# A UNIQUENESS RESULT FOR THE CONTINUITY EQUATION IN TWO DIMENSIONS. Dedicated to Constantine Dafermos on the occasion of his 70 th birthday

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1 A UNIQUENESS RESULT FOR THE CONTINUITY EQUATION IN TWO DIMENSIONS GIOVANNI ALBERTI, STEFANO BIANCHINI, AND GIANLUCA CRIPPA Dedicated to Constantine Dafermos on the occasion of his 7 th birthday Abstract. We characterize the autonomous, divergence-free vector fields b on the plane such that the Cauchy problem for the continuity equation tu+div bu) = admits a unique bounded solution in the weak sense) for every bounded initial datum; the characterization is given in terms of a property of Sard type for the potential f associated to b. As a corollary we obtain uniqueness under the assumption that the curl of b is a measure. This result can be extended to certain non-autonomous vector fields b with bounded divergence. Keywords: continuity equation, transport equation, uniqueness of weak solutions, weak Sard property, disintegration of measures, coarea formula. MSC 21): 35F1, 35L3, 28A5 1. Introduction In this paper we consider the continuity equation t u + div bu) =, 1.1) where b : [, T ) R d R d is a bounded, time-dependent vector field on R d with T possibly equal to + ). We recall that 1.1) is the equation satisfied by the density u of a continuous distribution of particles moving according to the velocity field b, that is, ut, x) is the number of particles per unit volume at time t and position x, and the trajectory of each particle satisfies the ordinary differential equation ẋ = bt, x). 1.2) Through this paper, the vector field b will not be any more regular than bounded and with bounded distributional) divergence. 1 Accordingly, solutions of the Cauchy problem for 1.1) are intended in the weak or distributional) sense: a function u : [, T ) R d R solves 1.1) with the initial condition u, ) = u if T t ϕ + b ϕ ) u dx dt + ϕ, ) u dx = R d R d ϕ Cc [, T ) R d ) 1.3). 1 In particular b is far from being Lipschitz in the space variable, which is the minimal regularity required to apply the method of characteristics to 1.1). 1

2 2 G. ALBERTI, S. BIANCHINI, AND G. CRIPPA Concerning the existence of solutions, it can be shown that if the divergence of b is bounded from below then, for every bounded initial datum u, a solution of the Cauchy problem for 1.1) exists for all times in the future and is bounded for finite times. 2 We focus therefore on the problem of uniqueness, and precisely on the following question: Under which assumption on b does the Cauchy problem for 1.1) admit a unique bounded solution for every bounded initial datum u? In the fundamental paper [13], R.J. DiPerna and P.-L. Lions have proved this uniqueness result under the assumption that b is in the Sobolev class W 1,1 locally in space), and later L. Ambrosio [4] generalized this result to vector fields b of class BV locally in space). 3 4 When b is a divergence-free autonomous vector field on the plane d = 2), uniqueness has been proved replacing the BV or Sobolev regularity of b by various assumptions on the direction of b [7, 17], see also [1, 8] for the bounded divergence case). In the opposite direction, N. Depauw [11] gave an example of time-dependent, divergence-free, bounded vector field b on the plane for which there is no uniqueness see also [1, 9]). We can now turn to the main result of this paper. Let b : R 2 R 2 be a bounded, divergence-free, autonomous vector field on the plane; then b admits a Lipschitz potential f : R 2 R, that is, b = f, where := 2, 1 ). In Theorem 4.7 we prove that the Cauchy problem for 1.1) admits a unique bounded solution for every bounded initial datum if and only if the potential f satisfies what we call weak Sard property. We recall that a differentiable function f : R 2 R satisfies the strong) Sard property if the image according to f of the critical set S the set of all x where fx) = ) is negligible, that is, L 1 fs)) =. 5 The weak Sard property is a measure theoretic version of the Sard property; the precise formulation requires a few additional definitions, and will be given in Conditions on f and therefore on b) that imply the weak Sard property are discussed in detail in 2 The bound div b m implies that the Jacobian determinant J of the flux associated to 1.2) satisfies J e mt, and therefore, according to the mechanical interpretation given above, the particle density u should satisfy ut, ) e mt u. This estimate can indeed be proved for smooth b, and then used to obtain solutions for non-smooth b by approximation. 3 Both uniqueness results, besides being deeply interesting per se, had relevant applications to other problems, among which we mention the Boltzmann [14] and Vlasov-Poisson equations [15], and the Keyfitz-Kranzer system [5]. 4 The mechanical interpretation given above suggests that uniqueness for 1.1) should be connected to the uniqueness of solutions of 1.2) with initial condition x) = x for a generic initial point x, but not necessarily for every x. This relation can be made rigorous in terms of uniqueness of the regular Lagrangian flow associated to 1.2) see [4]), and explains why the regularity assumption on b in the theorems of DiPerna-Lions and Ambrosio are distinctly weaker than those typically required for the uniqueness for 1.2) for every initial point. 5 Sard theorem states that functions of class C 2 have the Sard property see [2], or [18, Chapter 3, Theorem 1.3]); while this is not always the case for functions of class C 1.

