆u µ′ divu = 0, u = u . t L L L L t=0 0 − − ∇ | Of course, Proposition 1 guarantees that u belongs to E (T) for all T > 0. Hence, if T and L p R are small enough then any vector-field in B¯ (u ,R) satisfies (2.7). Ep(T) L We claim that if T is small enough (a condition which will be expressed in terms of the free solution u ) and if R is small enough (a condition which will depend only on the viscosity L coefficients and on p, n and P) then v B¯ (u ,R) = u B¯ (u ,R). ∈ Ep(T) L ⇒ ∈ Ep(T) L Indeed u := u u satisfies L − ∂ u µ∆u µ′ divu = I (v,v)+2µdivI (v,v)+λdivI (v,v) divI (v), t 1 2 3 4 e − − ∇ − ( ut=0 = 0. | e e e e COMPRESSIBLE NAVIER-STOKES EQUATIONS 7 So Proposition 1 yields1 kukEp(T) . kI1(v,v)kL1(B˙n/p−1)+kI2(v,v)kL1(B˙n/p)+kI3(v,v)kL1(B˙n/p)+TkI4(v)kL∞(B˙n/p). T p,1 T p,1 T p,1 T p,1 Inserting inequalities (2.8), (2.10) and (2.11), we thus get: e kukEp(T) . kDvk2L1(B˙n/p)+ ka0kM(B˙n/p−1)+kDvkL1(B˙n/p) k∂tvkL1(B˙n/p−1)+T(1+ka0kB˙n/p). T p,1 p,1 T p,1 T p,1 p,1 That is, keeping in mind tha(cid:0)t v is in B¯ (u ,R), (cid:1) e Ep(T) L kukEp(T) ≤ C ka0kM(B˙n/p−1)+kDuLkL1(B˙n/p)+R (R+k∂tuLkL1(B˙n/p−1)) p,1 T p,1 T p,1 (cid:16) (cid:0) +(cid:1) Du 2 +R2+T(1+ a ) . e k LkL1(B˙n/p) k 0kB˙n/p T p,1 p,1 (cid:17) So we see that, and if T satisfies (2.12) CT(1+ka0kB˙n/p) ≤ R/2 and kDuLkL1(B˙n/p)+k∂tuLkL1(B˙n/p−1) ≤ R p,1 T p,1 T p,1 then we have kukEp(T) ≤ 2C(ka0kM(B˙n/p−1)+2R)R+2CR2+R/2. p,1 Hence there exists a small constant η = η(n,p) such that if e (2.13) ka0kM(B˙n/p−1) ≤ η, p,1 and if R has been chosen small enough then u is in B¯ (u ,R). Of course, taking R and T Ep(T) L even smaller ensures that (2.7) is satisfied for all vector-field of B¯ (u ,R). Ep(T) L Third step: contraction properties. We claim that under Conditions (2.13) and (2.12) (with a smaller R if needed), the map Φ is 1/2-Lipschitz over B¯ (u ,R). So we are given v and Ep(T) L 1 v in B¯ (u ,R) and denote 2 Ep(T) L u := Φ(v ) and u := Φ(v ). 1 1 2 2 Let X and X be the flows associated to v and v . Set A = (DX )−1 and J := detDX 1 2 1 2 i i i i for i = 1,2. The equation satisfied by δu := u u reads 2 1 − ∂ δu µ∆δu µ′ divδu = δf := δf +δf +divδf +2µdivδf +λdivδf t 1 2 3 4 5 − − ∇ with δf := (1 ρ J )∂ δu, δf := ρ (J J )∂ u , 1 0 2 t 2 0 2 1 t 1 − − − δf := adj(DX )P(ρ J−1) adj(DX )P(ρ J−1), 3 1 0 1 − 2 0 2 δf := adj(DX )D (u ) adj(DX )D (u ) D(δu), 4 2 A2 2 − 1 A1 1 − δf := adj(DX )TA : u adj(DX )TA : u divδuId. 