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ASYMPTOTIC PROPERTIES OF ENTROPY SOLUTIONS TO FRACTAL BURGERS EQUATION NATHAELALIBAUD,CYRILIMBERT,ANDGRZEGORZKARCH Abstract. We study properties of solutions of the initial value problem for 0 thenonlinearandnonlocalequationut+(−∂x2)α/2u+uux=0withα∈(0,1], supplementedwithaninitialdatumapproachingtheconstantstatesu±(u−< 1 0 u+) as x → ±∞, respectively. It was shown by Karch, Miao & Xu (SIAM J. Math. Anal. 39 (2008), 1536–1549) that, for α ∈ (1,2), the large time 2 asymptotics of solutions is described by rarefaction waves. The goal of this n paper is to show that the asymptotic profile of solutions changes for α ≤ 1. a If α = 1, there exists a self-similar solution to the equation which describes J thelargetimeasymptotics ofother solutions. Inthecaseα∈(0,1), weshow 2 thatthenonlinearityoftheequationisnegligibleinthelargetimeasymptotic 2 expansionofsolutions. ] P A h. 1. Introduction t a In this work, we continue the study of asymptotic properties of solutions of the m Cauchy problem for the following nonlocal conservation law [ (1.1) u +Λαu+uu =0, x R, t>0, 3 t x ∈ (1.2) u(0,x)=u (x), v 0 4 whereΛα =( ∂2/∂x2)α/2 isthepseudodifferentialoperatordefinedviatheFourier 9 \− 3 transform (Λαv)(ξ) = ξ αv(ξ). This equation is referred to as the fractal Burgers 3 equation. | | 3. The initial datum u0 Lb∞(R) is assumed to satisfy: ∈ 0 9 (1.3) u <u with u u L1( ,0) and u u L1(0,+ ) − + 0 − 0 + 0 ∃ − ∈ −∞ − ∈ ∞ : (where u are real numbers). An interesting situation is where u BV(R), that v ± 0 ∈ is to say i X x r (1.4) u (x)=c+ m(dy) a 0 Z−∞ Date:January22,2010. 2000 Mathematics Subject Classification. 35K05,35K15. Keywordsandphrases. fractalBurgersequation,asymptoticbehaviorofsolutions,self-similar solutions,entropysolutions. The authors would like to thank the referee for suggestions that improved significantly the presentationoftheresults. ThefirstauthorwouldliketothanktheDepartmentofMathematics Prince of Songkla University (Hat Yai campus, Thailand) for having ensured a large part of his workingfacilities. ThesecondauthorwaspartiallysupportedbytheANRproject“EVOL”.The workofthethirdauthorwaspartiallysupportedbytheEuropeanCommissionMarieCurieHost Fellowship for the Transfer of Knowledge “Harmonic Analysis, Nonlinear Analysis and Proba- bility” MTKD-CT-2004-013389, and by the PolishMinistry of Science grant N201 022 32/0902. The authors were also partially supported by PHC-Polonium project no 20078TL ”Nonlinear evolutions equations withanomalous diffusions”. 1 2 NATHAELALIBAUD,CYRILIMBERT,ANDGRZEGORZKARCH with c R and a finite signed measure m on R. In that case Jourdain, Méléard, ∈ and Woyczyn´ski [11, 12] have recently given a probabilistic interpretation to problem (1.1)–(1.2). Assumption (1.3) holds true when (1.5) u =c and u u = m(dx)>0. − + − − R Z If c = 0 and if m is a probability measure, the function u defined in (1.4) is 0 the cumulative distribution function and this property is shared by the solution u(t) u(,t) for every t > 0 (see [11, 12]). As a consequence of our results, we ≡ · describetheasymptoticbehaviorofthefamily u(t) ofprobabilitydistribution t≥0 { } functions as t + (see the summary at the end of this section). → ∞ It was shown in [14] that, under assumptions (1.3)–(1.5) and for 1 < α 2, ≤ the large time asymptotics of solution to (1.1)–(1.2) is described by the so-called rarefaction waves. The goalofthis paperis tocomplete theseresultsandtoobtain universal asymptotic profiles of solutions for 0<α 1. ≤ 1.1. Known results. Let us first recall the results obtained in [14]. For α ∈ (1,2], the initial value problem for the fractal Burgers equation (1.1)–(1.2) with u L∞(R) has a unique, smooth, global-in-time solution (cf. [8, Thm. 1.1], [9, 0 ∈ Thm. 7]). If, moreover, the initial datum is of the form (1.4) and satisfies (1.3)– (1.5), the corresponding solution u behaves asymptotically when t + as the → ∞ rarefaction wave (cf. [14, Thm. 1.1]). More precisely, for every p (3−α,+ ] ∈ α−1 ∞ there exists a constant C >0 such that for all t>0, (1.6) u(t) wR(t) p Ct−12[α−1−3−pα]log(2+t) k − k ≤ ( is the standard norm in Lp(R)). Here, the rarefaction wave is the explicit p k·k (self-similar) function x u , u , − − t ≤ x x x (1.7) wR(x,t)=wR ,1 , u u , (cid:16)t (cid:17)≡ t x− ≤ t ≤ + u , u . + + t ≥ It is well-known that wR is the unique entropy solution of the Riemann problem for the nonviscous Burgers equation wR+wRwR =0. t x Thegoaloftheworkistoshowthat,forα (0,1],oneshouldexpectcompletely ∈ differentasymptoticprofilesofsolutions. Letusnoticethattheinitialvalueproblem (1.1)–(1.2) has a unique global-in-time entropy solution for every u L∞(R) and 0 ∈ α (0,1] due to the recent work by the first author [1]. We recall this result in ∈ Section 2. 1.2. Main results. Our twomainresults areTheorems1.3and1.6, statedbelow. Both of them are a consequence of the following Lp-estimate of the difference of two entropy solutions. Theorem 1.1. Let 0< α 1. Assume that u and u are two entropy solutions of (1.1)–(1.2) with initial cond≤itions u and u in L∞(R). Suppose, moreover, that u 0 0 0 is non-decreasing and u u L1(R). Then there exists a constant C =C(α)>0 0 0 e − ∈ such that for all p [1,+ ] and all t>0 e e ∈ ∞ (1.8) u(t)e u(t) p Ct−α1(1−p1) u0 u0 1. k − k ≤ k − k Remark 1.2. (1) It is worth mentioning that this estimate is sharper than the oneobtainedbyinterpeolatingthe L1-contractioneprincipleandL∞-bounds on the solutions. FRACTAL BURGERS EQUATION 3 (2) Mentionalsothatthisresultholdstrueforα (1,2]withoutadditionalBV- ∈ assumptiononu . Consequently,asanimmediatecorollaryof(1.6)and(1.8), 0 one can slightly complete the results from [14]. More precisely, let α (1,2], u L∞(R) satisfying (1.3) and u be the solution to (1.1)–(1.2)∈. 0 ∈ Then for every p (3−α,+ ] there exists a constant C >0 such that for ∈ α−1 ∞ all t>0 u(t) wR(t) p Ct−21[α−1−3−pα]log(2+t)+Ct−α1(1−p1), k − k ≤ even if u / BV(R). 0 ∈ In the case α<1, the linear part of the fractal Burgers equation dominates the nonlinear one for large times. In the case α = 1, both parts are balanced; indeed, self-similar solutions exist. Let us be more precise now. For α < 1, the Duhamel principle (see equation (3.3) below) shows that the nonlinearityinequation(1.1)isnegligibleintheasymptoticexpansionofsolutions. Theorem 1.3. (Asymptotic behavior as the linear part) Let 0 < α < 1 and u L∞(R) satisfying (1.3). Let u be the entropy solution to 0 ∈ (1.1)–(1.2). Denote by S (t) the semi-group of linear operators whose infini- α