3 UNIQUENESS FOR THE CONTINUITY EQUATION IN 2D ; here we just recall that this property holds whenever b is different than a.e. 2.15i)), or is of class BV ; 6 indeed it suffices that the curl of b is a measure 2.15vi)). We stress once again that while the results in the literature give only sufficient conditions for uniqueness, Theorem 4.7 gives a necessary and sufficient condition. Accordingly, the examples of functions without the weak Sard property constructed in the companion paper [2] immediately yield examples of divergence-free autonomous vector fields in the plane for which there is no uniqueness Corollary 4.8). Note that the weak Sard property, like the Sard property, is completely nonlinear in character, and is satisfied by a generic divergence-free vector field in the category sense, see 2.15i)). We conclude this introduction with an outline of the proof of Theorem 4.7. An essential ingredient is the description of generic) level sets of Lipschitz functions on the plane given in [2]; we refer to this paper for a detailed discussion of all related measure-theoretic and real analytic issues, and in particular for counterexamples. The first step is a dimension-reduction argument Theorem 3.1) that can be summarized as follows: a bounded function u solves the continuity equation 1.1) if and only if it solves a suitable one-dimensional variant of the same equation on every nontrivial connected component of the level set E y := f 1 y) for almost every y. 7 In other words, the problem of uniqueness for equation 1.1) is reduced to the uniqueness for the corresponding equations on the nontrivial connected components of a generic) level set E y. The proof of Theorem 3.1 relies on the notion of disintegration of a measure with respect to the level sets of a function, and on the coarea formula for Lipschitz functions, and holds in every space dimension. The rest of the proof of Theorem 4.7 is strictly two-dimensional. The next key fact is that for a.e. y every connected component C of E y is a rectifiable simple curve as shown in [2]), and more precisely it is possible to choose an interval I and a Lipschitz parametrization γ : I C so that, under the change of variable x = γs), the equation on C becomes t u1 + λ)) + s u =, 1.4) where λ is a suitable singular measure on I. 8 Note that, due to the particular choice of γ, the vector field b no longer appears in the equation. 6 Thus we recover the uniqueness results in [7, 17] in full generality, and those in [13, 4] limited to our particular class of b. 7 The notion of solving the equation on a subset of) a level set is properly defined in 3.6; a connected component is nontrivial if it contains more than one point. Note that this statement is a close relative of the method of characteristics: indeed the vector field b, being orthogonal to f, is tangent to the level set E y at a.e. point with respect to the length measure) and for a.e. y. Hence these level sets are the proper replacement for characteristic curves. 8 The precise definition of λ and the weak formulation of 1.4) are given in Lemma 4.4.

4 4 G. ALBERTI, S. BIANCHINI, AND G. CRIPPA Next we note that the Cauchy problem for 1.4) admits a unique bounded solution for every bounded initial datum if and only if the measure λ is trivial. Indeed, if λ = equation 1.4) reduces to t u + s u =, for which uniqueness is well-known. To understand why the converse holds, assume that I is the real line and λ is a Dirac mass at ; if u represents the density of a distribution of particles, then equation 1.4) means that each particle moves at constant speed 1 from left to right, except when it reaches the point, where it may stop for any given amount of time. Therefore, if u is an initial datum with support contained in, ), a solution of 1.4) with initial condition u, s) = u s) is the function u : [, + ) R R defined by { u s t) if s ut, s) := if s = no particle stops at ), while another solution is u s t) if s < ũt, s) := u s) ds if s = t if s > all particles stop at ). We conclude that uniqueness for the 1.1) holds if and only if λ = for every nontrivial connected component C of a.e. level set E y, which in turn is equivalent to the weak Sard property Lemma 2.14). Without entering into details, we just mention that the connection between the weak Sard property, which is related to the critical set S, and the measures λ lies in the fact that these measures are given by the disintegration with respect to the level sets of f) of the restriction of Lebesgue measure to S E, where E is the union of all nontrivial connected components of all levels sets of f. As one might expect, Theorem 4.7 can be extended in many different ways. In Theorem 5.2 we consider the case of a vector field on the plane of the form b = a f, where f is a Lipschitz function as above and a a scalar function depending also on time. Further extensions are mentioned in 6.1. It is indeed conceivable to apply the strategy outlined above even in higher space dimension see 6.2). This paper is organized as follows: Section 2 contains the measure-theoretic and real analysis results used in the rest of the paper: the disintegration of a measure with respect to the level sets of a function, the coarea formula for Lipschitz functions, the description of connected components of the level sets of a Lipschitz function on the plane, the definition of weak Sard property and some related results. Section 3 is devoted to the dimension-reduction argument Theorem 3.1); Section 4 and Section 5 contain the proof of the main result Theorem 4.7) and of the generalization mentioned above Theorem 5.2). Section 6 contains some additional remarks, and finally the Appendix contains a measurable selection lemma used in the proof of Theorem 3.1.

5 UNIQUENESS FOR THE CONTINUITY EQUATION IN 2D 5 Acknowledgements. This work has been partially supported by the italian Ministry of Education, University and Research MIUR) through the 28 PRIN Grant Trasporto ottimo di massa, disuguaglianze geometriche e funzionali e applicazioni, and by the European Research Council through the 29 Starting Grant Hyperbolic Systems of Conservation Laws: singular limits, properties of solutions and control problems. 2. Measure theoretic preliminaries We begin this section by recalling the notion of disintegration of a measure, focusing in particular on the disintegration of the Lebesgue measure with respect to Lipschitz maps 2.7 and Lemma 2.8). We then restrict our attention to Lipschitz functions on the plane Lemma 2.11); in this setting we introduce the notion of weak Sard property 2.13), prove a characterization of this property in terms of disintegration of the Lebesgue measure Lemma 2.14), and list some conditions which imply it 2.15). We conclude with a change-of-variable formula used in the proof of Lemma Basic notation. In this paper we follow the standard notation of measure theory. Sets and functions are always assumed to be Borel measurable, and measures are always defined on the Borel σ-algebra of some locally compact, separable metric space X. Unless otherwise stated, a measure on X is assumed to be positive and locally finite. We write λ µ when the measure λ is absolutely continuous with respect to the measure µ, and λ µ when λ is singular with respect to µ. 9 We say that the measure µ is supported on the set E if µx \ E) =. 1 Given a function ρ : X [, + ], we denote by ρ µ the measure on X defined by ρ µ)e) := ρ dµ for every Borel set E contained in X. Given a set A contained in X, we write 1 A : X {, 1} for the characteristic function of A, and therefore 1 A µ is the restriction of µ to A. Given a metric space Y and a map f : X Y, the push-forward of µ according to f is the measure f # µ on Y defined by E [f # µ]e) := µf 1 E)) for every Borel set E contained in Y. Thus ϕ df # µ) = ϕ f dµ Y 9 Recall that λ µ if λe) = whenever µe) =, or equivalently in the context of this paper) when λ can be written as λ = ρ µ for a suitable density ρ; λ is singular with respect to µ if it is supported on a µ-negligible set, or equivalently if λ and µ are supported on disjoint sets. 1 Note that E does not need to be closed, and does not necessarily contain the support of µ defined in the usual distributional sense). X

