A note on Kelso and Crawford's gross substitutes condition

In their 1982 article, Kelso and Crawford proposed a gross substitutes condition for the existence of core (and equilibrium) in a two-sided matching model. Since then, this condition has often been used in the literature on matching models and equilibrium models in the presence of indivisibilities. In this paper we prove that a reservation value (or utility) function satisfies the gross substitutes condition if and only if it is an M*-concave function defined on the unit-hypercube, which is a discrete concave function recently introduced by Murota and Shioura.