Abstract
In this note, we extend the regularity theory for monotone measure-preserving maps, also known as optimal transports for the quadratic cost optimal transport problem, to the case when the support of the target measure is an arbitrary convex domain and, on the low-regularity end, between domains carrying certain invariant measures.
1 Introduction
Given a pair of atom-free Borel probability measures
is measure preserving, i.e.,
In particular,
When
(Brenier’s theorem guaranteed the same conclusion as McCann’s theorem, but under some restrictive technical conditions on
The first general regularity result on these Brenier-McCann maps was proved by Caffarelli in [4]: provided that
Unfortunately, Caffarelli’s boundedness assumptions on the domains
1.1 Results
Our main theorem is an extension of Caffarelli’s theorem, on the strict convexity of
Definition 1.1
A nonnegative measure
for all ellipsoids
This notion of doubling was introduced by Jhaveri and Savin in [11].[2] That said, the first consideration of measures with a “doubling-like” property in the world of solutions to Monge-Ampère equations can be traced back to the work of Jerison [9] and then Caffarelli [3]. In particular, in [3], Caffarelli showed that Alexandrov solutions to
where the measure
We now state our main theorem.
Theorem 1.2
Let
Remark 1.3
It is well known that
Remark 1.4
While the target measure
Remark 1.5
A simple case to which Theorem 1.2 applies, but the corresponding results in [4, 5] do not, is when
With our main theorem in hand, our second and third theorems further extend the known regularity theory for monotone measure-preserving maps, completing the story started by Cordero-Erausquin and Figalli in [5] on monotone transports between unbounded domains.
Theorem 1.6
Let
Theorem 1.7
Let f and g be two functions on
Remark 1.8
We note that the proof of the Theorem 1.7, given the strict convexity of
1.2 Structure
This remainder of this article is structured as follows.
In Section 2, we prove Theorem 1.2. Our proof is self-contained apart from some facts in convex analysis; we provide explicit references to these used but unproved facts. We remark that our proof is inspired by the proof of the Alexandrov maximum principle in [11] (and, of course, Caffarelli’s original proof of the strict convexity of potential functions of optimal transports/solutions to Monge-Ampère equations). If the reader is familiar with [5] or [4], then they might consider directing their attention to Case 2. Case 2b is completely novel. Case 2a illustrates our argument in the setting of [4], which builds on the work of [2] and is the foundation for Case 2b.
Section 3 is dedicated to the proof of Theorem 1.6. Our proof here is similarly self-contained (and an adaptation of Caffarelli’s argument of the same result in [2], but, of course, using the line of reasoning developed to prove Theorem 1.2). The Hölder regularity of
2 Proof of Theorem 1.2
Before we begin, it will be convenient to replace the potential
Observe that
We shall denote the domain of
Let
and
is closed, as
and, because
(A proof of (2.1) can be found in [5].)
Recall that an exposed point
Lemma 2.1
(Mass balance formula) Let
Remark 2.2
The mass balance formula was originally proved for measures that are absolutely continuous with respect to Lebesgue measure [13, Lemma 4.6]. However, with respect to absolute continuity, the proof only relies on the measures in question not giving mass to the set of nondifferentiable points of a convex function. As observed in Section 1, such points are contained in a countable union of Lipschitz
Finally, recall that if a nonnegative measure is locally doubling (on ellipsoids), then it is locally doubling on all bounded convex domains [11, Corollary 2.5]. After this preliminary discussion, we can now prove our main theorem.
Proof of Theorem 1.2
Proving that
Case 1.
Case 2.
Since
for some
Case 2a:
for some
and define
Recalling that
In particular, if we let
then, by construction,
and
Now, let
Also, let
Notice that, by construction,
Also, if
and
Let
where
Then
and
Recall that affine transformations preserve the ratio of the distances between parallel planes; therefore, letting
In turn,
and considering the cone generated by
(For more details on this inclusion, see, e.g., [7, Theorem 2.8].) So if we let
then
On the other hand, since
In turn, for
Therefore, if we define
then there exists a dimensional constant
Also, by, for example, [7, Corollary A.23],
Thus, for all
where
for all
Now, let
By construction,
Hence, since
with
or, equivalently,
But this is impossible for small
Case 2b:
Like before, for all
for some
The function
for all
Now notice that
Moreover, we claim there exists an
Indeed, if not, then we can find a sequence of points
Therefore,
And so, arguing exactly like we did in Case 2a, we find that
where
Case 3.
Like before, let
Again,
for all
where the second equality follows from the mass balance formula. (Recall that
(cf. (2.1)). Thus, any open subset of
(Again, for more details on this inclusion, see, e.g., [7, Theorem 2.8].) This is a contradiction and concludes the proof.□
3 Proof of Theorem 1.6
Again, we replace
Part 1.
u
is continuously differentiable inside
X
.
We follow the argument used to prove [2, Corollary 1]. Assume for the sake of a contradiction that the result is false. Up to a translation, let
Now consider the function
Note that
then, by the strict convexity of
for some
for some
provided
and
where
Now, set
where
(cf. (2.6)), where
This proves that
By [7, Lemma A.24], for example, we know that differentiable convex functions are continuously differentiable. So we conclude that
Part 2.
∇
u
(
X
)
=
Y
when
X
is convex.
Because
Part 3.
∇
u
is locally Hölder continuous inside
X
.
Thanks to the strict convexity and
More precisely, if
-
Funding information: AF acknowledges the support of the ERC (Grant No. 721675 “Regularity and Stability in Partial Differential Equations (RSPDE)”) and the Lagrange Mathematics and Computation Research Center. YJ was supported in part by the NSF (Grant No. DMS-1954363).
-
Conflict of interest: The authors state no conflict of interest.
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