Christian Olivera
email: colivera@ime.unicamp.br

Ciprian A Tudor
email: tudor@math.univ-lille1.fr

The density of the solution to the stochastic transport equation with fractional noise

Keywords: Primary 60F05: Secondary 60H05, 91G70 transport equation, fractional Brownian motion, Malliavin calculus, method of characteristics, existence and estimates of the density

We consider the transport equation driven by the fractional Brownian motion. We study the existence and the uniqueness of the weak solution and, by using the tools of the Malliavin calculus, we prove the existence of the density of the solution and we give Gaussian estimates from above and from below for this density.

Introduction

The purpose of this paper is to study the probability law of the real-valued solution of the following stochastic partial differential equations du(t, x) + b(t, x)∇u(t, x) dt + ∇u(t, x) • dB H t + F (t, u) dt = 0, u(0, x) = u 0 (x), [START_REF] Aboura | Density estimates for solutions to one dimensional backward SDE's[END_REF] where B H t = (B H 1 t , ..., B H d t ) is a fractional Brownian motion (fBm) in R d with Hurst parameter H = (H 1 , ..., H d ) ∈ 1 2 , 1 d and the stochastic integration is understood in the symmetric (Stratonovich) sense. The equation ( 1) is usually called the stochastic transport equation and arises as a prototype model in a wide variety of phenomena.

Although we introduced (1) in a general form, we mention that some results will be obtained in dimension one.

The stochastic transport equation with standard Brownian noise has been first studied in the celebrated works by Kunita [START_REF] Kunita | First Order Stochastic Partial Differential Equations[END_REF], [START_REF] Kunita | Stochastic flows and stochastic differential equations[END_REF] and more recently it has been the object of study for many authors. We refer, among many others, to [START_REF] Catuogno | L p solutions of the stochastic transport equation[END_REF], [START_REF] Fedrizzi | Noise prevents singularities in linear transport equations[END_REF], [START_REF] Flandoli | Well-posedness of the transport equation by stochastic perturbation[END_REF], [START_REF] Maurelli | Wiener chaos and uniqueness for stochastic transport equation[END_REF], [START_REF] Mohammed | Sobolev Differentiable Stochastic Flows of SDE's with Measurable Drift and Applications[END_REF], [START_REF] Olivera | Well-posedness of first order semilinear PDEs by stochastic perturbation[END_REF].

Our aim is to analyze the stochastic partial equation [START_REF] Aboura | Density estimates for solutions to one dimensional backward SDE's[END_REF] when the driving noise is the fractional Brownian motion, including the particular case of the Brownian motion. We will first give, by interpreting the stochastic integral in (1) as a symmetric integral via regularization in the Russo-Vallois sense [START_REF] Russo | Forward, backward and symmetric stochastic integration[END_REF], an existence and uniqueness result for the weak solution to (1) via the so-called method of characteristics and we express the solution as the initial value applied to the inverse flow generated by the equation of characteristics. This holds, when H i = 1 2 , i = 1, .., d for any dimension d and in dimension d = 1 if the Hurst parameter is bigger than one half. Using this representation of the solution to [START_REF] Aboura | Density estimates for solutions to one dimensional backward SDE's[END_REF], we study the existence and the Gaussian estimates for its density via the analysis of the dynamic of the inverse flow. A classical tools to study the absolute continuity of the law of random variables with respect to the Lebesque measure is the Malliavin calculus. We refer to the monographs [START_REF] Nualart | Malliavin Calculus and Related Topics[END_REF] or [START_REF] Sanz-Solé | Malliavin Calculus. With applications to stochastic partial differential equations[END_REF] for various applications of the Malliavin calculus to the existence and smoothness of the density of random variables in general, and of solutions to stochastic equations in particular.

We will prove the Malliavin differentiability of the solution to [START_REF] Fedrizzi | Noise prevents singularities in linear transport equations[END_REF] by analyzing the dynamic of the inverse flow generated by the characteristics [START_REF] Flandoli | Generalized integration and stochastic ODEs[END_REF]. Using a result in [START_REF] Nourdin | Density formula and concentration inequalities with Malliavin calculus[END_REF] we obtain, in dimension d = 1 upper and lower Gaussian bounds for the density of the solution to the transport equation. We are also able to find the explicit form of the density in dimension d ≥ 2 when the driving noise is the standard Brownian motion and the drift is divergence-free (i.e. the divergence of the drift vanishes).

We organized our paper as follows. In Section 2 we recall the existence and uniqueness results for the solution to the transport equation driven by the standard Brownian motion. In Section 3, we analyze the weak solution to the transport equation when the noise is the fBm, via the method of characteristics. In Section 4 we study the Malliavin differentiability of the solution to the equation of characteristics and this will be applied in Section 4 to obtain the existence and the Gaussian estimates for the solution to the transport equation. In Section 6 we obtain an explicit formula for the density when the noise is the Wiener process and the drift is divergence-free.

Stochastic transport equation driven by standard Brownian motion

Throughout the paper, we will fix a probability space (Ω, F , P ) and a d-dimensional Wiener process (B t ) t∈[0,T ] on this probability space. We will denote by (F t ) t∈[0,T ] the filtration generated by B.

We will start by recalling some known facts on the solution to the transport equation driven by a standard Wiener process in R d .

