1. Introduction
Let
be 2
m + 2 vector fields on
ℝd with bounded derivatives at each order. Let
(1.1)
be an Hoermander′s type operator on
ℝ1+d. Let
(1.2)
be a second Hoermander′s operator on
ℝ1+d. Bismut [
1] considers the generator
(1.3)
and the Markov semi-group exp [
t𝔸]. This semi-group has a probabilistic representation. We consider a Brownian motion
t →
zt independent of the others Brownian motions
. Bismut introduced the solution of the stochastic differential equation starting at
x in Stratonovitch sense:
(1.4)
where
are
m independent Brownian motions.
Let us introduce the local time
t →
Lt associated with
t →
zt and its right inverse
t →
At (see [
2,
3]). Then,
(1.5)
Such operator is classically related to the Dirichlet Problem [
3].
Classically [
4],
(1.6)
where
is the solution of the Stratonovitch differential equation starting at
x:
(1.7)
The question is as following: is there an heat-kernel associated with the semi-group exp [
t𝕃1]? This means that
(1.8)
There are several approaches in analysis to solve this problem, either by using tools of microlocal analysis or tools of harmonic analysis. Malliavin [
5] uses the probabilistic representation of the semi-group. Malliavin uses a heavy apparatus of functional analysis (number operator on Fock space or equivalently Ornstein-Uhlenbeck operator on the Wiener space, Sobolev spaces on the Wiener space) in order to solve this problem.
Bismut [6] avoids using this machinery to solve this hypoellipticity problem. In particular, Bismut’s approach can be adapted immediately to the case of the Poisson process [7]. The main difficulty to treat in the case of a Poisson process is the following: in general the solution of a stochastic differential equation with jumps is not a diffeomorphism when the starting point is moving (see [8–10]).
The main remark of Bismut in [
1] is that if we consider the jump process
, then it is a diffeomorphism almost surely in
x. So, Bismut mixed the tools of the Malliavin Calculus for diffusion (on the process
) and the tools of the Malliavin Calculus for Poisson process (on the jump process
t →
At) in order to show that this isthe problem if
(1.9)
Developments on Bismut’s idea was performed by Léandre in [
9,
11]. Let us remark that this problem is related to study the regularity of the Dirichlet problem (see [
1, page 598]) (see [
12–
14] for related works).
Recently, we have translated into the language of semi-group theory the Malliavin Calculus of Bismut type for diffusion [15]. We have translated in semi-group theory a lot of tools on Poisson processes [16–22]. Especially, we have translated the Malliavin Calculus of Bismut type for Poisson process in semi-group theory in [17]. It should be tempting to translate in semi-group theory Bismut’s Calculus on boundary process. It is the object of this work.
On the general problematic on this work, we refer to the review papers of Léandre [23–25]. It enters in the general program to introduce stochastic analysis tools in the theory of partial differential equation (see [26–28]).
2. Statements of the Theorems
Let us recall some basis on the study of fractional powers of operators [
29]. Let
𝕃 be a generator of a Markovian semi-group
Ps. Then,
(2.1)
The results of this paper could be extended to generators of the type
(2.2)
where
and
g ≥ 0, but we have chosen the operator of the type (
1.3) to be more closely related to the original intuition on Bismut’s Calculus on boundary process. Let be
𝔼d =
ℝ1+d ×
𝔾d ×
𝕄d where
𝔾d is the space of invertible matrices on
ℝd and
𝕄d the space of symmetric matrices on
ℝd. (
s,
x,
U,
V) is the generic element of
𝔼d.
V is called the Malliavin matrix.
On
𝔼d, we consider the vector fields:
(2.3)
We consider the Malliavin generator
on
𝔼d:
(2.4)
We consider the Malliavin semi-group
associated and
.
We perform the same algebraic considerations on
𝕃2. We get
,
, and
. Let us consider the total generator
(2.5)
and the Malliavin semi-group
.
We get a theorem which enters in the framework of the Malliavin Calculus for heat-kernel.
Theorem 2.1. Let one suppose that the Malliavin condition in x is checked:
(2.6)
holds for all
p, then
(2.7)
where
qt(
s,
y) is the density of a probability measure on
ℝ1+d.
Theorem 2.2. If the quadratic form
(2.8)
is invertible in
x, then the Malliavin condition holds in
x.
Remark 2.3. We give simple statements to simplify the exposition. It should be possible by the method of this paper to translate the results of [9, part III], got by using stochastic analysis as a tool.
