1. ## Banach space

prove that, in a Banach space if $Sum||Xn||$ converges then $SumXn$ converges

2. ## Banach space

3. prove that , in a Banach space if $\sum^{\infty}_{n=1}||Xn||$ converges then $\sum^{\infty}_{n=1}Xn$ converges.

4. Originally Posted by cizzzi
prove that , in a Banach space if $\sum^{\infty}_{n=1}||Xn||$ converges then $\sum^{\infty}_{n=1}Xn$ converges.
Show that the sum is Cauchy, i.e. consider $S_k:=\sum_{n=1}^k X_n$, now consider $||S_k-S_l||$.

Hint: If $\sum_{n=1}^\infty x_n < \infty$ then $\sum_{n=k}^\infty x_n \rightarrow 0$ as k goes to infinity. (A fact that you can prove using the fact that $x_n \rightarrow 0$).

5. thank you I try to solve but I do not know very well functional analysis

6. Originally Posted by cizzzi
thank you I try to solve but I do not know very well functional analysis
Why don't you post what you have done so far (even if it is wrong)?

7. if X is complete and $\sum^{\infty}_{n=1}||X_n||<{\infty}$ then sequence $S_{k}=\sum^{k}_{n=1}X_{n}$ for $k\epsilon\aleph$ is Cauchy because for k>m
$||S_{k}-S_{m}||\leq\sum^{k}_{n=m+1}||X_{n}||\rightarrow0$ as $m,k\rightarrow0$
therefore , $S=\sum^{\infty}_{n=1}X_{n}= lim_{k\rightarrow\infty}\sum^{k}_{n=1}X_{n}$ exists in X.

is it true?

8. Originally Posted by cizzzi
if X is complete and $\sum^{\infty}_{n=1}||X_n||<{\infty}$ then sequence $S_{k}=\sum^{k}_{n=1}X_{n}$ for $k\epsilon\aleph$ is Cauchy because for k>m
$||S_{k}-S_{m}||\leq\sum^{k}_{n=m+1}||X_{n}||\rightarrow0$ as $m,k\rightarrow0$
therefore , $S=\sum^{\infty}_{n=1}X_{n}= lim_{k\rightarrow\infty}\sum^{k}_{n=1}X_{n}$ exists in X.

is it true?
The sum converges to zero as k and m tend to infinity (not zero).