# Measure theory problem

hellbike
let $$f_n$$ be series of borel functions. Explain why set B = {x: $$\sum_n f_n(x)$$ is not convergent} is borel set.

Proof, that if$$\int_R |F_n|dY \leq 1/n^2$$ for every n then Y(B) = 0.

Y is lebesgue measure.

for first part i thought that set of A={x: convergent} is borel, and B=X\A so it's also borel, but i got 0 points, so i'm wrong.

for second part - it seems quite obvious for me that for every $$x \neq 0$$
$$lim_n Y(f^{-1}_{n}[x])->0$$ and i think proving this would be enough.
I tried doing this using simple functions, but got 0 points.

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## Answers and Replies

ystael
For the first part, the problem is likely that you didn't show why the set of points where $$(f_n)$$ converges should be a Borel set. It's not completely obvious.

hellbike
and for the second part?