5 2 2 2 1 1 1 ∇ − ∇ − Once again, bounding δu in E (T) stems from Proposition 1, which ensures that p 2 5 (2.14) kδukEp(T) . kδfikL1(B˙n/p−1)+Tkδf3kL∞(B˙n/p)+ kδfikL1(B˙n/p). T p,1 T p,1 T p,1 i=1 i=4 X X In order to bound δf and δf , we just have to use the definition of the multiplier space 1 2 (B˙n/p−1), and (A.11),(A.20). We get M p,1 (2.15) kδf1kL1(B˙n/p−1) ≤ ka0kM(B˙n/p−1)+CkDv2kL1(B˙n/p) k∂tδukL1(B˙n/p−1), T p,1 p,1 T p,1 T p,1 (2.16) kδf2kL1(B˙n/p−1) ≤ C(cid:0) kρ0kM(B˙n/p−1)kDδvkL1(B˙n/p)k∂tu(cid:1)1kL1(B˙n/p−1). T p,1 p,1 T p,1 T p,1 1For simplicity, we donot track thedependency of thecoefficients with respect to µ and µ′. 8 R.DANCHIN Next, using the decomposition δf = (adj(DX ) adj(DX ))P(ρ J−1)+adj(DX )(P(ρ J−1) P(ρ J−1)), 3 1 − 2 0 2 1 0 1 − 0 2 together with composition inequalities (1.5), (1.6) and (A.19), and product laws in Besov space yields (2.17) kδf3kL∞(B˙n/p−1) . T(1+ka0kB˙n/p)kDδvkL1(B˙n/p) T p,1 p,1 T p,1 Finally, we have δf = (adj(DX ) adj(DX ))TA : u +adj(DX )T(A A ) : u +(adj(DX )TA Id) : δu, 5 2 1 2 2 1 2 1 2 1 1 − ∇ − ∇ − ∇ whence, by virtue of (A.9), (A.10), (A.18) and (A.19), (2.18) δf . Dδv Du + Dδu Dv . k 4kL1(B˙n/p) k kL1(B˙n/p)k 2kL1(B˙n/p) k kL1(B˙n/p)k 1kL1(B˙n/p) T p,1 T p,1 T p,1 T p,1 T p,1 Bounding δf worksexactly thesame. SoweseethatifConditions(2.13) and(2.12) aresatisfied 4 (with smaller η and larger C if need be) then we have 1 δu δv . k kEp(T) ≤ 2k kEp(T) Hence, the map Φ : B¯ (u ,R) B¯ (u ,R) is 1/2-Lipschitz. Therefore, Banach’ fixed Ep(T) L 7→ Ep(T) L point theorem ensures that Φ admits a unique fixed point in B¯ (u ,R). This completes the Ep(T) L proof of existence of a solution in E (T) for System (1.2). p A tiny variation over the proof of the contraction properties yields uniqueness and Lipschitz continuity of the flow map. We eventually get: Theorem 3. Let p [1,2n) (with n 1) and u be a vector-field in B˙n/p−1. Assume that the ∈ ≥ 0 p,1 initial density ρ satisfies a := (ρ 1) B˙n/p. There exists a constant c depending only on 0 0 0 − ∈ p,1 p and on n such that if (2.19) ka0kM(B˙n/p−1) ≤ c p,1 then System (1.2) has a unique local solution (ρ¯,u¯) with (a¯,u¯) ([0,T];B˙n/p) E (T). ∈ C p,1 × p Moreover, the flow map (a ,u ) (a¯,u¯) is Lipschitz continuous from Bn/p B˙n/p−1 to 0 0 7−→ p,1 × p,1 ([0,T];B˙n/p) E (T). C p,1 × p In Eulerian coordinates, this result recasts in: Theorem 4. Under the above assumptions, System (0.1) has a unique local solution (ρ,u) with density bounded away from vacuum and a ([0,T];B˙n/p−1) and u E (T). ∈C p,1 ∈ p We do not give here more details on how to complete the proof of Theorem 3 and its Eulerian counterpart,Theorem4asitwilldoneinthenextsectionundermuchmoregeneralassumptions. 