The equation ( 1) is interpreted in the strong sense, as the following stochastic integral equation

u(t, x) = u 0 (x) - t 0 b(s, x)∇u(s, x) ds - d i=0 t 0 ∂ x i u(s, x) • dB i s - t 0 F (t, u) ds (2)
for t ∈ [0, T ] and x ∈ R d . The solution to (1) is related with the so-called equation of characteristics. That is, for 0 ≤ s ≤ t and x ∈ R d , consider the following stochastic differential

equation in R d X s,t (x) = x + t s b(r, X s,r (x)) dr + B t -B s . (3)
and denote by

X t (x) := X 0,t (x), t ∈ [0, T ], x ∈ R d .
For m ∈ N and 0 < α < 1, let us assume the following hypothesis on b:

b ∈ L 1 ((0, T ); C m,α b (R d )) (4)
where C m,α (R d ) denotes the class of functions of class C m on R d such that the last derivative is Hölder continuous of order α.

Let us recall the definition of the stochastic flow (see e.g. [START_REF] Kunita | Stochastic differential equations and stochastic flows of diffeomorphisms[END_REF]).

Definition 1 A stochastic flow is a family of maps (Φ s,t : R d → R d ) 0≤s≤t≤T such that

• lim t→s + Φ s,t (x) = x for every x ∈ R d . • Φ u,t • Φ s,u = Φ s,t if 0 ≤ s ≤ u ≤ t.
Note that in [START_REF] Kunita | Stochastic differential equations and stochastic flows of diffeomorphisms[END_REF] the some measurability is also required in the definition of the flow but, since we are working later in the paper with non-semimartingales, we will omit it.

It is well known that under conditions (4), X s,t (x) is a stochastic flow of C m -diffeomorphism (see for example [START_REF] Kunita | Stochastic flows and stochastic differential equations[END_REF] and [START_REF] Kunita | Stochastic differential equations and stochastic flows of diffeomorphisms[END_REF]). Moreover, the inverse Y s,t (x) := X -1 s,t (x) satisfies the following backward stochastic differential equation

Y s,t (x) = x - t s b(r, Y r,t (x)) dr -(B t -B s ). ( 5
)
for every 0 ≤ s ≤ t ≤ T , see [START_REF] Flandoli | Well-posedness of the transport equation by stochastic perturbation[END_REF] or [START_REF] Kunita | Stochastic flows and stochastic differential equations[END_REF] pp. 234.

In order to get the solution of (1) via the stochastic characteristic method we considerer the following ordinary differential equation

Z t (r) = r + t 0 F (s, Z s (r)) ds. (6)
We have the following representation of the solution to the transport equation in terms of the inital data and of the inverse flow [START_REF] Chow | Stochastic Partial Differential Equations[END_REF]. We refer to e.g. [START_REF] Kunita | Stochastic differential equations and stochastic flows of diffeomorphisms[END_REF] or [START_REF] Chow | Stochastic Partial Differential Equations[END_REF], Section 3 for the proof.

Lemma 1 Assume (4) for m ≥ 3 and let

u 0 ∈ C m,δ (R d ), F ∈ L ∞ ((0, T ); C m b (R d ))
. Then the Cauchy problem (2) has a unique solution u(t, .) for 0 ≤ t ≤ T such that it is a C m -semimartingale which can be represented as

u(t, x) = Z t (u 0 (X -1 t (x))), t ∈ [0, T ], x ∈ R d where Z is the unique solution to (6) and X -1 t = X -1 0,t = Y 0,t for every t ∈ [0, T ].
3 The weak solution of the transport equation driven by fractional Brownian motion parameter bigger than one half. We refer to the last section (the Appendix) for the basic properties of this process. We will restrict throughout this section to the case d = 1 and and we will use the concept of weak solution. We explain at the end of this section (see Remark 1) why we need to assume these restrictions. Consider the following one-dimensional Cauchy problem: given an initial-data u 0 , find u(t, x; ω) ∈ R, satisfying

     ∂ t u(t, x; ω) + ∂ x u(t, x; ω) b(t, x) + dB H t dt (ω) = 0, u| t=0 = u 0 , (7)
with T > 0, (t, x) ∈ U T , ω ∈ Ω , where U T = [0, T ] × R, and b : [0, T ] + ×R → R is a given vector field. The noise B H is a fractional Brownian motion with Hurst parameter H > 1 2 and the stochastic integral in (7) will be understood in the symmetric sense via regularization [START_REF] Russo | Forward, backward and symmetric stochastic integration[END_REF] or [START_REF] Russo | Elements of stochastic calculus via regularization[END_REF]. The fBm B H is related to the Brownian moption B via (31).

Let us first recall the notion of weak solution to [START_REF] Fedrizzi | Noise prevents singularities in linear transport equations[END_REF].

Definition 2 A stochastic process u ∈ L ∞ (Ω×[0, T ]×R) is called a weak L p -solution of the Cauchy problem (7), when for any ϕ ∈ C ∞ c (R), R u(t, x)ϕ(x)
dx is an adapted real value proces which has a continuous modification, finite covariation, and for all t ∈ [0, T ], we have

P -almost surely R u(t, x)ϕ(x)dx = R u 0 (x)ϕ(x) dx + t 0 R u(s, x) b(s, x)∂ x ϕ(x) dxds + t 0 R u(s, x) b ′ (s, x) ϕ(x) dxds + t 0 R u(s, x) ∂ x ϕ(x) dxd • B H s . ( 8
)
where b ′ (s, x) denotes the derivative of b(s, x) with respect to the variable x.

