# prove this

• Feb 8th 2010, 07:59 PM
nzm88
prove this
if An>=0 and summation(An) converge, then summation ((An)^2) also converge.please...
• Feb 8th 2010, 08:12 PM
Drexel28
Quote:

Originally Posted by nzm88
if An>=0 and summation(An) converge, then summation ((An)^2) also converge.please...

This is a corollary of a more comprehensive theorem. If $\displaystyle \sum_{n\in\mathbb{N}}a_n$ converges absolutely then $\displaystyle \sum_{n\in\mathbb{N}}a_n^2$ converges as well.

To see this merely note that since $\displaystyle a_n\to0$ there exists some $\displaystyle N\in\mathbb{N}$ such that $\displaystyle N\leqslant n\implies |a_n|<1\implies a_n^2<|a_n|$ and so $\displaystyle \sum_{j=N}^{\infty}a_n^2<\sum_{j=N}^{\infty}|a_n|$ and the conclusion follows.
• Feb 8th 2010, 08:12 PM
Bruno J.
Well from some point on, say for $\displaystyle n>n_0$, we will have $\displaystyle |A_n|<1$. But then $\displaystyle A_n^2<|A_n|$; so just compare the tails of the two series.
• Feb 8th 2010, 08:13 PM
Bruno J.
Great minds think alike (Nerd)
• Feb 8th 2010, 08:14 PM
Drexel28
Quote:

Originally Posted by Bruno J.
Great minds think alike (Nerd)

Agreed. We'll probably be in a think-tank someday together. We can then go to bars and try to impress women with our bridling intellect and manish good looks.
• Feb 8th 2010, 08:20 PM
Bruno J.
It'd be an honour to wing-man you! (Rock)