Is there a perfect square n^2 such that
n^2 = -1 (mod p) for
Can you characterize the primes for which n^2 = -1 (mod p) has a solution?
Sol: If a = b (mod k), then b = a + mk, where m is an integer.
Here, n^2 = -1 (mod p) implies that -1 = n^2 + mp, where m is an ...