Let μ be a probability measure on a locally convex Hausdorff space
E and
A(μ) be the quasi-invariant set of μ. If μ
*(
A(μ))>0, then there exist a finite-dimensional subspace
L, a thick subgroup
G of
L and a countable subgroup {
xi} such that
A(μ)=∪\limits
i=1∞(
G+
xi). If
E is Souslin, then
A(μ) is a Borel subset. If
E is Souslin and if μ (
A(μ))>0, then
A(μ)=∪\limits
i=1∞(
L+
xi).
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