At this point, we need to recall the definition of the symmetric integral d • B H that appears in [START_REF] Flandoli | Well-posedness of the transport equation by stochastic perturbation[END_REF]. assume (X t ) t≥0 is a continuous process and (Y t ) t≥0 is a process with paths in

L 1 loc (R + ), i.e. for any b > 0, b 0 |Y t |dt < ∞ a.s.
The generalized stochastic integrals (forward, backward and symmetric) are defined through a regularization procedure see [START_REF] Russo | Forward, backward and symmetric stochastic integration[END_REF], [START_REF] Russo | Elements of stochastic calculus via regularization[END_REF]. That is, let I 0 (ǫ, Y, dX) be the ε-symmetric integral

I 0 (ǫ, Y, dX) = t 0 Y s (X s+ǫ -X s-ǫ ) 2ǫ ds t ≥ 0. The symmetric integral t 0 Y d • X is defined as t 0 Y d • X := lim ǫ→0 I 0 (ε, Y, dX)(t),
for every t ∈ [0, T ], provided the limit exist ucp (uniformly on compacts in probability).

Similarly to Lemma 1, we also have a representation formula for the weak solution in terms of the initial condition u 0 and the (inverse) stochastic flow associated to SDE [START_REF] Flandoli | Generalized integration and stochastic ODEs[END_REF].

Theorem 1 Assume that b ∈ L ∞ ((0, T ); C 1 b (R d )). Then there exists a C 1 (R) stochas- tic flow of diffeomorhism X s,t , 0 ≤ s ≤ t ≤ T that satisfies X s,t (x) = x + t s b(u, X s,u (x))du + B H t -B H s ( 9
)
for every x ∈ R d . Moreover, if d = 1, given u 0 ∈ L ∞ (R), the stochastic process u(t, x) := u 0 (X -1 t (x)), t ∈ [0, T ], x ∈ R (10)
is the unique weak L ∞ -solution of the Cauchy problem [START_REF] Fedrizzi | Noise prevents singularities in linear transport equations[END_REF], where X t := X 0,t for every t ∈ [0, T ].

Proof: We will proceed in several steps: first we show that ( 9) is a diffeomorphism flow, then we prove the uniquennes of the L ∞ weak solution to [START_REF] Fedrizzi | Noise prevents singularities in linear transport equations[END_REF] and then we show that (10) satisfies the transport equation [START_REF] Fedrizzi | Noise prevents singularities in linear transport equations[END_REF].

Let us first show that (9) generates a flow of diffeomorphism. By doing the linear transformation Z s,t = X s,t (x) -(B H t -B H s ) we deduce that the equation ( 9) is equivalent to the random equation

Z s,t (x) = x + t s b(r, Z s,r (x) + B H r -B H s ) dr (11) for 0 ≤ s ≤ t ≤ T .
From the classical theory for ordinary differential equations (see e.g. [START_REF] Abraham | Manifolds, tensor analysis, and applications[END_REF]) we have that

Z s,t (x) with 0 ≤ s ≤ t ≤ T is a C 1 (R d ) diffeomorphism flow. Thus we deduce that X s,t (x) is a C 1 (R d ) diffeomorphism flow.
In a second step, we will show that the transport equation with fBm noise admits a unique L ∞ weak solution. By linearity we have to show that a weak L ∞ -solution with initial condition u 0 = 0 vanishes identically. Applying the Itô-Ventzel for the symmetric integral formula (see Proposition 9 of [START_REF] Flandoli | Generalized integration and stochastic ODEs[END_REF]) to F (y) = u(t, x)ϕ(x -y) dx (which depends on ω), we obtain that

R u(t, x)ϕ(x -B H t )dx = t 0 R b(s, x)∂ x ϕ(x -B H s )u(s, x)dxds + t 0 R b ′ (s, x)ϕ(x -B H s )u(s, x)dxds + t 0 R u(s, x)∂ x ϕ(x -B H s )dxd • B H s + t 0 R u(s, x)∂ y [ϕ(x -B H s )]dxd • B H s . (12)
We observe that

∂ y [ϕ(x -B H s )] = -∂ x ϕ(x -B H s )
. Thus the process

V (t, x) := u(t, x + B H t ) verifies R V (t, x)ϕ(x)dx = t 0 R b(s, x + B H s )∂ x ϕ(x)V (s, x)dxds + t 0 R b ′ (s, x + B H s )ϕ(x)V (s, x)dxds.
Let φ ε be a standard mollifier and let

V ε (t, x) := V (t, .) * φ ε . Then it holds R V (t, z)φ ε (x -z)dz = t 0 R V (s, z) b(s, z + B H s ) ∂ z φ ε (x -z)dzds + t 0 R u(s, z)b ′ (s, z + B H s ) φ ε (x -z)dzds
From an algebraic convenient manipulatio we get

dV ε dt -b(t, x -B H t )∂ x V ε = R ε (b, u)
where R ε (b, u) is the commutator defined as

