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I have ## 120 \sum \limits_1^\infty (\sigma_{3}(n))^{2} ## , where ## \sigma_{3}(n) ## is a divisor function.

And I want to show that this can be written as ##120\sum \limits_{k=1}^{n-1} \sigma_{3}(k) \sigma_{3}(n-k) ##

I'm pretty stuck on ideas starting of to be honest, since the sum is infinite, any help much appreciated.

Many thanks in advance.