Question

prove that....

Answer 1

Let \[\sum_{n=0}^{\infty}a_n\]
be not absolutely convergent, and L be any real number. Then there exists a bijection:
\[f:N \rightarrow N\]
such that \[\sum_{m=0}^{\infty} a_{f(m)}\] converges to L

Answer 2

or perhaps any site? its a real analysis optional question