3. Integration by Parts on the Underlying Diffusion
We consider the vector fields on
ℝ1+d+1,
(3.1)
where
(
ϕ(
x) is a convenient matrix on
ℝm which depends smoothly on
x and whose derivatives at each order are bounded.
does not depend on
x, and
belong to
ℝm). Let
be a smooth function on
ℝ1+d+1,
denotes its gradient, and
denotes its Hessian.
We consider the generator
acting on smooth functions on
ℝ1+d+1,
(3.2)
In (
3.2), the generator is written under Itô’s form. It generates a time inhomogeneous in the parameter
s semi-group
. We can consider
(3.3)
We put
(3.4)
It generates a semi-group
.
Let us consider the Hoermander’s type generator associated with the smooth Lipschitz vector fields on
ℝ1+d+d((
s,
x,
U) on
ℝ1+d+d):
(3.5)
We consider the heat semi-group associated with
(3.6)
Let us recall [
15, Theorem 2.2] that
(3.7)
where
f depends only on (
s,
x). In the left-hand side of (
3.7), we apply the enlarged semi-group to the test function (
s,
x,
u) →
f(
s,
x)
u and in the right-hand side we apply the semi-group to the test function (
s,
x,
U) → 〈
Df,
U〉.
u belongs to
ℝ and
U belongs to
ℝd. From this, we deduce the following.
Lemma 3.1. One has the relation
(3.8)
Let us consider the semi-group
associated with
(3.9)
We get, with the same notations for (
s,
x,
u,
U) the following.
Theorem 3.2. For f bounded continuous with compact support in (s, x), one has the following relation:
(3.10)
Proof. For the integrability conditions, we refer to the appendix.
We remark that ∂/∂u commute with , therefore with . We deduce that
(3.11)
By the method of variation of constants,
(3.12)
In order to show that, we follow the lines of (2.17) and (2.18) in [
15]. We apply
to (
3.11).
By Lemma 3.1,
(3.13)
Let us consider the vector fields on
ℝ1+d ×
𝔾d,
(3.14)
We consider the Hoermander’s type operator associated with these vector fields:
(3.15)
We consider the generator
(3.16)
It generates a semi-group
. By lemma 3.2 of [
15], we have
(3.17)
By [
15, Equation (3.18)],
(3.18)
In [
15, Equation (3.18)], we consider the semi-group
instead of the semi-group
and the test function
Df instead as of the test function
here.
is considered as an element of
ℝ and not as a one-order differential operator:
(3.19)
Therefore,
(3.20)
We write
(3.21)
where
(3.22)
The Volterra expansion (see [
15, Equation (3.17)]) if it converges gives the following formula:
(3.23)
But
is linear in
u0. Therefore:
(3.24)
In this last formula,
are considered as differential operators.
Therefore, does not depend on U0 and is equal to
(3.25)
where
are considered as elements of
ℝd. We deduce as in [
15, Equation (3.17)],
(3.26)
But
is linear. Therefore,
(3.27)
It remains to replace
f by
Df in this last equation and to compare (
3.26) with (
3.13) and (
3.20).
We consider the Malliavin generator . We can perform the same algebraic construction as in Theorem 3.2. We get two semi-groups and . and are smooth with bounded derivatives in . We get by the same procedure the following.
Theorem 3.3. If is bounded with bounded derivatives and with compact support in s, then one gets
(3.28)
where one take does not derivative in the direction of
s in
.
We can perform the same improvements as in [
15, page 512]. We define on
some vectors fields:
(3.29)
where
(3.30)
where
have derivatives bounded at each order and
has derivative with polynomial growth.
We can consider the generator associated with these vector fields and perform the same algebraic computations as in Theorem 3.2. We get two semi-groups and and are smooth with bounded derivatives in . We get by the same procedure the following.
Theorem 3.4. If is bounded with bounded derivatives and with compact support in s, then one gets
(3.31)
where
does not include derivative in the direction of
s.
We refer to the appendix for the proof and the subsequent estimates.
Remark 3.5. Let us show from where come these identities, by using (1.4): we consider a time interval [At−, At]. On this random time interval, we do the following translation on the leading Brownian motion :
- (i)
if zs > 0 on this time interval, then is transformed in for a small parameter λ,
- (ii)
if zs < 0 on this time interval, then is transformed in for a small parameter λ.
