Necessary and sufficient conditions for a probability measure Ρ defined on a product Χ χ Y of Polish spaces to be extremal in the set of the measures with same marginals as Ρ are found. The paper presents results that extend that by Lindenstrauss (1965), Letac (1966), Denny (1980 and Benes, Stepan (1987, 1990.
IntroductionLet X be a Polish space. We shall denote by M(X) (Λίι(Χ)) the set of bounded Borel signed (probability) measures defined on X. Having a pair of Polish spaces Χ, Y and a measure m £ M(X x Y) we denote by m x and m y the projections of τη to X and Y, respectively (the marginal measures). Putting MARG(m) = (m"m y ) we define a map M(X χ Υ) -» M{X) X M(Y). We shall say that Ρ £ M^X χ Y) is a simplicial measure if it is an extremal point in C(P) = {Q e M,{X χ Y): MARG{Q) = MARG(P)}. A Borel set D C X x Y will be called a set of marginal uniqueness or an MU-set if MARG: Mi(D) Μι(Χ) χ M X {Y) is an injection. Denote by 5, MU, MUC, the set of simplicial measures, the family of all MU-sets and the family of all compact MU-sets, respectively.The purpose of the present paper is to discuss topological and measure-theoretical properties of sets that support a simplicial measure. Up to now only the case of at most countable X and. Y has been solved in a satisfactory manner. A brief summary to this research (Letac (1966), Diego, Germani (1972), Denny (1980, Mukerjee (1985) reads as follows:1.1 Suppose that the spaces X and Y are at most countable. Then: