I have to prove that f(n) = 2n is not an onto function when the domain and co-domain are a set of all of the integers.

I graph it and see it isn't an onto function. However, everytime I do the math, it says it is. Can someone step through how to solve this one for me?

This is what I get.

First substitute m for f(n)

m = 2m

Solve for one side

(2/m) = m

If f(n) = m then,

f(n) = f(2/m) = 2(2/m) = m

therefore, f(n) = m.

What am I doing wrong. Thanks...