# Freaky Function!

Algebra Level 3

A function $$f: \mathbb{N} \rightarrow \mathbb{N}$$ is such that

1) $$f(xy)= f(x)f(y)$$

2) The function is strictly increasing.

3) $$f(2)=2$$

Find $$\sum _{ k=1 }^{ 20 }{ f(k) }$$

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