6 6 G. ALBERTI, S. BIANCHINI, AND G. CRIPPA for every function ϕ : Y [, + ], and therefore also for every possibly vectorvalued ϕ in L 1 f # µ). As usual, L d is the Lebesgue measure on domains in) R d while H d is the d- dimensional Hausdorff measure on every metric space the usual d-dimensional volume for d-dimensional surfaces of class C 1 in some Euclidean space. When the measure is not specified, it is assumed to be the Lebesgue measure; along this line we often write gx) dx instead of g dl d Borel family of measures. Let Y be a metric space and {µ y : y Y } a family of measures on a locally compact, separable metric space X. We say that such family, or more precisely the map y µ y, is Borel if the function y ϕ dµ y 2.1) X is Borel for every test function ϕ : X R which is continuous and compactly supported. 11 It then follows that the function in 2.1) is Borel measurable also for every Borel test function ϕ : X Y [, + ] which is either positive or such that ϕ, y) belongs to L 1 µ y ) for every y Disintegration of measures. Let X and Y be locally compact, separable metric spaces, µ a measure on X, f : X Y a Borel map, and ν a measure on Y such that f # µ ν. Then there exists a Borel family {µ y : y Y } of measures on X such that i) µ y is supported on the level set E y := f 1 y) for every y Y ; ii) µ can be decomposed as µ = Y µ y dνy), which means that µa) = µ y A) dνy) 2.2) Y for every Borel set A contained in X. Any family {µ y } satisfying i) and ii) is called a disintegration of µ with respect to f and ν. The disintegration is unique in the following sense: for any other disintegration { µ y } there holds µ y = µ y for ν-a.e. y Y. The existence and uniqueness of the disintegration is a standard result in case X is compact, µ is finite, and ν := f # µ see for instance [12, Chapter III, 7 and 71]). The statement given above can be easily derived from this particular case Properties of disintegration. We list here some general properties of the disintegration that will be used often tacitly) through the paper. i) Formula 2.2) implies that ϕ dµ = X Y [ E y ϕ dµ y ] dνy) 2.3) 11 This is equivalent to the notion of Borel measurability for maps with values in the space of locally finite measures on X, when the latter has been endowed with the weak* topology as dual of the space of continuous functions with compact support.

7 UNIQUENESS FOR THE CONTINUITY EQUATION IN 2D 7 for every Borel function ϕ : X [, + ]. If ϕ is a real- or vector-valued function in L 1 µ), by applying identity 2.3) with ϕ in place of ϕ we obtain that ϕ belongs to L 1 µ y ) for ν-a.e. y, and the function y ϕ dµ y is in L 1 ν). Thus both sides of 2.3) make sense, and the equality holds even for such ϕ. ii) If A is a set in X with µa) < +, formula 2.2) implies that µ y A) is finite for ν-a.e. y. In particular, if µ is finite then µ y is finite for ν-a.e. y. iii) Formula 2.2) shows that a set A in X is µ-negligible if and only if it is µ y -negligible for ν-a.e. y. We infer the following: a) µf 1 N)) = for every set N such that νn) = ; b) µ is supported on a set F if and only if µ y is supported on F E y for ν-a.e. y; c) if P x) is a proposition that depends on the variable x X, then P x) holds for µ-a.e. x if and only if it holds for µ y -a.e. x and ν-a.e. y. iv) If µ is a measure on the metric space X, we write µ µ for the product measure on X X and we consider the function f : X X Y defined by fx, x ) = fx). Then, the disintegration of µ µ with respect to f and ν is {µ y µ } Lipschitz maps. For the rest of this section d, k are integers such that k < d, and f : R d R d k is a Lipschitz map. For every y R d k we write E y for the level set f 1 y). By Rademacher theorem f is differentiable at almost every point in R d, and in all such points we define the Jacobian J := [det f t f)] 1/2. We call critical set of f the set S of all points in R d where either f is not differentiable or J =, that is, the rank of f is strictly less than d k. A connected component C of E y is a connected subset of E y which is maximal with respect to inclusion. Notice that every such connected component is a closed set. For every y R d k we denote by C y the family of all connected components C of E y such that H k C) > ; we then define Ey as the union of all C in C y, and E as the union of all Ey with y R d k. Note that the sets E and Ey are Borel see [2, Proposition 6.1]) Lemma. In the context of the previous paragraph, the following statements hold: i) if µ := J L d then f # µ is absolutely continuous with respect to L d k and its disintegration with respect to f and L d k is µ y := 1 Ey H k ; ii) for a.e. y R d k there holds H k E y S) =, and for every bounded set A in R d we have H k A E y ) < + and 1 A E y J dh k < + ;

