Tuesday, 15 January 2019

sequences and series - Convergent sum of reciprocals?

Let n denote a positive integer and let σ(n) denote the sum
of all divisors of n, so that σ(n) is larger than n (for n>1)
but not by much since it's bounded above by c nloglogn .



This invites comparison between situations in which the two are
interchanged. As an example, consider the following:



Let A be a subset of the positive integers. Suppose that
the sum of 1/n , for all n in A , converges. Then the sum

of 1/σ(n) , over all n in A, clearly converges as well.



Is the converse true?

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