Series of product

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Assuming that you have a series of a product $\sum_{l=0}^{\infty} f(l) g(l)$ and you know what $\sum_{l=0}^{\infty} f(l) $ is. Does this help, finding an approximate form for the whole series?

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No, let $f(l)=1/(l^2)$, then your summation doesn't converge for $g(l)=l$, but it does for $g(l)=1$.

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No. For example $f(\ell)=\dfrac{1}{\ell^2}$ and $g(1)=$anything, $g(\ell)=0, \; \forall \; \ell>1$.
Then $\sum_{\ell=0}^\infty f(\ell)g(\ell)$ can be anything.