8 8 G. ALBERTI, S. BIANCHINI, AND G. CRIPPA iii) if µ := 1 R d \S L d then f # µ is absolutely continuous with respect to L d k, and its disintegration with respect to f and L d k is given by µ y := 1/J) 1 Ey H k ; iv) the family C y is countable and H k E y \ E y) = for L d k -a.e. y; v) L d R d \ E S)) =. Proof. Statement i) is just a reformulation of the coarea formula for Lipschitz maps, see for instance [16, ], [21, 1], or [19, Corollary 5.2.6]. Using statement i) and formula 2.2) we obtain = µs) = µ y S) dy = H k E y S) dy, R d k R d k and therefore H k E y S) = for a.e. y. Similarly, + > µa) = µ y A) dy = H k A E y ) dy R d k R d k implies that H k E y A) is finite for a.e. y. The last part of the statement follows by applying 2.3) with ϕ := 1/J) 1 A\S : [ ] + > L d A \ S) = ϕ dµ = ϕ dµ y dy R d k E y [ ] 1 = R d k A E y J dh k dy. Similarly, statement iii) follows by applying 2.3) with ϕ := 1/J) 1 A\S and A an arbitrary Borel set in R d. Statement iv) is proved in [2, Theorem 2.5iii)]. Statement v) follows from statements iii) and iv): L d R d \ E S)) = µr d \ E ) = µ y R d \ E ) dy R d k [ ] 1 = J dh k dy =. R d k E y\e y 2.7. Disintegration of Lebesgue measure. We take f as above and choose a measure ν s on R d k so that ν s is singular with respect to L d k and f # L d ν with ν := L d k + ν s. For the rest of this section we denote by {µ y } the disintegration of L d with respect to f and ν Lemma. In the context of the previous paragraph, the following statements hold: i) for ν s -a.e. y the measure µ y is supported on S E y ;

9 UNIQUENESS FOR THE CONTINUITY EQUATION IN 2D 9 ii) for L d k -a.e. y the measure µ y can be decomposed as µ y = 1/J) 1 Ey\S H k + µ s y with µ s y a measure supported on E y S; moreover µ s y is singular with respect to 1/J) 1 Ey\S H k, and the latter measure agrees with 1/J) 1 Ey H k. Proof. Note that L d can be written as the sum of µ 1 := 1 S L d and µ 2 := 1 R d \S L d, and therefore its disintegration with respect to f and ν) is obtained by summing the disintegrations of µ 1 and µ 2. Now µ 1 is supported on S and then µ 1 y is supported on S E y for ν-a.e. y; on the other hand, Lemma 2.6iii) implies that µ 2 y = for ν s -a.e. y and µ 2 y = 1 Ey /J) H k for L d k -a.e. y. Putting together these facts we immediately obtain statement i) and the first part of ii). We have that µ s y is singular with respect to 1 Ey\S/J) H k because these measures are supported on the disjoint sets E y \S and E y S. The latter measure agrees with 1 Ey /J) H k because H k E y S) = Lemma 2.6ii)) Lipschitz functions on the plane. For the rest of this section we assume d = 2 and k = 1, that is, f is a Lipschitz function on R 2. Therefore J = f, S is the set of all points where either f is not differentiable or f =, and C y is the family of all nontrivial connected components of the level set E y, namely those which contain more than one point Simple curves. We denote by [, L] the space obtained by identifying the endpoints of the interval [, L], endowed with the distance dx, y) := x y L x y ). The canonical measure on [, L] is the Lebesgue measure, which we denote simply by L. We say that a set C in R d is a closed, simple curve with finite length if there exist L > and a Lipschitz bijection γ : [, L] C. We call any such γ a parametrization of C Lemma. Assume that f has compact support, and take µ y, µ s y as in Lemma 2.8. The following statements hold for L 1 -a.e. y R and every connected component C C y : i) C is a closed simple curve with finite length; ii) C admits a parametrization γ such that γ = fγ) a.e. recall that := 2, 1 )); iii) the push-forward of L according to γ is 1/J) 1 C H 1 ; iv) there exists a measure λ on [, L] such that λ is singular with respect to L, and the push-forward of λ according to γ is the restriction of µ s y to C; v) the push-forward of L + λ according to γ is the restriction of µ y to C.

10 1 G. ALBERTI, S. BIANCHINI, AND G. CRIPPA Remark. i) The assumption that f has compact support was made for the sake of simplicity, and it can be easily removed. In that case statement i) should be modified, allowing for connected component that are simple curves with end points at infinity. ii) It is always possible to choose the parametrization γ in such a way that the measure λ has no atom at, that is, λ{}) =. This assumption makes certain proofs much simpler, and will be always tacitly made in the following. Proof. By Lemma 2.8ii) we can assume that H 1 S C) = and 1 J dh 1 < ) Statement i) is proved in [2, Theorem 2.5iv)]. C The same theorem shows that C admits a parametrization ϕ : [, L ] C such that Jϕ) and ϕ = fϕ) fϕ) = fϕ) a.e. 2.5) Jϕ) Thus the parametrization in statement ii) is given by γ := ϕ σ 1 where σ : [, L ] [, L] is the function defined by σs) := s 1 Jϕs )) ds and L := L 1 Jϕs )) ds. 2.6) Using 2.4) and the fact that the push-forward of the Lebesgue measure according to ϕ is the restriction of H 1 to C note that ϕ = 1 a.e.) we obtain that L < +, which implies that the function σ is well-defined, continuous, strictly increasing, and surjective because of the choice of L. Since J m where m is the Lipschitz constant of f) we also have that σs 2 ) σs 1 ) 1 m s 2 s 1 ) whenever s 1 < s 2, which implies that σ 1 is a Lipschitz function, and therefore so is γ. Moreover σ is of class W 1,1 and σ = 1/Jγ) a.e., and since σ maps Lebesgue-) negligible sets in negligible sets, the image of the set of points where either σ is not differentiable or σ = is negligible. Thus σ 1 ) = Jγ) = fγ) a.e. This equation and 2.5) yields γ = fγ) a.e., concluding the proof of statement ii). Statement iii) follows by the fact that γ = fγ) = Jγ) a.e. To prove statement iv), let λ be the push-forward according to γ 1 of 1 C µ s y. In order to prove that λ L, note that µ s y is supported on S C, and therefore λ is supported on γ 1 S C). Moreover S C is H 1 -negligible, and therefore γ 1 S C) = σϕ 1 S C)) is L -negligible because both ϕ 1 and σ maps negligible sets in negligible sets.

