# borel measurability

• May 27th 2008, 12:18 AM
squarerootof2
borel measurability
if (S,d) is a metric space and B(d) is the sigma algebra on S generated by all open sets, how would i prove that every continuous function f:S->R is (B(d),B(R))-measurable?
• May 27th 2008, 09:22 AM
Opalg
Quote:

Originally Posted by squarerootof2
if (S,d) is a metric space and B(d) is the sigma algebra on S generated by all open sets, how would i prove that every continuous function f:S->R is (B(d),B(R))-measurable?

If U is open in R, then $f^{-1}(U)$ is open, hence Borel, in S. Now suppose that $V_n\ (n=1,2,3,\ldots)$ are sets in R such that $f^{-1}(V_n)$ is Borel for each n. Then $f^{-1}\Bigl(\bigcup_nV_n\Bigr) = \bigcup_nf^{-1}(V_n)$, which is a countable union of Borel sets and in therefore Borel in S. By a similar argument, $f^{-1}\Bigl(\bigcap_nV_n\Bigr)$ is Borel.

That shows that the class of subsets V ⊆ R for which $f^{-1}(V)$ is Borel in S is a σ-algebra containing all the open sets in R. Therefore this class contains all the Borel sets in R. Thus f is Borel measurable.