Is this an onto function?

Question

Is f(m,n) = m^2 - 4 an onto function for a function that goes from Z x Z -> Z (Where Z means the set of all integers).
I think it is an onto function, but I am not sure how to go about proving it. Can I set an integer y to = m^2 - 4 and then show that if I set n = 0, it will work...?

(Sorry for any formatting errors).

Answer 1

It is not an onto function.

$$f(m,n) \geq -4$$

Hence there is no $(m,n)$ such that $f(m,n)=-5$ but $-5 \in \mathbb{Z}$