3. The fully nonhomogeneous case For treating the general case where ρ need not satisfy (2.19), just resorting to Proposition 1 0 is not enough because the term I (v,v) in the r.h.s of (2.6) need not be small. One has first to 1 establish a similar statement for a Lam´e system with nonconstant coefficients. More precisely, keeping in mind that ρ= J−1ρ (we still drop the bars for notational simplicity), we recast the u 0 velocity equation of (1.2) in: L (u) = I (u,u)+ρ−1div I (u,u)+I (u,u)+I (u,u)+I (u) ρ0 1 0 2 3 4 5 with (cid:0) (cid:1) (3.1) L (u) := ∂ u ρ−1div 2µ(ρ )D(u)+λ(ρ )divuId ρ0 t − 0 0 0 (cid:0) (cid:1) COMPRESSIBLE NAVIER-STOKES EQUATIONS 9 and I (v,w) := (1 J )∂ w 1 v t − I (v,w) := (adj(DX ) Id) µ(J−1ρ )(DwA +TA w)+λ(J−1ρ )(TA : w)Id 2 v − v 0 v v∇ v 0 v ∇ I (v,w) := (µ(J−1ρ ) µ(ρ(cid:0)))(DwA +TA w)+(λ(J−1ρ ) λ(ρ ))(TA : w)(cid:1)Id 3 v 0 − 0 v v∇ v 0 − 0 v ∇ I (v,w) := µ(ρ ) Dw(A Id)+T(A Id) w +λ(ρ )(T(A Id) : wId 4 0 v v 0 v − − ∇ − ∇ I (v) := adj((cid:0)DX )P(ρ J−1). (cid:1) 5 − v 0 v Therefore, in order to solve (1.2) locally, it suffices to show that the map (3.2) Φ : v u 7−→ with u the solution to L (u) = I (v,v)+ρ−1div I (v,v)+I (v,v)+I (v,v)+I (v) , ρ0 1 0 2 3 4 5 ( u = u (cid:0) (cid:1) t=0 0 | has a fixed point in E (T) for small enough T. p As a first step, we have to study the properties of the linear Lam´e operator L . This is done ρ0 in the following subsection. 3.1. Linear parabolic systems with rough coefficients. As a warm up, we consider the following scalar heat equation with variable coefficients: (3.3) ∂ u adiv(b u)= f. t − ∇ We assume that (3.4) α:= inf (ab)(t,x) > 0. (t,x)∈[0,T]×Rn Let us first consider the smooth case. Proposition 2. Assume that a and b are bounded functions satisfying (3.4) and such that b a ∇ and a b are in L∞(0,T;B˙n/p) for some 1 < p < . There exist two constants κ = κ(p) and ∇ p,1 ∞ C = C(s,n,p) such that the solutions to (3.3) satisfy for all t [0,T], ∈ C t u +κα u u + f exp (b a,a b) 2 dτ k kL∞t (B˙ps,1) k kL1t(B˙ps,+12) ≤ k 0kB˙ps,1 k kL1t(B˙ps,1) (cid:18)α Z0 k ∇ ∇ kB˙pn,/1p (cid:19) (cid:0) (cid:1) whenever min(n/p,n/p′) <s n/p. − ≤ Proof. We first rewrite the equation for u as follows: ∂ u div(ab u) = f b a u, t − ∇ − ∇ ·∇ then localize the equation in the Fourier space, according to Littlewood-Paley decomposition: ∂ u div(ab u )= f ∆˙ (b a u)+R t j j j j j − ∇ − ∇ ·∇ with u := ∆˙ u, f := ∆˙ f and R := div([∆˙ ,ab] u). j j j j j j ∇ Next, we multiply the above equation by u u p−2 and integrate over Rn. Taking advantage j j | | of Lemma 8 in the appendix of [12] (here 1 < p < comes into