# Show that f(n)=gcd(a,n)f(n) = \gcd(a,n) is a multiplicative function [on hold]

I want to show that f(n)=gcd(a,n)f(n) = \gcd(a,n) where a is any natural number, is a multiplicative function.

I know I need to show that f(mn)=f(n)∗f(m)f(mn)=f(n)*f(m), but I do not know how to do this.

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