We generalize Pym’s
decomposition w = μE∗ wH∗ μF of idempotent probability measures to the
decomposition μE∗ℋ(wH) ∗ μF of the maximal groups of units in semigroup of
probability measures on a compact semitopological semigroup. We also prove
that ℋ(w)≅ℋ(wH)≅N(H)∕H algebraically and topologically. With these
characterizations, we verify Rosenblatt’s necessary and sufficient condition for the
convergence of a convolution sequence (νn)n≧1 of a probability measure ν on a
compact topological semigroup.