Cauchy_Criterion
Theorem
A sequence converges if and only if it is a Cauchy_sequence.
Proof
$\left. ( \Rightarrow ) \right.$:
$\left. ( \Leftarrow ) \right.$:
A sequence converges if and only if it is a Cauchy_sequence.
$\left. ( \Rightarrow ) \right.$:
$\left. ( \Leftarrow ) \right.$: