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Homework Statement
Suppose [tex]\{a_n\}[/tex] is a sequence of positive numbers such that [tex]\sum_na_nb_n < \infty[/tex] whenever [tex]b_n \ge 0[/tex] and [tex]\sum_nb_n^2 < \infty[/tex]. Prove that [tex]\sum_na_n^2 < \infty[/tex].
Homework Equations
There's a suggestion in the text:
If [tex]\sum_na_n^2 = \infty[/tex], then there are disjoint sets [tex]E_k \, (k=1,2,3,\dots)[/tex] so that
[tex]\[\sum_{n\in E_k}a_n^2 > 1.\][/tex]
Define [tex]b_n[/tex] so that [tex]b_n = c_ka_n[/tex] for [tex]n \in E_k[/tex]. For suitably chosen [tex]c_k[/tex], [tex]\sum_na_nb_n = \infty[/tex] although [tex]\sum_nb_n^2 < \infty.[/tex]
The Attempt at a Solution
Using the hint, we've got
[tex]
\[b_n^2 = c_k^2a_n^2; \qquad\qquad a_nb_n = c_ka_n^2\]
\begin{align*}
S1 = \sum_n b_n^2 &= \sum_{n\in E_1} c_1^2 a_n^2 + \sum_{n\in E_2} c_2^2 a_n^2 + \dots \\
&= \sum_k \sum_{n_k\in E_k} c_k^2 a_{n_k}^2 < \infty\\
S2 = \sum_n a_n b_n &= \sum_k \sum_{n_k\in E_k} c_k a_{n_k}^2 = \infty.
\end{align*}
[/tex]
From this, it seems clear that we want to choose the [tex]c_k[/tex] to be "sufficiently dampening" so that [tex]S1[/tex] really is finite but [tex]S2[/tex] is not. I'm not entirely sure how to go about choosing the [tex]c_k[/tex] for arbitrary [tex]a_n[/tex] and [tex]b_n[/tex]. Thanks very much, any hints would be greatly appreciated! Sorry about the formatting, I'm new here..