Hi!

Can any one make this a little bit more clear please?

Let be a metric space, and a probability measure on borel sets of E.

I do not understand the next implication:

Since

then

for (at most) a numerable number of

is the topological boundary

I think i may be missing a "core" property of finite measures but i dont remember!

Thanx in advance

EDIT: Bad title i can't fix this sorry