According to the fact that f has compact support (this means that we consider bounded values of At), the transformed Brownian motion has an equivalent law through the Girsanov exponential to the original Brownian motions. The term in u in Theorem 3.2 come that from the fact we take the derivative in λ = 0 of the Girsanov exponential. When we do this transformation, we get a random process . Derivation of it in λ = 0 is done classically according to the stochastic flow theorem, which leads to the study of generators of the type and of the type 𝕃j,3.
4. Integration by Parts on the Subordinator
Let us consider diffusion type generator of the previous part:
(4.1)
Let us consider the semi-group
(4.2)
and the semi-group
(4.3)
We have classically
(4.4)
where the smooth vector fields are Lipschitz.
Therefore, we can write
(4.5)
We consider a diffeomorphsim
fλ(
s) of [0,
∞[ with bounded derivative of first order in
λ equal to
s if
s <
ϵ and equals to
s if
s > 2 (we suppose
λ small). We can write
(4.6)
We do this operation on the two operators on
ℝ1+d giving
𝔸. We get a generator
𝔸λ.
According the line of stochastic analysis, we consider a generator
𝔸λ,1 on
ℝ1+d+1. If
𝕃1 is a generator on
ℝ1+d with associated semi-group
Ps, then we consider
𝔸λ,1 the generator on
ℝ1+d+1,
(4.7)
where
Jλ(
s) is the Jacobian of the transformation
s →
fλ(
s). By doing this procedure in (
1.3), we deduce a global generator
𝔸λ,1 and a semi-group
associated with it.
It is not clear that
is a Markovian semi-group. We decompose
(4.8)
where
(4.9)
𝔸λ,1,ϵ generates a Markovian semi-group
. But
is a bounded operator on the set of bounded continuous functions on
ℝ1+d+1 endowed with the uniform norm. The Volterra expansion converges on this set:
(4.10)
Theorem 4.1 (Girsanov). For f with compact support in (s, x), one has
(4.11)
Proof. By linearity,
(4.12)
But by an elementary change of variable,
(4.13)
The result holds by the unicity of the solution of the parabolic equation associated with
𝔸. To state the integrability of
u, we refer to [
16].
Remark 4.2. Let us show from where this formula comes. In the previous part, we have done a perturbation of the leading Brownian motion . Here, we do a perturbation of ΔAs into . By standard result on Levy processes, the law of the Levy process is equivalent to the law of At. Moreover, and are independents. Therefore, the result.
Bismut’s idea to state hypoellipticity result is to take the derivative in λ of
(4.14)
in order to get an integration by parts.
First of all, let us compute in λ = 0. It is fulfilled by the next considerations. Let us consider a generator written under Hoermander’s form:
(4.15)
where
gλ are smooth and where the vector fields
Yi are smooth Lipschitz on
. We consider the semi-group
associated with it. Let us introduce the vector fields on
:
(4.16)
Let us consider the diffusion generator
(4.17)
Associated with it there is a semi-group
.
Proposition 4.3. For f smooth with compact support, one has
(4.18)
Proof. Let us introduce the vector fields on :
(4.19)
and the generator
(4.20)
Associated with it there is a semi-group
.
If the Volterra expansion converges, then
(4.21)
But
is linear in
and therefore the quantity
does not depend on
. Therefore the Volterra expansion reads
(4.22)
Let us compute
𝕃λ,1 −
𝕃λ,2. It is equal to
(4.23)
We use the relation (see [
15, Lemma 3.2])
(4.24)
and the relation
(4.25)
Therefore,
(4.26)
We insert this formula in the right-hand side of (
4.23) and we see that
satisfies the same parabolic equation as
.
Remark 4.4. Let us show from where this formula comes. Classically,
(4.27)
where
is the solution of the Stratonovitch equation starting at
x:
(4.28)
Therefore,
is solution starting at 0 of the Stratonovitch differential equation:
(4.29)
which can be solved classically by using the method of variation of constant [
4].
Let us introduce the generator on ℝ1+d+1+1+d𝔸λ,2:
(4.30)
It generates a semi-group
. In order to see that, we split the generator by keeping the values od
s〈
ϵ or
s〉
ϵ and we proceed as for
𝔸λ,1 (see (
4.10)).
We get the following.
Theorem 4.5. For f smooth with compact support in s and with derivatives of each order bounded, one has the relation if one takes only derivatives in (s0, x0) of the considered expressions:
(4.31)
Proof. We have
(4.32)
But by [
15, Lemma 3.2.]:
(4.33)
Therefore
satisfies the parabolic equation associated with
. Only the integrability of
U puts any problem. It is solved by the appendix since
f has compact support in
s.
