I have a puzzling question about this statement : consider four numbers $a,b,c,d \in \Bbb N$, $d\ \notin \Bbb N^2$, is it possible to state that this relation : $d=a^2 + b^2 -c^2$ is always true ? So that it is possible to write that number $d$ in that way ?
(please note the sign minus before the $c^2$)
Thanks guys !