R ε (b, u) = (b∂ x )(φ ε * u) -φ ε * ((b∂ x )u). Since b(s, x + B H s ) belongs a.s to L ∞ ((0, T ); C 1 b (R)) then by the Commuting Lemma (see Lemma II.1 of [6]), the process V ε (t, x) = V (t, .) * φ ε satisfies lim ε→0 dV ε dt -b(t, x -B H t )∂ x V ε = 0 a.s. in L 1 ([0, T ], L 1 loc (R)).
We deduce that if β ∈ C 1 (R) and β ′ is bounded, then

dβ(V ) dt -b(t, x -B H t )∂ x β(V ) = 0. ( 13
)
Now, by Theorem II. 2 of [START_REF] Diperna | Ordinary differential equations, transport theory and Sobolev spaces[END_REF], we define for each M ∈ [0, ∞) the function β M (t) = (|t| ∧ M) p and obtain that

d dt β M (V (t, x))dx ≤ C β M (V (t, x))dx.
Taking expectation we have that

d dt E(β M (V (t, x)))dx ≤ C E(β M (V (t, x)))dx.
From Gronwall Lemma we conclude that β M (V (t, x)) = 0. Thus u = 0. Let us finally show that ( 10) satisfies [START_REF] Fedrizzi | Noise prevents singularities in linear transport equations[END_REF]. We have that, by the change of variables

X -1 t (y) = x R u 0 (X -1 t )(y) ϕ(y)dy = R d u 0 (x) X ′ t (x)ϕ(X t (x))dx, (14)
for each t ∈ [0, T ], where X ′ t (x) denotes the derivative with respect to x of X t (x). Notice that

X ′ t (x) = 1 + t 0 b ′ (s, X s (x))X ′ s (x)ds for every t ∈ [0, T ],
x ∈ R. By applying Itô's formula (see [START_REF] Russo | Elements of stochastic calculus via regularization[END_REF], [START_REF] Russo | Forward, backward and symmetric stochastic integration[END_REF]) to the product

X ′ t (x)ϕ(X t (x))
and using the fact that B H has zero quadratic variation when H > 1 2 we obtain that

R u 0 (X -1 t (x))ϕ(x)dx = R u 0 (x)dx + t 0 R u 0 (x)b(s, X s (x))X ′ t (x) • ϕ ′ (X s (x))dxds (15) + t 0 R u 0 (x)b ′ (s, X s (x))X ′ t (x)ϕ ′ (X s (x))dxds + t 0 R u 0 (x)X ′ t (x)ϕ ′ (X s (x))dyd • B H s . (16)
Note that the Itô formula in [START_REF] Russo | Elements of stochastic calculus via regularization[END_REF] guarantees the existence of the symmetric stochastic integrals in [START_REF] Nourdin | Density formula and concentration inequalities with Malliavin calculus[END_REF] above. Now, by the change variable y = X t (x) we have that

R u 0 (X -1 t (x))ϕ(x)dx = R u 0 (x)dx + t 0 R u 0 (X -1 s (x))b(s, y) • ϕ ′ (y)dyds + t 0 R u 0 (X -1 s (x))b ′ (s, y)ϕ ′ (y)dyds + t 0 R u 0 (X -1 s (x)) ϕ ′ (y)dyd • B H s .
From this we conclude que u(t, x) = u 0 (X -1 t (x)) is a weak solution of [START_REF] Fedrizzi | Noise prevents singularities in linear transport equations[END_REF]. Its adaptedness is a consequence of (31). Thus the unique solution to ( 7) is u(t, x) = u 0 (X -1 t (x)) for every t ∈ [0, T ] and for every x ∈ R.

Remark 1

• We need to restrict to the situation d = 1 in order to get the existence of the symmetric integral in [START_REF] Nourdin | Density formula and concentration inequalities with Malliavin calculus[END_REF] or [START_REF] Kunita | Stochastic flows and stochastic differential equations[END_REF]. Here we also used the hypothesis H > 1 2 that ensures that there is not a second derivative term in the Itô formula. • The uniqueness of the weak solution can be obtined with weaker assumption on the drift b by following the proof of Theorem 3.1 in [START_REF] Catuogno | L p solutions of the stochastic transport equation[END_REF].

Fractional Brownian flow

In this section we will analyze the properties of the stochastic flow generated by the fractional Brownian motion. We will call it the fractional Brownian flow in the sequel. 9) generates a C 1stochastic flow of diffeomorphism. We next describe the dynamic of the inverse flow of [START_REF] Flandoli | Generalized integration and stochastic ODEs[END_REF].

if b ∈ L ∞ ((0, T ), C 1 b (R d )), (
Lemma 2 Let b ∈ L ∞ ((0, T ), C 1 b (R d ))
and denote, for every 0 ≤ s ≤ t ≤ T and for every

x ∈ R d Y s,t (x) = X -1 s,t (x) ( 17
)
the inverse of the stochastic flow given by [START_REF] Flandoli | Generalized integration and stochastic ODEs[END_REF]. Then the inverse flow satisfies the backward stochastic equation

Y s,t (x) = x - t s b(r, Y r,t )dr -(B H t -B H s ) ( 18
)
for every x ∈ R d .

