Let the cardinal number of N, the set of all natural numbers, be
א0. Prove that the product set N × N = {(m,n);m ∈ N,n ∈ N} has the
same cardinal number. Further prove that Q+, the set of all
positive rational numbers, has the cardinal number N_0.
Hint: You may use the formula 2^(m−1)(2n − 1) to define a
function from N × N to N, see the third example on page 214 of the
textbook.