t>0 { } tesimal generator is Λα. Consider the initial condition − u , x<0, − (1.9) U (x) 0 ≡(cid:26)u+, x>0. Then, there exists a constant C = C(α) > 0 such that for all p 1 ,+ and ∈ 1−α ∞ all t>0, (cid:0) (cid:3) u(t) Sα(t)U0 p Ct−α1(1−p1) u0 U0 1 (1.10) k − k ≤ k − k +C(u+ u−)max u+ , u− t1−α1(1−p1). − {| | | |} Remark 1.4. (1) ItfollowsfromtheproofofTheorem1.3thatinequality(1.10) is valid for every p [1,+ ]. However, its right-hand-side decays only for ∈ ∞ p 1 ,+ . ∈ 1−α ∞ (2) Let us recall here the formula S (t)U = p (t) U where p = p (x,t) (cid:0) (cid:3) α 0 α ∗ 0 α α denotes the fundamental solution of the equation u +Λαu = 0 (cf. the t beginning of Section 3 for its properties). Hence, changing variables in the convolution p (t) U , one can write the asymptotic term in (1.10) in the α 0 ∗ self-similarform(S (t)U )(x)=H (xt−1/α)whereH (x)=(p (1) U )(x) α 0 α α α 0 ∗ is a smooth and non-decreasing function satisfying lim H (x) = u x→±∞ α ± and ∂ H (x)=(u u )p (x,1). x α + − α − In the case α = 1, we use the uniqueness result from [1] combined with a standard scaling technique to show that equation (1.1) has self-similar solutions. In Section 4, we recall this well-known reasoning which leads to the proof of the following theorem. Theorem 1.5. (Existence of self-similar solutions) Assume α = 1. The unique entropy solution U of the initial value problem (1.1)– (1.2) with the initial condition (1.9) is self-similar, i.e. it has the form U(x,t) = U x,1 for all x R and all t>0. t ∈ (cid:0)Our(cid:1)secondmainconvergenceresultstatesthattheself-similarsolutiondescribes the large time asymptotics of other solutions to (1.1)–(1.2). Theorem 1.6. (Asymptotic behavior as the self-similar solution) Letα=1andu L∞(R)satisfying (1.3). Letubetheentropysolutiontoproblem 0 ∈ 4 NATHAELALIBAUD,CYRILIMBERT,ANDGRZEGORZKARCH (1.1)–(1.2). Denote by U the self-similar solution from Theorem 1.5. Then there exists a constant C =C(α)>0 such that for all p [1,+ ] and all t>0, ∈ ∞ (1.11) u(t) U(t) p Ct−(1−p1) u0 U0 1. k − k ≤ k − k 1.3. Properties of self-similar solutions. Let us complete the result stated in Theorem 1.6 by listing main qualitative properties of the profile U(1). Theorem 1.7. (Qualitative properties of the self-similar profile) The self-similar solution from Theorem 1.5 enjoys the following properties: p1. (Regularity) The function U(1)=U(,1) is Lipschitz-continuous. · p2. (Monotonicity and limits) U(1) is increasing and satisfies lim U(x,1)=u . ± x→±∞ p3. (Symmetry) For all y R, we have ∈ u +u − + U(c+y,1)=2c U(c y,1) where c . − − ≡ 2 p4. (Convex/concave) U(1) is convex (resp. concave) on ( ,c] (resp. on −∞ [c,+ )). ∞ p5. (Decay at infinity) We have u u U (x,1) +− − x−2 as x + . x ∼ 2π2 | | | |→ ∞ Actually, the profile U(1) is expected to be C∞ or analytic, due to recent regu- b larity results [16, 7, 18] for the critical fractal Burgers equation with α=1. It was shown that the solution is smooth whenever u is either periodic or from L2(R) or 0 from a critical Besov space. Unfortunately, we do not know if those results can be adapted to any initial condition from L∞(R). Property p3 implies that U(x(t),t) is a constant equal to c along the character- istic x(t)=ct, with the symmetry U(ct+y,t)=2c U(ct y,t) − − for all t > 0 and y R. Thus, the real number c can be interpreted as a mean ∈ celerity of the profile U(t), which is the same mean celerity as for the rarefaction wave in (1.7). In property p5, we obtain the decay at infinity which is the same as for the fundamentalsolutionsp (x,t)=t−1p xt−1,1 ofthelinearequationu +Λ1u=0, 1 1 t given by the explicit formula (cid:0) (cid:1) 2 (1.12) p (x,1)= . 1 1+4π2x2 Followingtheterminologyintroducedin[6],onemaysaythatpropertyp5expresses afarfieldasymptoticsandissomewhereinrelationwiththeresultsin[6]forfractal conservationlawswithα (1,2),wheretheDuhamelprincipleplaysacrucialrole. ∈ This principle is lessconvenientinthe criticalcaseα=1,and ourproofofp5 does not use it. Finally,ifu =0 andu u =1,propertyp2 meansthatU(1)isthe cumula- − + − − tivedistributionfunctionofsomeprobabilitylaw withdensityU (1). Propertyp3 x L ensures that is symmetrically distributed around its median c; notice that any L random variable with law has no expectation, because of property p5. Proper- L ties p4-p5 make precise that the density of decays around c with the same rate L at infinity as for the Cauchy law with density p (x,1). 1 The probability distributions of both laws around their respective medians can be compared as follows. FRACTAL BURGERS EQUATION 5 Theorem 1.8. (Comparison with the Cauchy law) Let be the probability law with density U (1), where U is the self-similar solution x L defined in Theorem 1.5, with u = 0 and u = 1. Let X (resp. Y) be a real − + random variable on some probability space (Ω, ,P) with law (resp. the Cauchy A L law (1.12) (with zero median)). Then, we have for all r >0 P(X c <r)<P(Y 0 <r) | − | | − | where c denotes the median of X. Remark 1.9. More can be said in order to compare random variables X c and − Y. Indeed, their cumulative distributionfunctions satisfy F (x)=F (x) g(x) X−c Y − where g is an explicit positive function (on the positive axis) depending the self- similar solution of (1.1) (see equation (6.26)). 1.4. Probabilistic interpretation of results for α (0,2]. To summarize, let ∈ us emphasize the probabilistic meaning of the complete asymptotic study of the fractal Burgers equation we have now in hands. We have already mentioned that the solution u of (1.1)–(1.2) supplemented with the initial datum of the form (1.4) with c = 0 and with a probability measure m on R is the cumulative distribution functionforeveryt 0. This familyofprobabilitiesdefinedby problem(1.1)-(1.2) ≥ behaves asymptotically when t + as → ∞ the uniform distribution on the interval [0,t] if 1 < α 2 (see the result • ≤ from [14] recalled in inequality (1.6) above); the family of laws constructed in Theorem 1.5 if α=1 (see Theo- t t≥0 • {L } rem 1.6); the symmetric α-stable laws p (t) if 0 < α < 1 (cf. Theorem 1.3 and α • Remark 1.4). 1.5. Organization of the article. The remainder of this paper is organized as follows. In the next section, we recall the notion of entropy solutions to (1.1)- (1.2) with α (0,1]. Results on the regularized equation (i.e. equation (1.1) ∈ with an additional term εu on the left-hand-side) are gathered in Section 3. xx − The convergence of solutions as ε 0 to the regularized problem is discussed in → Section 4. The main asymptotic results for (1.1)-(1.2) are proved in Section 5 by passageto the limit as ε goes to zero. Section 6 is devotedto the qualitative study ofthe self-similarprofileforα=1. Forthe reader’sconvenience,sketchesofproofs of a key estimate from [14] and Theorem4.1 are givenin appendices; the technical lemmata are also gathered in appendices. 2. Entropy solutions for 0<α 1 ≤ 2.1. Lévy-Khintchine’s representation of Λα. It is well-known that the operator Λα = ( ∂2/∂x2)α/2 for α (0,2) has an integral representation: for every Schwartz fun−ction ϕ (R) and∈each r >0, we have ∈S (2.1) Λαϕ=Λ(α)ϕ+Λ(0)ϕ, r r where the integro-differential operators Λ(α) and Λ(0) are defined by r r ϕ(x+z) ϕ(x) ϕ (x)z (2.2) Λ(α)ϕ(x) G − − x dz, r ≡ − α z 1+α Z|z|≤r | | ϕ(x+z) ϕ(x) (2.3) Λ(0)ϕ(x) G − dz, r ≡ − α z 1+α Z|z|>r | | αΓ(1+α) where G 2 > 0 and Γ is Euler’s function. On the basis of this α ≡ 2π21+αΓ(1−α) 2 formula, we can extend the domain of definition of Λα and consider Λ(0) and Λ(α) r r 6 NATHAELALIBAUD,CYRILIMBERT,ANDGRZEGORZKARCH as the operators Λ(0) :C (R) C (R) and Λ(α) :C2(R) C (R); r b → b r b → b hence, Λα :C2(R) C (R). b → b Let us recall some properties on these operators. First, the so-called Kato inequality can be generalized to Λα for each α (0,2]: let η C2(R) be convex and ϕ C2(R), then ∈ ∈ ∈ b (2.4) Λαη(u) η′(u)Λαu. ≤ Note that for α=2 we have (η(u)) = η′′(u)u2 η′(u)u η′(u)u since η′′ 0. − xx − x− xx ≤− xx ≥ Ifα (0,2),inequality(2.4)isthedirectconsequenceoftheintegralrepresentation ∈ (2.1)–(2.3) and of the following inequalities (2.5) Λ(0)η(u) η′(u)Λ(0)u and Λ(α)η(u) η′(u)Λ(α)u, r ≤ r r ≤ r resulting from the convexity of the function η. Finally, these operators satisfy the integration by parts formula: for all u C2(R) and ϕ (R), we have ∈ b ∈D (2.6) ϕΛudx= uΛϕdx, R R Z Z where Λ Λ(0),Λ(α),Λα for every α (0,2] and all r > 0. Notice that Λϕ r r ∈ { } ∈ ∈ L1(R), since it is obvious from (2.2)-(2.3) that Λ(α) :W2,1(R) L1(R) and Λ(0) : r r L1(R) L1(R). → → Detailedproofsofallthesepropertiesarebasedontherepresentation(2.1)–(2.3) and are written e.g. in [1]. 2.2. Existence and uniqueness of entropy solutions. Itwasshownin[2](see also[16])thatsolutionsoftheinitialvalue problemforthe fractalconservationlaw (2.7) u +Λαu+(f(u)) =0, x R, t>0, t x ∈ (2.8) u(0,x)=u (x), 0 where f : R R is locally Lipschitz-continuous, can become discontinuous in → finite time if 0 < α < 1. Hence, in order to deal with discontinuous solutions, the notion of entropy solution in the sense of Kruzhkov was extended in [1] to fractal conservation laws (2.7)–(2.8) (see also [15] for the recent generalization to Lévymixedhyperbolic/parabolicequations). Here,thecrucialroleisplayedbythe Lévy-Khintchine’s representation (2.1)–(2.3) of the operator Λα. Definition 2.1. Let 0<α 1 andu L∞(R). A function u L∞(R (0,+ )) 0 isan entropysolutionto(2.7≤)–(2.8)iffo∈rallϕ (R [0,+ ))∈,ϕ 0,×η C2∞(R) convex, φ:R R such that φ′ =η′f′, and r >∈0D, we×have ∞ ≥ ∈ → +∞ η(u)ϕ +φ(u)ϕ η(u)Λ(α)ϕ ϕη′(u)Λ(0)u dxdt ZRZ0 (cid:16) t x− r − r (cid:17) + η(u (x))ϕ(x,0)dx 0. 