11 UNIQUENESS FOR THE CONTINUITY EQUATION IN 2D 11 Statement v) follows from statements iii) and iv) The weak Sard property. Let f : R 2 R be a Lipschitz function. 12 We say that f has the weak Sard property if the push-forward according to f of the restriction of the Lebesgue measure to S E where S and E have been defined in 2.5) is singular, that is f # 1S E L 2) L ) Lemma. The following statements are equivalent: a) f satisfies the weak Sard property; b) µ s ye y) = for L 1 -a.e. y R; c) µ s yc) = for L 1 -a.e. y R and every C C y. Proof. Take ν as in 2.7 and µ s y as Lemma 2.8. Set µ := 1 S E L 2, and let { µ y } be the disintegration of µ with respect to f and ν. Thus 2.7) can be restated as µ y = for L 1 -a.e. y. On the other hand Lemma 2.8ii) implies that µ y = 1 S E µ s y for L 1 -a.e. y, and therefore µ y = if and only if = µ s ys E ) = µ s ye y) for the second equality use that µ s y is supported on E y S and E y E = E y). We have thus proved that 2.7) holds if and only if µ s ye y) = for L 1 -a.e. y, that is, statements a) and b) are equivalent. The equivalence of b) and c) is immediate because Ey is the union of all C C y, and C y is countable for a.e. y Lemma 2.6iv)) About the weak Sard property. We list here some conditions on f which imply the weak Sard property. Indeed all of them imply the slightly stronger property f # 1S L 2) L ) Since in Theorem 4.7 the function f is the potential associated to a given vector field b by the relation f = b, we will express the following conditions also in terms of b. i) The weak Sard property is implied by the condition L 2 S E ) =, which in turn is implied by f a.e., or, equivalently, b a.e. Note that the set of all vector fields b that verify the last condition is residual 13 in the Banach space X of divergence-free, bounded vector fields endowed with the L norm this is a reformulation of [2, Proposition 4.1]). In particular the set of all b whose potential f has the weak Sard property is residual in X. ii) The strong) Sard property implies the weak Sard property. Indeed the measure f # 1 S E L 2 ) is supported on the set fs), and if this set is negligible Sard property), then the measure must be singular weak Sard property). 12 The weak Sard property can be defined in the more general context of Lipschitz maps from R d into R d k, but only the case d = 2, k = 1 is relevant to this paper. 13 A set in a topological space is residual if it contains a countable intersection of open dense sets.

12 12 G. ALBERTI, S. BIANCHINI, AND G. CRIPPA iii) A function f on R 2 is said to have the Lusin property of order k if there exists a sequence of functions f n of class C k on R 2 and Borel sets A n such that f = f n on A n, and the sets A n cover almost all of R A Lipschitz function with the Lusin property of order 2 satisfies 2.8) and therefore has the weak Sard property. Let indeed S n be the critical set of f n. Then S A n is contained in S n A n up to a negligible set, and S is contained in the union of S n A n up to a negligible set. Hence the measure 1 S E L 2 is supported on the union of S n A n, and its push-forward according to f is supported on the union of all fs n A n ) = f n S n A n ), which is negligible because it is contained in the union of the negligible sets f n S n ). iv) Following [23, 3.5.5] we say that f admits an L 1 -Taylor expansion of order 2 at x if fx + h) = P x h) + R x h) for every h, where P x is a polynomial of degree 2, and the remainder R x satisfies R x h) dh = or 2 ) Br) the barred integral stands for the average over the ball Br) with center and radius r). If f admits an L 1 -Taylor expansion of order 2 at a.e. point, then it has the Lusin property of order 2 [23, Theorem 3.6.3], and therefore also the weak Sard property. v) If f is locally in the Sobolev class W 2,1 or, equivalently, b is locally in the class W 1,1 ) then f admits an L 1 -Taylor expansion of order 2 at a.e. point [23, Theorem 3.4.2], and therefore it has the weak Sard property. vi) The proof of [23, Theorem 3.4.2] can be easily modified to show that f admits an L 1 -Taylor expansion of order 2 at a.e. point which implies the weak Sard property even when the second order distributional derivative of f is locally a vector-valued measure that is, when b is locally of class BV ). In [3] we prove that the same conclusion holds under the weaker assumption that the distributional Laplacian of f that is, the curl of b) is locally a signed measure. vii) In [2, Section 4] we construct a function on the plane without the weak Sard property and of class C 1,α for every α < 1. We conclude this section with a particular change of variable that will be used in the proof of Lemma The functions σ and ˆσ. In what follows I is the interval [, L], L is the Lebesgue measure on I, λ is a measure on I which is singular with respect to L, and J is the set of atoms of λ points with positive measure). We set L := L + λ)i) and Î := [, L], and denote by L the Lebesgue measure on Î.. 14 Thus Lusin s theorem states that every Borel function has the Lusin property of order

13 UNIQUENESS FOR THE CONTINUITY EQUATION IN 2D 13 We denote by ˆσ the multi-valued function from I to Î that to every s I associates the interval ˆσs) := [σ s), σ + s)] where ˆσ s) := L + λ)[, s)) and ˆσ + s) := L + λ)[, s]), and define σ : Î I as the inverse of σ. Then ˆσ is surjective on Î, strictly increasing, and uni-valued for every s / J because ˆσ and ˆσ + are strictly increasing, and ˆσ s) = ˆσ + s) whenever s / J. Moreover s 2 s 1 ŝ 2 ŝ 1 2.9) for every s 1, s 2 I with s 1 < s 2, and every ŝ 1 ˆσs 1 ), ŝ 2 ˆσs 2 ). Accordingly σ is surjective from Î onto I, uni-valued and 1-Lipschitz because of 2.9), constant on the interval σs) for every s J and strictly increasing at every point outside σj) Lemma. Let F be an L -negligible set in I which supports the measure λ, and set F := ˆσF ). Then i) the push-forward of the measure L according to σ is L + λ, and this means that for every function g : I [, + ] which is either positive or belongs to L 1 L + λ) there holds g dl + λ) = g σ d L ; 2.1) I ii) the derivative of σ agrees with 1b I\ b F a.e. in Î. Proof. Let E := s 1, s 2 ) be an open interval in I. By the definition of σ and ˆσ we have that σ 1 E) = ˆσE) = σ + s 1 ), σ s 2 )) and therefore L σ 1 E)) = σ s 2 ) σ + s 1 ) = L + λ)e). Then a standard argument shows that the identity L σ 1 E)) = L + λ)e) holds for every open set E, and hence also for every Borel set E. Thus i) is proved. In order to prove statement ii), we first notice that by the choice of F we have that 1 F L + λ) = λ and 1 I\F L + λ) = L, and therefore statement i) yields σ # 1 b F L ) = L and σ# 1b I\ b F L ) = λ. 2.11) Since λ is singular with respect to L, it is well known that the density of λ with respect to L is equal to at L -a.e. point, and equal to + at λ-a.e. point. This means that the derivative of the monotone functions σ and σ + are equal to 1 at L -a.e. point, and equal to + at λ-a.e. point. Therefore, taking into account 2.11), we obtain that the derivative of σ is equal to 1 at L -a.e. point in Î \ F, and equal to at L -a.e. point in F. bi

14 14 G. ALBERTI, S. BIANCHINI, AND G. CRIPPA Remark. The definitions of σ and ˆσ and Lemma 2.17 can be extended with few obvious modification to the case where the intervals [, L] and [, L] are replaced by the quotients [, L] and [, L], see Dimension reduction We now begin to consider the question of the uniqueness for the Cauchy problem for the continuity equation 1.1). In this section, we still work in general space dimension. The existence of a Lipschitz function associated to the vector field as in 3.1 gives the possibility to relate the uniqueness for the Cauchy problem in R d with the uniqueness for suitably defined Cauchy problems on the level sets of such Lipschitz function Theorem 3.1) Assumptions on the vector field. In this section b : [, T ) R d R d is a bounded vector field and we assume that there exists a Lipschitz function f : R d R d k with compact support such that fx) bt, x) = for a.e. t, x) [, T ) R d. 3.1) The following objects are then defined as in 2.5 and 2.7: J, S, E y, E, ν, ν s, µ y. Further assumptions on b and f will be introduced when needed The uniqueness problem. By linearity, the uniqueness for the Cauchy problem for 1.1) is equivalent to the fact that every solution in the sense of distributions with initial value u = is trivial, that is, a.e. null in [, T ) R d. Therefore, in this and in the following sections, we only consider solutions of the Cauchy problem { t u + div bu) = 3.2) u, ) =. We recall that a bounded function u : [, T ) R d R solves 3.2) in the sense of distributions if T t ϕ + b ϕ ) u dx dt = ϕ C c [, T ) R d ). 3.3) R d 3.3. Remark. In the following we will often tacitly) use that the test functions in 3.3) can be equivalently taken in any of the following classes: i) ϕ : [, T ) R d R Lipschitz with compact support; ii) ϕ of the form ϕ 1 t) ϕ 2 x), with ϕ 1 C c [, T )) and ϕ 2 C c R d ); iii) ϕ of the form ϕ 1 t) ϕ 2 x), with ϕ 1 : [, T ) R Lipschitz with compact support and ϕ 2 : R d R Lipschitz with compact support Remark. Here and in the rest of the paper u and b are functions defined at every point of their domain, and not equivalence classes. 15 The reason is that it 15 Accordingly, we avoid the notations u L and b L.

15 UNIQUENESS FOR THE CONTINUITY EQUATION IN 2D 15 is way more convenient to use the disintegration formula 2.3) when functions defined everywhere instead than almost everywhere Measure solutions. For technical reasons it is convenient to introduce the following notion of generalized solution of 3.2). 16 We say that a signed locally finite measure σ on [, T ) R d is a measure solution of 3.2) if t ϕ + b ϕ ) dσ = ϕ C c [, T ) R d ). 3.4) [,T ) R d 3.6. Solutions on level sets. Take y R d k and let C be a subset of the level set E y. We say that a bounded function u : [, T ) C R is a solution of 3.2) on C if the measure σ = u 1 C L 1 µ y is a measure solution in the sense of 3.5, that is if T t ϕ + b ϕ ) u dµ y dt = ϕ Cc [, T ) R d ). 3.5) C It is important to keep in mind that this notion of solution does not involve only the function u and the set C, but also the disintegration measure µ y. The following lemma shows that any bounded solution of 3.2) is also a solution on a generic level set Lemma. Let u : [, T ) R d R be a bounded solution of 3.2). Then u solves 3.2) on the level set E y for ν-a.e. y R d k. Proof. Fix a function ρ Cc R d k ). We consider in 3.3) Lipschitz test functions of the form ϕt, x)ρfx)), where f : R d R d k is the Lipschitz function introduced in 3.1 recall Remark 3.3i)). After immediate computations we obtain T [ ] = ρf) t ϕ + ρf) b ϕ + ϕ f b ; ρf) u dx dt = R d T R d ρf) t ϕ + b ϕ ) u dx dt =, where the last equality follows from 3.1). Using 2.3), the above integral rewrites as T = ρf) t ϕ + b ϕ ) u dµ y x) dt dνy) R d k E y [ T = ρy) t ϕ + b ϕ ) ] u dµ y x) dt dνy) =. R d k E y 16 This notion is similar but not equivalent to that of measure solution considered for instance in [6].

