We will prove that this expression is of the form 4k+3 and hence not a perfect square.

is a multiple of 4.
Hence we only need to prove that the following expression is a multiple of 4:
So, we only have to prove that for all n

is divisible by 4.
If n is a multiple of 4, we are done. Else we write the expression as

We know that if n is not even, then n2-1 is divisible by 4 and hence in this case too the expression is a multiple of 4.
Thus, the polynomial is of the form 4k+3 for all n, and therefore not a perfect square