Proof: It follows from Kunita [START_REF] Kunita | Stochastic differential equations and stochastic flows of diffeomorphisms[END_REF]. Indeed, Lemms 6.2, page 235 in [START_REF] Kunita | Stochastic differential equations and stochastic flows of diffeomorphisms[END_REF] says that for any continuous function in two variables g we have t s g(r, X s,r (y))dr| y=X -1 s,t (x) = t s g(r, X -1 r,t (x))dr and it suffices to apply the above identity to [START_REF] Flandoli | Generalized integration and stochastic ODEs[END_REF]. We need the following useful lemma.

Lemma 3 Let us introduce the notation, for t ∈ [0, T ] and

x ∈ R d , R t,x (u) = Y t-u,t (x), if u ∈ [0, t]. (19)
Then we have, for every t

∈ [0, T ], u ∈ [0, t] and x ∈ R d R t,x (u) = x - u 0 b(a, R t,x (a))da -(B H t -B H t-u ). ( 20
)
Proof: In ( 17) we use the change of notation u = t -s and we get for every

y ∈ R d , R t,y (u) = y - t t-u b(r, Y r,t (x))dr -(B H t -B H t-u )
and then, with the change of variables a = t -r in the integral dr, we can writye

R t,y (u) = y - u 0 b(a, Y t,y (a))da -(B H t -B H t-u )
with R u,y (u) = y. As a consequence of the above Lemma 3, we get the uniqueness of solution to the backward equation [START_REF] Nualart | Gaussian density estimates for solutions to quasi-linear stochastic partial differential equations[END_REF] satisfied by the inverse flow.

Corollary 1 If ( Ỹs,t ) 0≤s≤t≤T is another two parameter process that satisfies (17) with Ỹs,s (x) = x and b is Lipschitz in x uniformy with respect to t, then Ỹs,t (x) = Y s,t (x) for every 0 ≤ s ≤ t and for every x ∈ R d .

Proof: If Ỹ satisfies [START_REF] Nualart | Malliavin Calculus and Related Topics[END_REF], then, if we denote Rt,x (u) = Ỹt-u,t (x), we get from Lemma that R satisfies [START_REF] Olivera | Well-posedness of first order semilinear PDEs by stochastic perturbation[END_REF] and the Gronwall lemma and the Lipschitz assumption on the drift b imply the conclusion.

We denote by D the Malliavin derivative with respect with the fBm B H (see the Appendix).

Proposition 1 Assume b ∈ L ∞ ((0, T ), C 1 b (R d )
) and let X s,t be given by [START_REF] Flandoli | Generalized integration and stochastic ODEs[END_REF]. Then, for every 0 ≤ s ≤ t ≤ T and for every x ∈ R d , the components of inverse flow Y i s,t

(1 ≤ i ≤ d) are Malliavin differentiable and for every α ∈ [s, t] D α Y i s,t (x) = - t s d j=1 ∂b i ∂x j (r, Y r,t )D α Y j r,t (x)dr -1 and D α Y i s,t (x) = 0 if α / ∈ [s, t].
We denoted by b i (1 ≤ i ≤ d) the components of the vector mapping b.

Proof:

It suffices to show that the random variable R t,x (u) defined by ( 19) is Malliavin differentiable for any x ∈ R d and for every 0 ≤ u ≤ t ≤ T . We will give the sketsch of the proof which follows by a routine fix point argument. Fix x ∈ R d , t ∈ [0, T ] and define the iterations

R (0) t,x (u) = x, for every u ∈ [0, t] and for n ≥ 1, R (n) t,x (u) = x - u 0 b(a, R (n-1) t,x (a))da -(B H t -B H t-u ).
By induction, we can prove by standard arguments (see e.g. [START_REF] Nualart | Malliavin Calculus and Related Topics[END_REF], Theorem 2.2.1) that for every p ≥ 1 sup

0≤u≤t E R (n) t,x (u) p < ∞, R (n),j t,x (u) ∈ D 1,∞ , j = 1, .., d, and sup n≥1 sup α∈[0,T ] E D α R (n),j t,x (u) p < ∞ where R (n),j t,x (u) denotes the j th component of R (n)
t,x (u). Moreover, the sequence of random variables (R n t,x (u)) n≥1 converges in L p to R t,x (u) which is the unique solution to [START_REF] Pipiras | Integration questions related to the fractional Brownian motion[END_REF]. It follows from Lemma 1.2.3 in [START_REF] Nualart | Malliavin Calculus and Related Topics[END_REF]

that R t,x (u) belongs to D 1,∞ .
Remark 2 Note that, when the noise is the standard Brownian motion, the Malliavin differentiability of Y is also claimed in [START_REF] Mohammed | Sobolev Differentiable Stochastic Flows of SDE's with Measurable Drift and Applications[END_REF].

Existence and Gaussian bounds for the density of the solution to the transport equation in dimension one

In this section we will assume that d = 1. On the other hand, the results in these section (except Theorem 4) will hold for every H ∈ (0, 1). We also mention that we will use the notation c, C.. for generic positive constants that may vary from line to line.

From Proposition 1 we immediately obtain the explicit expression for the Malliavin derivative of the inverse flow.

Proposition 2 If b ∈ L ∞ ((0, T ), C 1 b (R d ))
and Y s,t is defined by ( 17), we have for every α and for every 0 [START_REF] Russo | Forward, backward and symmetric stochastic integration[END_REF] with b ′ (t, x) the derivative of b(t, x) with respect to x.

≤ s ≤ t ≤ T D α Y s,t (x) = -1 [s,t] (α)e -α s b ′ (r,Yr,t)dr
Proof: For every α, we have

D α Y s,t (x) = - t s b ′ (s 1 , Y s 1 ,t (x))D α Y s 1 ,t (x)ds 1 -1 [s,t] (α)
and by iterating the above relation we can write, for every 0 ≤ s ≤ t ≤ T and for evey α ∈ [0, T ],

D α Y s,t (x) = -1 [s,t] (α) n≥0 (-1) n α s ds 1 α s 1 ds 2 ... α s n-1 ds n ×b ′ (s 1 , Y s 1 ,t (x))b ′ (s 2 , Y s 2 ,t (x))..b ′ (s n , Y sn,t (x)) = -1 [s,t] (α) n≥0 (-1) n n! t s drb ′ (r, Y r,t (x)) n = -1 [s,t] (α)e -α s b ′ (r,Yr,t)dr .
The main tool in order to obtain the Gaussian estimates for the density of the solution to the trasport equation is the following result given in [START_REF] Nourdin | Density formula and concentration inequalities with Malliavin calculus[END_REF].

Proposition 3 I If F ∈ F 1,2 , let g F (F ) = ∞ 0 dθe -θ E E ′ DF, DF H |F
where for any random variable X, we denoted

X(ω, ω ′ ) = X(e -θ w + 1 -e -2θ ω ′ ).
Here X is defined on a product probability space (Ω × Ω ′ , F ⊗ F , P × P ′ ) and E ′ denotes the expectation with respect to the probability measure P ′ . If there exists two constants γ min and γ max such that almost surely

0 ≤ γ min ≤ g F (F ) ≤ γ max
then F admits a density ρ. Moreover, for every z ∈ R,

E|F -EF | 2γ 2 max e - (z-EF ) 2 2γ 2 min ≤ ρ(z) ≤ E|F -EF | 2γ 2 min e -(z-EF ) 2 2γ 2 max
To apply the above result, we need to controll the Malliavin derivative of the inverse flow. This will be done in the next result. Notice that a similar method has been used in e.g. [START_REF] Aboura | Density estimates for solutions to one dimensional backward SDE's[END_REF], [START_REF] Besalú | Gaussian type lower bounds for the density of solutions of SDEs driven by fractional Brownian motions[END_REF] or [START_REF] Nualart | Gaussian density estimates for solutions to quasi-linear stochastic partial differential equations[END_REF] for various types of stochastic equations. In the sequel H denotes the canonical Hilbert space associated to the fractional Brownian motion (see the Appendix).

Proposition 4 Assume H > 1 2 and b ∈ L ∞ ((0, T ); C 1 b (R)).
Then there exist two positive constants c < C such that for every t ∈ [0, T ] and for every x ∈ R

ct 2H ≤ DY 0,t (x), DY 0,t (x) H ≤ Ct 2H (22)
where Y 0,t is given by [START_REF] Nualart | Gaussian density estimates for solutions to quasi-linear stochastic partial differential equations[END_REF].

Proof: Assume H = 1 2 . Then H = L 2 ([0, T ]) and DY 0,t (x), DY 0,t (x) H = t 0 dαe -α 0 b ′ (r,Yr,t(x))dr e -α 0 b ′ (r, Yr,t(x))dr
and since

e -T b ′ ∞ ≤ e -α s b ′ (r,Yr,t)dr ≤ e T b ′ ∞ (23)
(and a similar bound holds for the tilde process) we obtain ct ≤ DY 0,t (x), DY 0,t (x) H ≤ Ct with two positive constant c and C.

Assume H > 1 2 . Then by (30)

DY 0,t (x), DY 0,t (x) H = α H t 0 dα t 0 dβe -α 0 b ′ (r,Yr,t(x))dr e -β 0 b ′ (r, Yr,t(x))dr |α -β| 2H-2
and inequality [START_REF] Sanz-Solé | Malliavin Calculus. With applications to stochastic partial differential equations[END_REF] implies that

c t 0 dα t 0 dβ|α -β| 2H-2 ≤ DY 0,t (x), DY 0,t (x) H ≤ C t 0 dα t 0 dβ|α -β| 2H-2
which immediately gives [START_REF] Russo | Elements of stochastic calculus via regularization[END_REF]. Assume H < 1 2 . Then Proposition 23 in [START_REF] Besalú | Gaussian type lower bounds for the density of solutions of SDEs driven by fractional Brownian motions[END_REF] implies the lower bound in [START_REF] Russo | Elements of stochastic calculus via regularization[END_REF]. Concerning the upper bound, it suffices again to follow [START_REF] Besalú | Gaussian type lower bounds for the density of solutions of SDEs driven by fractional Brownian motions[END_REF], Section 3.4 and to note that for every α, β ∈ [0, T ] with α > β we have

|D α Y 0,t (x) -D β Y 0,t (x)| ≤ e -β 0 b ′ (r,Yr,t)dr e -α β b ′ (r,Yr,t)dr -1 ≤ c(α -β)
and the same bound holds for the Y 0,t .

Denote by m := Eu(t, x) (it satisfies a parabolic equation, see e.g. [START_REF] Flandoli | Well-posedness of the transport equation by stochastic perturbation[END_REF]). We are ready to state our main result.

Theorem 2 Let u(t, x) be the solution to the transport equation [START_REF] Fedrizzi | Noise prevents singularities in linear transport equations[END_REF]. Assume that u 0 ∈ C 1 (R) such that there exist 0 < c < C with c ≤ u ′ 0 (x) < C for every x ∈ R and b ∈ L ∞ ((0, T ); C 1 b (R)). Then, for every t ∈ [0, T ] and for every x ∈ R, the random variable u(t, x) is Malliavin differentiable. Moreover u(t, x) admits a density ρ u(t,x) and there exist two positive constants c 1 , c 2 such that

E|u(t, x) -m| 2c 1 t 2H e -(y-m) 2 2c 2 t 2H ≤ ρ u(t,x) ≤ E|u(t, x) -m| 2c 2 t 2H e -(y-m) 2 2c 1 t 2H (24)
Proof: Since by Theorem 1, u(t, x) = u 0 (Y 0,t (x)), we get the Malliavin differentiability of u(t, x) from Proposition 1 and the chain rule for the Malliavin derivative (see e.g. [START_REF] Nualart | Malliavin Calculus and Related Topics[END_REF]). Moreover, the chain rule implies

D α u(t, x) = u ′ 0 (Y 0,t (x))D α Y 0,t (x)
and thus

Du(t, x), Du(t, x) H = u ′ 0 (Y 0,t (x)) u ′ 0 (Y 0,t (x)) DY 0,t (x), DY 0,t (x) H .
By Proposition 4 and the asumption u 0 ∈ C 1 b , there exists two strictly positive constants c < C such that ct 2H ≤ Du(t, x), Du(t, x) H ≤ Ct 2H for every t ∈ [0, T ] and for every x ∈ R. Now, Proposition 3, point 2. implies that, if

F = u(t, x) then c 1 t 2H ≤ g F (F ) ≤ c 2 t 2H
and Proposition 3, point 1. gives the conclusion.

6 Explicit expression of the density when the noise is the Brownian motion in R d

We obtained above the existence and Gaussian estimate for the solution to the transport equation in dimension 1 and for H ≥ 1 2 . In this section, we will assume d ≥ 2, H = 1 2 , that is, the transport equation is driven a standard Brownian motion in R d . We obtain the followin explicit expression for the density of the solution when the divergence of the drift b vanishes.

Theorem 3 Assume d ≥ 2and let u 0 be a C m,δ (R d ) diffeomorphism. Assume (4) for m ≥ 3. Moreover, suppose that div b = 0. ( 25
)
Fix t ∈ [0, T ] and x ∈ R d . Then the law of the solution of (1), has a density ρ with respect to the Lebesgue measure. Moreover the density ρ admits the representation

ρ = Ju 0 (Z -1 t (y))JZ t ρ(u -1 0 (Z -1 t (y)), t, x) (26)
where ρ denotes the density of the solution to (3).

Proof: Let u(t, x) solution of the SPDE [START_REF] Aboura | Density estimates for solutions to one dimensional backward SDE's[END_REF]. By Lemma 1 we have that u(t, x) has the representation

u(t, x) = Z t (u 0 (X -1 t (x))). Let φ ε be a standard mollifier and consider a smooth function ϕ ∈ C ∞ c (R d ). Then E[ϕ(u(t, x))] = E[ϕ(Z t (u 0 (X -1 t (x))))] = E [lim ǫ→0 R d φ ε (y -x)ϕ(Z t (u 0 (X -1 t (y))))dy] = lim ǫ→0 E [ R d φ ε (y -x)ϕ(Z t (u 0 (X -1 t (y))))dy].
The assumption (25) implies that JX t = 1, where JX t denote of the Jacobian map of X t . By doing one more time a chamge of variable, we can write

E[ϕ(u(t, x))] = lim ǫ→0 E[ R d φ ε (y -x)ϕ(Z t (u 0 (X -1 t (y))))dy] = lim ǫ→0 E[ R d φ ε (X t (y) -x)ϕ(Z t (u 0 (y)))dy] = lim ǫ→0 R d E[φ ε (X t (y) -x)]ϕ(Z t (u 0 (y)))dy] (27)
The random variable X t (x) admits a density ρ in any dimension d. This is an easy consequence of equation ( 9) (see e.g. [START_REF] Nualart | Malliavin Calculus and Related Topics[END_REF]). Therefore, ( 27) becomes

E[ϕ(U(t, x))] = lim ǫ→0 R d E[φ ε (X t (y) -x)]ϕ(Z t (u 0 (y)))dy] = lim ǫ→0 R d R d φ ε (u -x)ρ(u, t, y)du ϕ(Z t (u 0 (y))
)dy and by calculating the limit above when ε → 0 we get

E[ϕ(u(t, x))] = R d
ρ(y, t, x)ϕ(Z t (u 0 (y)))dy.

Finally, the making succesively the changes of variables w = u 0 (y) and y = Z t (w) we obtain

E[ϕ(u(t, x))] = R d Ju 0 ρ(u -1 0 (w), t, x) ϕ(Z t (w))dw = R d Ju 0 (Z -1 t (y)) JZ t ρ(u -1 0 (Z -1 t (y)), t, x) ϕ(y) dy
and thus relation ( 26) is obtained.