16 16 G. ALBERTI, S. BIANCHINI, AND G. CRIPPA The arbitrariness of ρ gives the existence of a ν-negligible set N ϕ R d k with the property that T t ϕ + b ϕ ) u dµ y x) dt = for all y N ϕ. E y Finally, we consider a countable dense set D Cc [, T ) R d ) and set N = ϕ D N ϕ. Since D is countable the set N is ν-negligible. Moreover, for all ϕ D there holds T t ϕ + b ϕ ) u dµ y x) dt = for all y N; E y this fact and the density of D in Cc [, T ) R d ) imply the thesis Lemma. Take y R d k with y and let u : [, T ) E y R be a bounded solution of 3.2) on E y. Then u solves 3.2) on every connected component C of E y. Proof. The fact that u is a solution of 3.2) on E y means that recall 3.6) T t ϕ + b ϕ ) u dµ y dt = ϕ C c [, T ) R d ). 3.6) E y Given a connected component C of E y, it is possible to find a decreasing sequence of bounded open sets U n contained in R d such that the boundaries U n do not intersect E y, and the intersection of the closures U n is C a proof of this fact is briefly sketched in [2, 2.8]). Let ε n be the distance between the sets U n and E y it is positive because these sets are closed and disjoint). Given a smooth convolution kernel ρ supported in the unit ball of R d we define χ n := 1 Un ρ εn/2. Considering in 3.6) test functions of the form ϕt, x)χ n x), after standard computations we obtain T χ n t ϕ + b ϕ ) T u dµ y dt + ϕ b χ n u dµ y dt =. 3.7) E y E y Now we pass to the limit as n + in this equality. The definition of χ n and the properties of the sets U n give that χ n = on E y for every n, so that the second integral in 3.7) vanishes; concerning the first integral, we have that χ n converge to 1 C everywhere in E y, and therefore, using the dominated convergence theorem, T t ϕ + b ϕ ) u dµ y dt = C for all ϕ C c [, T ) R d ), that is, u solves 3.2) on C Lemma. Given a bounded function u : [, T ) R d R, the following statements are equivalent:

17 a) u solves 3.2); UNIQUENESS FOR THE CONTINUITY EQUATION IN 2D 17 b) there exists an L d k -negligible set N R d k such that N supports ν s, and setting F := f 1 N) R d \ E ) with E given in 2.5 there holds b1) u solves 3.2) on C for every C C y and every y N; b2) 1 F u solves 3.2). Proof. Step 1: a) implies b). By Lemma 3.7 and Lemma 2.6ii) we can choose an L d k -negligible set N so that i) N; ii) N supports ν s ; iii) u solves 3.2) on E y for every y N; iv) H k E y ) < + for every y N. Then, b1) follows from Lemma 3.8. It remains to prove b2). Let y N be fixed for the time being. Because of iv), C y is countable, and therefore, by summing 3.5) over all C C y and recalling that their union is Ey, we obtain T [ t ϕ + b ϕ ) ] u dµ y dt = ϕ Cc [, T ) R d ), 3.8) E y that is, u solves 3.2) on Ey. By integrating 3.8) over all y N with respect to ν we obtain T [ = t ϕ + b ϕ ) ] u dµ y dνy) dt = T [ R d k \N R d \F E y t ϕ + b ϕ ) ] u dx dt. The second equality in the previous formula follows by the fact that {µ y } is the disintegration of L d with respect to f and ν, and by the fact that Ey is equal to R d \ F ) E y for all y N. Thus we have proved that u1 R d \F solves 3.2), and by linearity we obtain that also u1 F = u u1 R d \F solves 3.2), that is, we obtain b2). Step 2: b) implies a). Let N be the set of all y N such that H k E y ) = +. We know by Lemma 2.6ii) that L d k N ) =, and therefore νn ) = because ν s is supported on N). Fix y N N. By b1), u solves 3.2) on every C C y, and since H k E y ) is finite there are only countably many such C. Hence, proceeding as in Step 1, we obtain that u solves 3.2) on Ey and u1 R d \F solves 3.2). Finally b2) and the linearity of 3.2) imply that u = u1 F + u1 R d \F solves 3.2). We can now state the main result of this section, namely that, under the additional assumption 3.9), uniqueness for the Cauchy problem 3.2) in R d is equivalent to uniqueness on the connected components C in C y for a.e. y.

18 18 G. ALBERTI, S. BIANCHINI, AND G. CRIPPA 3.1. Theorem. Take b and f as in 3.1 and assume that bt, x) = for a.e. t, x) [, T ) S. 3.9) Then the following statements are equivalent: a) if u : [, T ) R d R is a bounded solution of 3.2) then u = for a.e. t, x) [, T ) R d ; b) for L d k -a.e. y R d k and every C C y the following implication holds: if u : [, T ) C R is a bounded solution of 3.2) on C, then u = for L 1 -a.e. t [, T ) and µ y -a.e. x C. Proof. Step 1: b) implies a). Let u : [, T ) R d R be a bounded solution of 3.2). We want to show that u = a.e. From Lemma 3.9 we deduce the existence of an L d k -negligible set N such that ν s is supported in N and u solves 3.2) on C for every C C y and every y N. By assumption b) we can find an L d k -negligible set N such that u = for L 1 -a.e. t [, T ) and µ y -a.e. x C, for every C C y and every y N N. By Lemma 2.6ii) we can also find an L d k -negligible set N such that y N we have H k E y ) < + and therefore C y is countable. The previous considerations imply that u = for L 1 -a.e. t [, T ) and µ y -a.e. x E y for every y N N N. Recalling that N support ν s we obtain that u = a.e. in [, T ) E \ f 1 N N N ) ). 3.1) By Lemma 3.9 that u 1 F solves 3.2), where F := f 1 N) R d \ E ). Lemma 2.8i) together with the fact that N is an L d k -negligible set supporting ν s implies that f 1 N) S up to negligible sets. Using Lemma 2.6v) we deduce that F S up to negligible sets, thus assumption 3.9) implies that b = for a.e. t, x) [, T ) F. Therefore, the fact that u1 F solves 3.2) rewrites as T t ϕ u dx dt = ϕ C c [, T ) R d ). 3.11) F Now, since every φ Cc, T ) R d ) can be written as φ = t ϕ for some ϕ Cc [, T ) R d ), we see that 3.11) implies that u = a.e. in [, T ) F. 3.12) Taking into account 3.1) and 3.12), to complete the proof it suffices to show that f 1 N N ) \ N ) is negligible. This follows from the fact that ν N N ) \ N ) = recall 2.4iii)), which in turn follows from the fact that N and N are L d k -negligible and ν s is supported on N. Step 2: a) implies b). Let H be the set of all y R d k such that there exist C y C y and u y : [, T ) C y R a nontrivial bounded solution of 3.2) on C y. We have to show that L d k H) =. Assume by contradiction that L d k H) >. Then we can find a constant m < + and a subset H H with L d k H ) > for which the functions u y

