Kenneth Rosen Edition 7 Exercise 2.5 Question 31 (Page No. 177)

Question

Show that $Z^{+} \times Z^{+}$ is countable by showing that the polynomial function $f : Z^{+} \times Z^{+}\rightarrow Z^{+}$ with $f(m, n) = \dfrac{(m + n − 2)(m + n − 1)}{2} + m$ is one-to one and onto.