play) and of H¨older inequality, ∞ we get for some constant c depending only on p: p 1 d u p +c α22j u p u p−1 f + ∆˙ (b a u) + R , pdtk jkLp p k jkLp ≤ k jkLp k jkLp k j ∇ ·∇ kLp k jkLp (cid:0) (cid:1) 10 R.DANCHIN which, after time integration, leads to (3.5) kujkL∞t (Lp)+cpα22jkujkL1t(Lp) ≤ ku0,jkLp t + f + ∆˙ (b a u) + R dτ. k jkL1t(Lp) k j ∇ ·∇ kLp k jkLp Z0 (cid:16) (cid:17) AccordingtoLemmas4and5inAppendix,thereexistsapositiveconstant C andsomesequence (cj)j∈Z with c ℓ1(Z) = 1, satisfying k k (3.6) ∆˙ (b a u) + R Cc 2−js b a + a b u . k j ∇ ·∇ kLp k jkLp ≤ j k ∇ kB˙pn,/1p k ∇ kB˙pn,/1p k∇ kB˙ps,1 Then inserting (3.6) in (3.5), multiplying by 2js a(cid:0)nd summing up over j y(cid:1)ields (3.7) u +c α u u + f k kL∞t (B˙ps,1) p k kL1t(B˙ps+,12) ≤ k 0kB˙ps,1 k kL1t(B˙ps,1) t +C (b a,a b) u dτ. Z0 k ∇ ∇ kB˙pn,/1pk kB˙ps,+11 From the interpolation inequality 1/2 1/2 (3.8) u u u , k kB˙ps,+11 ≤ k kB˙ps,1k kB˙ps,+12 we gather that αc C2 C (b a,a b) u p u + (b a,a b) 2 u . k ∇ ∇ kB˙pn,/1pk kB˙ps,+11 ≤ 2 k kB˙ps+,12 2αcpk ∇ ∇ kB˙pn,/1pk kB˙ps,1 So plugging this in (3.7) and applying Gronwall lemma completes the proof of the proposition. (cid:3) In the rough case where the coefficients are only in B˙n/p, the above proposition has to be p,1 modified as follows: Proposition 3. Let a and b be bounded positive and satisfy (3.4). Assume that b a and a b ∇ ∇ are in L∞(0,T;B˙n/p−1) with 1 < p < . There exist three constants η, κ and C such that if p,1 ∞ for some m Z we have ∈ (3.9) inf(t,x)∈[0,T]×RnS˙m(ab)(t,x) α/2, ≥ (3.10) k(Id−S˙m)(b∇a,a∇b)kL∞(B˙n/p−1) ≤ ηα T p,1 then the solution to (3.3) satisfies for all t [0,T], ∈ C t u +ακ u u + f exp S˙ (b a,a b) 2 dτ k kL∞t (B˙ps,1) k kL1t(B˙ps,+12) ≤ k 0kB˙ps,1 k kL1t(B˙ps,1) (cid:18)α Z0 k m ∇ ∇ kB˙pn,/1p (cid:19) (cid:0) (cid:1) whenever (3.11) min(n/p,n/p′) < s n/p 1. − ≤ − Proof. Given the new assumptions, it is natural to replace (3.6) by the inequality (3.12) k∆˙j(b∇a·∇u)kLp +kRjkLp ≤ Ccj2−js kb∇akB˙pn,/1p−1 +ka∇bkB˙pn,/1p−1 k∇ukB˙ps+,11, which may be obtained by taking σ = 1 and ν(cid:0)= 1 in Lemmas 4 and 5. (cid:1)However, when bounding R , in addition to (3.11), one has to assume that p n. Also, as it involves the j ≤ highest regularity of u, we cannot expect to absorb this “remainder term” any longer, unless a b and b a are small in B˙n/p−1 (which would correspond to the case that has been treated ∇ ∇ p,1 in the previous section). So we rather rewrite the heat equation as follows: ∂ u div(S˙ (ab) u) = f +div((Id S˙ )(ab) u) S˙ (b a) u (Id S˙ )(b a) u. t m m m m − ∇ − ∇ − ∇ ·∇ − − ∇ ·∇