Define an arithmetic function

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Do you need something like the following?

def f_k(k):
    return 2^gcd(4,k) * 3^gcd(2,k)

print(f_k(720))

f_k(720)= 2^gcd(4,k) * 3^gcd(2,k) is not a function definition. Please define what is f_k(n).