Real Analysis Show that the Series a k from k=1 to infinity converges if and only if given Epsilon>0 there exists N in the Natural numbers such that Absolute value (Series a k from k=m+1 to n) m>=N) I...

Real Analysis

Show that the Series a_k
from k=1 to infinity converges if and only if given Epsilon>0 there exists N in the Natural numbers such that

Absolute value (Series a_k
from k=m+1 to n) m>=N)

I am told that this is proving the Caucy criterion for series.

Please help.