necessary and sufficient conditions for a normed vector space to be a Banach space
Theorem 1 - Let be a normed vector space. is a Banach space
if and only if every absolutelyconvergent series in is convergent, i.e., whenever converges in .
Theorem 2 - Let be normed vector spaces, . Let be the space of bounded operators . Then is a Banach space if and only if is a Banach space.