Theorem 4.6. For f with compact support in in .
(4.34)
if
,
belong to
.
Proof. If the Volterra expansion converges, then
(4.35)
But
is linear in
and therefore the quantity
do not depend of
. Therefore the Volterra expansion reads
(4.36)
But the last term in the right-hand side of (
4.26) is equal to
by the end of the proof of the Proposition
4.3.
Remark 4.7. Analogous formula works for D exp [t𝔸]f.
Let us compute . We remark that
(4.37)
Namely, the generator of
does not act on the
u0 component such that the two sides of (
4.37) satisfy the same parabolic equality.
Therefore,
(4.38)
where
, Therefore,
(4.39)
a3(
t) in the previous expression is the only term which contains a derivative of
f, because by Proposition
4.3,
(4.40)
Let 𝔸3 be the generator on ℝ1+d+1+d:
(4.41)
It generates a semi-group,
. We get the following.
Theorem 4.8. For f with compact support in s and with bounded derivatives at each order, we have
(4.42)
Proof. If the Volterra expansion converges, then
(4.43)
But
is linear in (
u0,
U0). Let us explain the details of that. We have to compute
(4.44)
By the technique of the beginning of the proof of Proposition
4.3, it does not depend on (
u0,
U0). Therefore, the Volterra expansion reads:
(4.45)
But
(4.46)
does not depend on (
u0,
U0). Therefore, the right-hand side of formula (
4.45) is equal to
(4.47)
But
(4.48)
because
is linear in (
u0,
U0) and because the vector fields which give the generator of
are linear in
u0,
U0. Therefore,
(4.49)
But by an analog of Theorem
4.5,
(4.50)
We can summarize the previous considerations in the next theorem.
Theorem 4.9. If fλ(s) is a diffeomorphism of [0, ∞[ equal to s if s ∈ [0, ϵ[ and if s > 1, then one has the following integration by part formula if f is with compact support in s, bounded with bounded derivatives at each order:
(4.51)
where
.
Theorem 4.10. Let one suppose that fλ(s) = s + λs5 near 0. Then, (4.51) is still true.
Proof. It is enough to show that wecan approximate fλ(s) by a function equal to s if s < ϵ. Let us give some details on this approximation. We consider a smooth function g from ℝ into [0,1] equal to zero if s ≤ 1/2 and equal to 1 if s > 1. We put
(4.52)
We remark that
- (i)
if s ≤ ϵ/2, then g′(s/ϵ)s5/ϵ = 0,
- (ii)
if s > ϵ, then g′(s/ϵ)s5/ϵ = 0,
- (iii)
if s ∈ [ϵ/2, ϵ], then |g′(s/ϵ)s5/ϵ | ≤ Cs4.
is the semi-group associated with 𝔸3,ϵ where we replace in the construction of (4.41) fλ(s) by :
(4.53)
By the appendix,
(4.54)
if
h is compact support in
s. Let us consider the generator
A3,ϵ associated with
. If
g = 〈
df,
u,
U〉, then we have by Duhamel principle
(4.55)
By the proof of Theorem
4.8,
is affine in (
u0,
U0). Namely, in the proof of this theorem, we have removed the
which is equal to zero in
u0 = 0,
U0 = 0 because this expression is linear in
u0,
U0. Its component in (
u0,
U0) is smooth with bounded derivatives at each order. By Theorem
4.6,
is still affine in (
u0,
U0) and its components in (
u0,
U0) are smooth with bounded derivatives at each order. Moreover, if
g1 is affine in (
u0,
U0) with components in (
u0,
U0) smooth with bounded derivatives at each order, then we get that, for
s ≤ 1,
(4.56)
where
C(
ϵ) → 0 when
ϵ → 0. This can be seen as an appliation of the Duhamel formula applied to the two semi-groups
and
. Then, the result arises from the Duhamel formula (
4.55).
We can consider vector fields at the manner of (3.30) and fλ(s) = s + λs5 in a neighborhood of 0. We get a generator 𝔸tot and semi-groups and . We have with the extension of Theorem 4.10 the following.
Theorem 4.11 (Bismut). If fλ(s) = s + λs5 in a neighborhood of 0 and is equal to 1 if s > 1, then one has the following integration by parts: let ftot be a function with compact support in s, bounded with bounded derivatives at each order. Then,
(4.57)
5. The Abstract Theorem
The proof of Theorem
2.1 follows the idea of Malliavin [
5]. If there exist
Cl such that, for function
f with compact support in [0,1] × [0,
l]
d,
(5.1)
then the heat kernel
qt(
s,
y) exists.
There are two partial derivatives to treat:
- (i)
the partial derivative in the time of the subordinator s,
- (ii)
the partial derivatives in the space of the underlying diffusion x.
Let us begin by the most original part of Bismut′s Calculus on boundary process, that is, the integration by parts in the time s.
We look at (
4.42). We remark (see the next part) that
(5.2)
for all
p. So, we take
ftot(
s,
x,
u) =
f(
s,
x)1/
u and we apply (
4.42) for this convenient semi-group. We get
(5.3)
R can be estimated by using the appendix by
Cl∥
f∥
∞ for
f with compact support in [0,
l] × [0,
l]
d and by (
5.2).
Lemma 5.1. For a conveniently enlarged semi-group in the manner of Theorem 3.4, one has for f with compact support in s
(5.4)
Proof. If is a function with compact support depending only of s, xtot and V, we have
(5.5)
We do the change of variable
U →
U and
V →
UV on the Malliavin generator
. By using Lemma 3.7 of [
15], it is transformed in
where for
𝔸2,tot we consider the same type of operator as
𝔸2 but with the modified vector fields:
(5.6)
It remains to use the appendix to show the Lemma.
We consider
. By the previous Lemma and Malliavin hypothesis,
(5.7)
for all
p if
g(
s) has compact support (
V is a matrix). After we consider a test function of the type of Bismut, we consider the component
ui of
U in (
5.3). We consider the Bismut function
fV−1(
ui/
u). We integrate by parts as in Theorem
3.4. We deduce under Malliavin assumption that
(5.8)
if
f has compact support in [0,
l] × [0,
l]
d.
By the same way, we deduce that if
f has compact support in [0,
l] × [0,
l]
dthen
(5.9)
Therefore, the result is obtained.
Remark 5.2. We could do integration by parts to each order in order to show that the semi-group exp [t𝔸] has a smooth heat-kernel under Malliavin assumption.
6. Inversion of the Malliavin Matrix
Proof of Theorem 2.2. Let s1 < s2 and let ξ be of modulus 1. Then,
(6.1)
These two quantities are equal in
t = 0 when we consider the semi-group
. Let us compute their derivative in time
t. The derivative of the left-hand side is bigger than the derivative of the right-hand side because
(6.2)
(These two quantities are negative.)
By the result of the appendix,
(6.3)
for all
p.
Lemma 6.1. If |ξ| = 1, then there exist C and C0 independent of ξ such that
(6.4)
Proof. We consider a convex function decreasing from [0, ∞[ into [0,1] equal to 1 in 0 and tending to 0 at infinity. Let us introduce
(6.5)
In order to consider the derivative in
s of
αs, we study the expression
(6.6)
We have only to consider by (
6.3) the case where
s′ is small enough, |
x′ −
x| is small enough, |
U −
I| is small enough, and the positive matrix
V′ is small enough. For that we have to estimate
(6.7)
for
u between 0 and
ϵ. The first derivative of
γu has an equivalent −
Cϵ−1 when
ϵ → 0, and its second derivative has a bound
Cϵ−2 when
ϵ → 0. Therefore,
(6.8)
on [0,
ϵ] and
(6.9)
We deduce from that that
(6.10)
Remark 6.2. We could improve (6.4) by showing that
(6.11)
if
s′ is small enough, |
x′ −
x| is small enough, |
U′ −
I| is small enough, and the positive matrix
V′ is small enough.
We consider a very small α. We slice the time interval [0, ϵα] in ϵα−1 intervals of length ϵ. We have
(6.12)
for a small
C0. This last quantity is smaller than
Cϵp for all
p by the previous lemma if
α is small enough. The proof of Theorem
2.2 follows from
(6.13)
for all
p by using the result of the appendix. The result follows by standard methods (see [
15, Equations (4.8) and (4.9)].
It remains to show the following.
Theorem 6.3. For all p > 0,
(6.14)
Proof. We remark that if we consider only functions of u, then
(6.15)
where
is a Lévy semi-group with generator
(6.16)
where
g(
s) = 1 on a neighborhood of 0, is with compact support and is positive. The result follows from the adaptation in [
17,
18] of the proof of [
7] in semi-group theory. We remark that
(6.17)
By using the adaptation in semi-group theory of the exponential martingales of Levy process of [
17,
18], we have
(6.18)
The result holds from the Tauberian theorem of [
7,
17,
18].