Codensity, profiniteness and algebras of semiring-valued measures

We show that, if S is a finite semiring, then the free profinite S-semimodule on a Boolean Stone space X is isomorphic to the algebra of all S-valued measures on X, which are finitely additive maps from the Boolean algebra of clopens of X to S. These algebras naturally appear in the logic approach to formal languages as well as in idempotent analysis. Whenever S is a (pro)finite idempotent semiring, the S-valued measures are all given uniquely by continuous density functions. This generalises the classical representation of the Vietoris hyperspace of a Boolean Stone space in terms of continuous functions into the Sierpinski space. We adopt a categorical approach to profinite algebra which is based on profinite monads. The latter were first introduced by Adamek et al. as a special case of the notion of codensity monads.Comment: 21 pages. Presentation improved. To appear in the Journal of Pure and Applied Algebr