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If [itex]\ast : (f \ast g)(n) = \sum\limits_{d|n}f(d)g(\frac{n}{d})[/itex], show that [itex]\ast[/itex] is commutative. Note that [itex]d|n[/itex] says [itex]d[/itex] divides [itex]n[/itex]. Now I was not sure how to do this from an abstract algebra point of view although when I stare at it my though process was to maybe rewrite it somehow, which will then be easier to show its commutative.

## Homework Statement

## Homework Equations

## The Attempt at a Solution

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