Optimal transport, Euler equations, Mather and DiPerna-Lions theories

1.5. THE (ANISOTROPIC) ISOPERIMETRIC INEQUALITY 21
1.5.1 Stability of isoperimetric problems
A quantitative version of the anisotropic
Whenever 0 < |E| < ∞, we introduce the isoperimetric deficit of E,
δ(E) :=
PK(E)
n|K| 1/n − 1 .
|E| (n−1)/n
This functional is invariant under translations, dilations and modifications on a set of measure
zero of E. Moreover, δ(E) = 0 if and only if, modulo these operations, E is equal to K (as
a consequence of the characterization of equality cases of isoperimetric inequality). Thus δ(E)
measures, in terms of the relative size of the perimeter and of the measure of E, the deviation of
E itself from being optimal in (1.5.3). The stability problem consists in quantitatively relating
this deviation to a more direct notion of distance from the family of optimal sets. To this end
we introduce the asymmetry index of E,
A(E) := inf
x∈Rn
|E∆(x + rK)|
|E|
: r n
|K| = |E| ,
where E∆F denotes the symmetric difference between the sets E and F . The asymmetry is
invariant under the same operations that leave the deficit unchanged. We look for constants C
and α, depending on n and K only, such that the following quantitative form of (1.5.3) holds
true:
PK(E) ≥ n|K| 1/n |E| (n−1)/n
1 +
A(E)
α
, (1.5.8)
C
i.e. A(E) ≤ C δ(E) 1/α . This problem has been thoroughly studied in the Euclidean case K = B,
starting from the two dimensional case, already considered by Bernstein [33] and Bonnesen [35].
They prove (1.5.8) with the exponent α = 2, that is optimal concerning the decay rate at zero of
the asymmetry in terms of the deficit. Concerning the higher dimensional case, it was recently
shown in [54] that (1.5.8) holds with the sharp exponent α = 2.
The main technique behind these proofs is to use quantitative symmetrization inequalities,
that of course reveal useful due to the complete symmetry of B. However, if K is a generic convex
set, then the study of uniqueness and stability for the corresponding isoperimetric inequality
requires the employment of different ideas. The first stability result for (1.5.3) is due to Esposito,
Fusco and Trombetti in [53] with some constant C = C(n, K) and for the exponent
α(2) = 9
n(n + 1)
, α(n) = , n ≥ 3.
2 2
This remarkable result leaves however the space for a substantial improvement concerning the
decay rate at zero of the asymmetry index in terms of the isoperimetric deficit. In collaboration
with Francesco Maggi and Aldo Pratelli, we could indeed prove the result with the sharp decay
rate [19]: