According to a source online, the method to prove a statement $P(m,n)$ to be true the base case is to show that both $$P(1,n) \ \ P(m,1) $$ hold for all $m,n \in \mathbb N$. The inductive step is that if both $$P(m+1,n) \ \ P(m,n+1)$$ hare assumed to hold then $P(m+1,n+1)$ holds for all $m,n \in \mathbb N$.
So if my $P(1,n) \ \ P(m,1)$ imply that only odd integer solutions exist and if $P(m+1,n) \ \ P(m,n+1)$ also implies that only odd integer solutions exist and that since they are odd intger solutions then $P(m+1,n+1)$ holds. But I'm not sure what would imply that only odd integer solutions exist for $P(m+1,n+1)$