Remark 3

The assumption divb = 0 can be interpreted as follows (see [START_REF] Lions | I: incompressible models[END_REF]): in fluid mechanics or more generally in continuum mechanics, incompressible flow (isochoric flow) refers to a flow in which the material density is constant within a fluid parcelan infinitesimal volume that moves with the velocity of the fluid. This is equivalent to the condition that the divergence of the fluid velocity is zero.

Appendix

We present here some basic element on the fractional Brownian motion and on the Malliavin calculus.

Fractional Brownian motion

Consider (B H t ) t∈[0,T ] a fractional Brownian motion with Hurst parameter H ∈ (0, 1). Recall that is it a centered Gaussian process with covariance function

EB H t B H s := R H (t, s) = 1 2 (t 2H + s 2H -|t -s| 2H , s, t ∈ [0, T ]. (28)
The fractional Brownian motion can be also defined as the only self-similar Gaussian process with stationary increments.

Denote by H its canonical Hilbert space . If H = 1 2 then B 1 2 is the standard Brownian motion (Wiener process) W and in this case H = L 2 ([0, T ]). Otherwise H is the Hilbert space on [0, T ] extending the set of indicator function 1 [0,T ] , t ∈ [0, T ] (by linearity and closure under the inner product) the rule

1 [0,s] ; 1 [0,t] H = R H (s, t) := 2 -1 s 2H + t 2H -|t -s| 2H .
The followings facts will be needed in the sequel (we refer to [START_REF] Pipiras | Integration questions related to the fractional Brownian motion[END_REF] or [START_REF] Nualart | Malliavin Calculus and Related Topics[END_REF] for their proofs):

• If H > 1 2
, the elements of H may be not functions but distributions; it is therefore more practical to work with subspaces of H that are sets of functions. Such a subspace is

|H| = f : [0, T ] → R T 0 T 0 |f (u)||f (v)||u -v| 2H-2 dvdu < ∞ .
Then |H| is a strict subspace of H and we actually have the inclusions

L 2 ([0, T ]) ⊂ L 1 H ([0, T ]) ⊂ |H| ⊂ H. (29)
• The space |H| is not complete with respect to the norm • H but it is a Banach space with respect to the norm • If H > 1 2 and f, g are two elements in the space |H|, their scalar product in H can be expressed by

f, g H = α H T 0 T 0 dudv|u -v| 2H-2 f (u)g(v) (30)
where α H = H(2H -1).

• when H < 1 2 then the canonical Hilbert space is a space of functions. We have

C γ ⊂ H ⊂ L 2 ([0, T ])
for all γ > 1 2 -H where C γ denotes the class of Hölder continuous functions of order γ.

• The fBm admits a representation as Wiener integral of the form

B H t = t 0 K H (t, s)dW s , (31)
where W = {W t , t ∈ T } is a Wiener process, and K H (t, s) is the kernel

K H (t, s) = d H (t -s) H-1 2 + s H-1 2 F 1 t s , (32)
d H being a constant and

F 1 (z) = d H 1 2 -H z-1 0 θ H-
R H i (t, s) = EB H i t B H i s = 1 2 (t 2H i + s 2H i -|t -s| 2H i )
for every 1 ≤ i ≤ d.

The Malliavin derivative

Here we describe the elements from the Malliavin calculus that we need in the paper. We refer [START_REF] Nualart | Malliavin Calculus and Related Topics[END_REF] for a more complete exposition. Consider H a real separable Hilbert space and (B(ϕ), ϕ ∈ H) an isonormal Gaussian process on a probability space (Ω, A, P ), which is a centered Gaussian family of random variables such that E (B(ϕ)B(ψ)) = ϕ, ψ H . We denote by D the Malliavin derivative operator that acts on smooth functions of the form F = g(B(ϕ 1 ), . . . , B(ϕ n )) (g is a smooth function with compact support and ϕ i ∈ H, i = 1, ..., n) DF = n i=1 ∂g ∂x i (B(ϕ 1 ), . . . , B(ϕ n ))ϕ i .

It can be checked that the operator D is closable from S (the space of smooth functionals as above) into L 2 (Ω; H) and it can be extended to the space D 1,p which is the closure of S with respect to the norm F p 1,p = EF p + E DF p H . We denote by D k,∞ := ∩ p≥ D k,p for every k ≥ 1. In our paper, H will be the canonical Hilbert space associated with the fractional Brownian motion, as defined in the previous paragraph.

  ||f (v)||u -v| 2H-2 dvdu.

  A a d dimensional fractional Brownian motion B H = (B H 1 , . . . , B H d with Hurst parameter H = (H 1 , . . . , H d ) ∈ (0, 1) d is a centered Gaussian process in R d with independent components and the covariance of the ith component is given by

					3 2	1 -(θ + 1) H-1 2	dθ.
	If H > 1 2 , the kernel K 1 2 -H	t	(u -s) H-3 2 u H-1 2 du	(33)
			s
			1
	where t > s and c H =	H(H-1) β(2-2H,H-1 2 )	2 .

H has the simpler expression

K H (t, s) = c H s

Supported by FAPESP 2012/18739-0 † Supported by the CNCS grant PN-II-ID-PCCE-2011-2-0015. Associate member of the team Samos, Université de Panthéon-Sorbonne Paris 1