On the Ultrafilter Semigroup of an Abelian Topological Group

The ultrafilter semigroup of a topological group G, denoted Ult(G), consists of all nonprincipal ultrafilters on G converging to the identity and is a closed subsemigroup in the Stone-Cech compactification β G_d of G as a discrete semigroup. We show that for every countable nondiscrete Abelian topological group G not containing an open Boolean subgroup, the structural group of the smallest ideal of Ult(G) has cardinality 2^c.