0 R ≥ Z Note that, due to formula (2.3), the quantity Λ(0)u in the above inequality is r well-defined for any bounded function u. The notion of entropy solutions allows us to solve the fractal Burgers equation for the range of exponent α (0,1]. ∈ FRACTAL BURGERS EQUATION 7 Theorem 2.2 ([1]). Assume that 0 < α 1 and u L∞(R). There exists 0 ≤ ∈ a unique entropy solution u to problem (2.7)–(2.8). This solution u belongs to C([0,+ );L1 (R)) and satisfies u(0)=u . Moreover, we have the following max- ∞ loc 0 imum principle: essinfu u esssupu . 0 0 ≤ ≤ If α (1,2], all solutions to (2.7)–(2.8) with bounded initial conditions are ∈ smooth and global-in-time (see [8, 16, 17]). On the other hand, the occurrence of discontinuities in finite time of entropy solutions to (2.7)–(2.8) with α = 1 seems to be unclear. As mentioned in the introduction, regularity results have recently been obtained[16, 7, 18]for a largeclass ofinitial conditions which, unfortunately, does not include general L∞-initial data. Nevertheless, Theorem 2.2 provides the existenceandtheuniquenessofaglobal-in-timeentropysolutionevenforthecritical case α=1. 3. Regularized problem In this section, we gather properties of solutions to the Cauchy problem for the regularized fractal Burgers equation with ε>0 (3.1) uε+Λαuε εuε +uεuε =0, x R, t>0, t − xx x ∈ (3.2) uε(x,0)=u (x). 0 Our purpose is to derive asymptotic stability estimates of a solution uε = uε(x,t) (uniforminε)thatwillbevalidfor(1.1)–(1.2)afterpassingtothelimitε 0. Most → of the results of this section are based on a key estimate from [14]; unfortunately, this estimate is not explicitely stated as a lemma in [14]. Hence, for the sake of completeness, we have recalled this key estimate in Lemma A.1 in Appendix A as well as the main lines of its proof. Below, we will use the following integral formulation of the initial value problem (3.1)-(3.2) t (3.3) uε(t)=Sε(t)u Sε(t τ)uε(τ)uε(τ)dτ, α 0− α − x Z0 where Sε(t) is the semi-group generated by Λα+ε∂2. { α }t>0 − x If, for each α (0,2], the function p denotes the fundamental solution of the α ∈ linear equation u +Λαu=0, then t (3.4) Sε(t)u =p (t) p (εt) u . α 0 α ∗ 2 ∗ 0 It is well-known that p = p (x,t) can be represented via the Fourier transform α α (w.r.t. the x-variable) p (ξ,t)=e−t|ξ|α. In particular, α (3.5) pα(x,t)=t−α1Pα(xt−α1), b whereP istheinverseFouriertransformofe−|ξ|α. Foreveryα (0,2]thefunction α ∈ P is smooth, non-negative, P (y)dy = 1, and satisfies the estimates (optimal α R α for α=2) 6 R (3.6) 0<P (x) C(1+ x)−(α+1) and ∂ P C(1+ x)−(α+2) α x α ≤ | | | |≤ | | for a constant C and all x R. ∈ One can see that problem (3.1)–(3.2) admits a unique global-in-time smooth solution. Theorem 3.1 ([8]). Let α (0, 2], ε>0 and u L∞(R). There exists a unique 0 ∈ ∈ solution uε to problem (3.1)–(3.2) in the following sense: uε C (R (0,+ )) C∞(R (a,+ )) for all a>0, • uε ∈satisbfies×equatio∞n (3∩.1)bon R× (0,+∞ ), • lim uε(t)=u in L∞(R) wea×k- and∞in Lp (R) for all p [1,+ ). • t→0 0 ∗ loc ∈ ∞ 8 NATHAELALIBAUD,CYRILIMBERT,ANDGRZEGORZKARCH Moreover, the following maximum principle holds true: (3.7) essinfu uε esssupu . 0 0 ≤ ≤ Proof. Here,theresultsfrom[8]canbeeasilymodifiedinordertogettheexistence and the regularity of solutions to (3.1)–(3.2) with ε>0. (cid:3) Herearesomeelementaryproperties(comparisonprinciple,L1-contractionprin- ciple and non-increase of the BV-semi-norm) of fractal conservation laws that will be needed. Proposition 3.2 ([8]). Let ε > 0 and uε and uε be solutions to (3.1)–(3.2) with respective initial data u and u in L∞(R). Then: 0 0 if u u then uε uε, f 0 0 • if u0 ≤u0 L1(R)≤thefn uε uε L∞(0,+∞,L1) u0 u0 1, •• if u0−∈BfV∈(R) then kfuεx(kt)kL−∞(0,+k∞,L1) ≤|u0|B≤Vk − k where L∞(0,+f∞,L1) and BV denotefrespectively thenorminfL∞(0,+ ,L1(R)) and thke·skemi-norm in BV(|R·|). ∞ Sketch of the proof. As explained in [8, Remarks 1.2 & 6.2], these properties are immediateconsequencesofthesplittingmethoddeveloppedin[8]andthefactsthat boththehyperbolicequationu +uu =0andthefractalequationu +Λαu εu = t x t xx 0 satisfy these properties. − (cid:3) The next proposition provides an estimate on the gradient of uε. Proposition 3.3. Let 0<α 1and u L∞(R) be non-decreasing. For each ε> 0 ≤ ∈ 0, denote by uε the solution to (3.1)–(3.2). Then: uε(x,t) 0 for all x R and t>0, • x ≥ ∈ there exists a constant C = C(α) > 0 such that for all ε > 0, p [1,+ ] • ∈ ∞ and t>0, (3.8) kuεx(t)kp ≤Ct−α1(1−p1)|u0|BV. Proof. Foranyfixedrealh,thefunctionuε( +h, )isthesolutionto(3.1)–(3.2)with · · the initial datum u ( +h). Consequently,for non-decreasingu andfor h>0,the 0 0 · inequalityu ( +h) u ()andthecomparisonprincipleimplyuε( +h, ) uε(, ) 0 0 · ≥ · · · ≥ · · which gives uε 0. x ≥ ToshowthedecayoftheLp-norm,oneslightlymodifiestheargumentsfrom[14, Proof of Lemma 3.1]. One shall use Lemma A.1 with v uε. It is clear that v ≡ x satisfies the required regularity: for all a>0 v C∞(R (a,+ )) L∞(0,+ ,L1(R)), ∈ b × ∞ ∩ ∞ thanks to Proposition 3.2 ensuring that kvkL∞(0,+∞,L1) =kuεxkL∞(0,+∞,L1) ≤|u0|BV. Itthusreststoshowthatv satisfies(A.1). Byinterpolationofthe inequalityabove and the L∞-bound on v from Theorem 3.1, one sees that for all p [1,+ ] and ∈ ∞ all t>0, v(t) Lp(R) and v (t),Λαv(t),v (t),v (t) L∞(R). t x xx ∈ ∈ Hence, for p [2,+ ), one can multiply the equation for v ∈ ∞ v +Λαv εv +(uεuε) =0, t − xx x x by vp−1 to obtain after integration: p 1 (3.9) v vp−1dx+ vp−1Λαvdx ε v vp−1dx+ − vp+1dx=0; t xx R R − R p R Z Z Z Z FRACTAL BURGERS EQUATION 9 here one has used that lim v(x,t) = 0 (since v(t) C∞(R) L1(R)) to |x|→+∞ ∈ b ∩ droptheboundarytermsprovidingfromintegrationbyparts. Integratingagainby parts, one sees that ε v Φ(v)dx=ε v2Φ′(v)dx 0 − R xx R x ≥ Z Z forallnon-decreasingfunctionΦ C1(R)withΦ(0)=0;ChoosingΦ(v)= v p−2v, ∈ | | one gets (3.10) ε v v p−2vdx 0. xx − R | | ≥ Z We deduce from (3.9), (3.10) and the non-negativity of v that v v p−2vdx+ v p−2vΛαvdx 0 t R | | R| | ≤ Z Z for all p [2,+ ) and t > 0. This is exactly the required differential inequation in (A.1).∈Lemm∞a A.1 thus completes the proof. (cid:3) We can now give asymptotic stability estimates uniform in ε. Theorem 3.4. Let α (0,2]. Consider two initial data u and u in L∞(R) such 0 0 that u is non-decreas∈ing and u u L1(R). For each ε > 0, denote by uε 0 0 0 − ∈ and uε the corresponding solutions to (3.1)–(3.2). Then, thereeexists a constant C =Ce(α)>0 such for all ε>0, p e[1,+ ] and t>0 ∈ ∞ (3.11f) uε(t) uε(t) p Ct−α1(1−p1) u0 u0 1. k − k ≤ k − k Proof. The proof follows the arguments from [14, Proof of Lemma 3.1] by skip- ping the additional term profviding from εuε . Thateis to say, one uses again − xx Lemma A.1 with v = uε uε. First, the L1-contraction principle (see Proposi- − tion 3.2) ensures that v satisfies the required