19 UNIQUENESS FOR THE CONTINUITY EQUATION IN 2D 19 above satisfy u y m on [, T ) C y for every y H. At this point we would like to claim that the function u defined by { u y t, x) if x C y for some y H ut, x) := otherwise is a nontrivial bounded solution of 3.2), in contradiction with assumption a) of the present theorem. Indeed, if the function u were Borel, we could use the implication b) a) in Lemma 3.9 to prove such claim. Unfortunately this is not the case, and therefore we proceed in a slightly) different way. We take m as above and consider the set C of all couples y, σ) with y R d k and σ a nontrivial signed measure on [, T ) R d of the form σ = u L 1 µ y where u solves 3.2) on E y, satisfies u m everywhere. By construction, the projection G of C on R d k contains H because for every y H the couple y, σ y ) with σ y := u y 1 Cy L 1 µ y. belongs to C. Hence L d k G) >, and by Corollary 7.2 we can find a Borel set G G with νg ) L d k G ) = L d k G) > and a Borel family of measure {σ y : y G } so that y, σ y ) C for every y G. We denote by u y the bounded solution of 3.2) on E y associated to σ y. Now we set σ := σ y dνy) = u y L 1 µ y dνy). G G It is clear that σ is a nontrivial measure solution of 3.2), and we deduce from 2.4iii), iv) that σ is absolutely continuous with respect to L 1 L d, Therefore we can find a function u : [, T ) R d R such that σ = u L 1 L d. Moreover, passing through the disintegration of σ with respect to f, we recover that for L d k -a.e. y G there holds u = u y a.e. with respect to L 1 µ y, and u = a.e. in [, T ) A, where A is the complement of the union of E y over all y G. Since u y m everywhere for every y G we conclude that u is a nontrivial solution of 3.2) which satisfies u m a.e., and this contradicts a) Remark. i) The converse of Lemma 3.7 also holds: if a bounded function u : [, T ) R d R solves 3.2) on the level set E y for ν-a.e. y R d k, then u is a bounded solution of 3.2). This is an immediate consequence of 2.3). ii) The converse of Lemma 3.8 does not hold. It is not true in general that a bounded function u : [, T ) E y R which solves 3.2) on every connected component of the level set E y is also a solution of 3.2) on the level set E y. This is related to the fact that the connected components of a level set which are H k -negligible can in principle be more than countable.

20 2 G. ALBERTI, S. BIANCHINI, AND G. CRIPPA iii) Assumption 3.9) in Theorem 3.1 is used in the proof to show that every solution of 3.2) is a.e. null on the set F defined in Lemma 3.9 of must be trivial. Presumably we could dispense with assumption 3.9); however, a proof of the corresponding variant of Theorem 3.1 would require a more refined disintegration formula, to take into account all the connected components of the level sets, including those which are H k -negligible see the previous remark). We decided not to pursue this refinement because we did not see any specific use for it in the two dimensional context considered in Sections 4 and 5 assumption 3.9) is always verified). 4. Uniqueness in the autonomous, divergence-free case 4.1. Assumptions on the vector field. Through this section we assume that d = 2 and b : R 2 R 2 is a bounded, autonomous, divergence-free vector field with compact support. It follows that there exists a Lipschitz function f : R 2 R with compact support such that b = f a.e. in R ) Such f is unique and is called the potential associated to b. Note that b and f satisfy the assumptions of Theorem 3.1, namely 3.1) and 3.9). Theorem 3.1 shows that uniqueness for the Cauchy problem 3.2) is equivalent to uniqueness for the same problem on every nontrivial connected component C of a generic level set E y of f. Using the parametrization of C given in Lemma 2.11, we show that solving 3.2) on C is equivalent to solve the Cauchy problem with zero initial datum for the equation t u1 + λ)) + s u = on the parametrization domain [, L] Lemma 4.4), and for such problem there is uniqueness if and only if λ = Lemma 4.5). In Theorem 4.7 we put together these results and prove that if b is taken as above then uniqueness for 3.2) is equivalent to the weak Sard property for the potential f Assumptions on the set C. In the following C is a nontrivial connected component of a level set E y of f, and µ y and µ s y are the measures on E y given in 2.7 and Lemma 2.8. Taking Lemma 2.11 into account, we assume in addition that i) C is a simple curve with finite length; ii) γ : [, L] C is a one-to-one Lipschitz parametrization of C such that γ a.e. with respect to L the Lebesgue measure on [, L] ); iii) the restriction of µ s y to C agrees with the push-forward according to γ of some measure λ on [, L] which is the singular with respect to L. The next lemma is a particular case of [2, Corollary 7.3], and states that the test functions of the form ϕ := φ γ with φ Cc R 2 ) are dense in a suitable sense) in the class of Lipschitz functions on [, L]. 17 Since div b =, by rotating b by 9 clockwise we obtain a curl-free vector field, which can be rewritten as the gradient of a Lipschitz scalar function.

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