Approximation of borel sets from the top with closed sets.
Hello,
I have this problem that I don't know how to approach.
Suppose we have a topological space
.
Consider the Borel
-algebra
, generated by the open of T.
Fix a probability measure )
It is known that for every
, and for every
, there exist an open set
and a closed set
,
such that
In other words, every Borel set
, can be approximated up-to an arbitrary
from the top (with an open set) and from the bottom (with a closed set).
What I'm trying to understand if also the contrary is possible, i.e. it is also possible to approximate any
from the top with a CLOSED set, and from the bottom with an OPEN.
More formally I wonder if for every
, and for every
, there exist an open set
and a closed set
,
such that
Any suggestions?
Thank you in advance,
Matteo