regularity with uε fuε L∞(0,+∞,L1) u0 u0 1. k − k ≤k − k In particular,once againthe interpolationofthe L1- and L∞-norms implies thatv is Lp in space for all time andfall p [1,+ ]. Secondf, one takes p [2,+ ) (so ∈ ∞ ∈ ∞ that all the integrands below are integrable) and one multiplies the difference of the equations satisfied by uε and uε by v p−2v. One gets after integration: | | (3.12) v v p−2vdx+ v p−f2vΛαvdx t R | | R| | Z Z 1 ε v v p−2vdx+ v2+2vuε v p−2vdx=0. − R xx| | 2 R x| | Z Z (cid:0) (cid:1) Thelasttermoftheleft-handsideofthisequalityisnon-negatfive,sinceintegrations by parts give v2+2vuε v p−2vdx R x| | Z (cid:0) (cid:1) = 2v vfpdx+ 2uεv v p−2vdx+ 2uε v pdx, R x| | R x| | R x| | Z Z Z 1 =2 1 2uε vfpdx 0 f − p R x| | ≥ (cid:18) (cid:19)Z (onceagaintheboundarytermscfanbeskippedsincev vanishesforlargex). More- overthe third termof (3.12)is alsonon-negativeby (3.10). Oneeasily deducesthe desired inequality (A.1) and completes the proof by Lemma A.1. (cid:3) 10 NATHAELALIBAUD,CYRILIMBERT,ANDGRZEGORZKARCH Theorem 3.5. Let 0<α<1 and u L∞(R) be non-decreasing. For each ε>0, 0 ∈ denote by uε the solution to (3.1)–(3.2). Then, there exists C = C(α) > 0 such that for all ε>0, p [1,+ ] and t>0 ∈ ∞ kuε(t)−Sαε(t)u0kp ≤Cku0k∞|u0|BVt1−α1(1−p1) (where Sε(t) is generated by Λα+ε∂2). { α }t>0 − x Proof. Using the integral equation (3.3) we immediately obtain t (3.13) uε(t) Sε(t)u Sε(t τ)uε(τ)uε(τ) dτ. k − α 0kp ≤ k α − x kp Z0 Now,weestimatetheintegralintheright-handsideof(3.13)usingtheLp-decayof the semi-groupSε(t) as well as inequalities (3.7) and (3.8). Indeed, it follows from α (3.5)-(3.6) that p2(εt) 1 =1 and pα(t) r =t−α1(1−r1) pα(1) r k k k k k k for every r [1,+ ]. Hence, by the Young inequality for the convolution and ∈ ∞ inequalities (3.7), (3.8), we obtain Sε(t τ)uε(τ)uε(τ) k α − x kp p (t τ) (uε(τ)uε(τ)) , ≤k α − ∗ x kp (3.14) ≤C(t−τ)−α1(q1−p1)kuε(τ)k∞kuεx(τ)kq, C(t τ)−α1(q1−p1) u0 ∞ u0 BVτ−α1(1−q1), ≤ − k k | | for all 1 q p + , t > 0, τ (0,t), where the constant C only depends ≤ ≤ ≤ ∞ ∈ on max p (1) and the constant in (3.8). r∈[1,+∞] α r k k Next, we decompose the integral on the right-hand side of (3.13) as follows t t/2 t ...dτ = ...dτ+ ...dτ andweboundbothintegrandsbyusinginequality 0 0 t/2 (3.14) either with q =1 or with q =p. This leads to the following inequality R R R uε(t) Sε(t)u k − α 0kp t/2 t (3.15) ≤Cku0k∞|u0|BV Z0 (t−τ)−α1(1−p1) dτ +Zt/2τ−α1(1−p1) dτ!, 2β 1 =C u u − tβ, k 0k∞| 0|BV β2β−1 where β 1 1 1 1 . It is readily seen that β R 2β−1 is positive and ≡ − α − p ∈ → β2β−1 continuous and th(cid:16)at p (cid:17)[1,+ ) 1 1 1 1 is bounded. This completes the ∈ ∞ → − α − p proof of Theorem 3.5. (cid:16) (cid:17) (cid:3) 4. Entropy solution: parabolic approximation and self-similarity Inthissection,westatetheresultontheconvergence,asε 0,ofsolutionsuεof → (3.1)–(3.2)towardtheentropysolutionuof(1.1)–(1.2). WealsoproveTheorem1.5 about self-similar entropy solutions in the case α=1. Together with the general fractal conservation law (2.7)–(2.8), we study the associated regularized problem (4.1) uε+Λαuε εuε +(f(uε)) =0, x R, t>0, t − xx x ∈ (4.2) uε(x,0)=u (x) 0 where f C∞(R). Hence, by results of [8] (see also Theorem 3.1), problem (4.1)- ∈ (4.2) admits a unique, global-in-time, smooth solution uε.