Ising systems, measures on the sphere, and zonoids
Résumé
We give an interpretation of a class of discrete-to-continuum results for Ising systems
using the theory of zonoids. We define the classes of rational zonotopes and zonoids, as
those of the Wulff shapes of perimeters obtained as limits of finite-range homogeneous
Ising systems and of general homogeneous Ising systems, respectively. Thanks to the
characterization of zonoids in terms of measures on the sphere, rational zonotopes,
identified as finite sums of Dirac masses, are dense in the class of all zonoids. Moreover,
we show that a rational zonoid can be obtained from a coercive Ising system if and
only if the corresponding measure satisfies some ‘connectedness’ properties, while it is
always a continuum limit of ‘discrete Wulff shapes’ under the only condition that the
support of the measure spans the whole space. Finally, we highlight the connection
with the homogenization of periodic Ising systems and propose a generalized definition
of rational zonotope of order N , which coincides with the definition of rational zonotope
if N=1.