The square of a number not divisible by is congruent to modulo (prove this). And no square is congurent to modulo (prove this also)

Suppose and are not divisible by , then the left hand side of:

is congruent to modulo while the right hand side cannot be congurent to modulo , a contradiction, hence at least one of and is divisible by .

If both and are divisible then the left hand side is divisible by but the right hand side cannot be divisivle by because is a primitive Pythagorean triple and so share no common factor. This is a contradiction, hence both and cannot be divisible by .

Hence exactly one of is divisible by .

(note we have not used the fact that is even)

CB