Course notes (chap. 1 Number Theory, chap. 2 ... - McGill University

1.4 Quadratic Residues
Quadratic residues modulo n are the integers with an integer square root
modulo n (Maple quadres):
QR n = {a :gcd(a, n) =1, ∃r[a ≡ r 2
QNR n = {a :gcd(a, n) =1, ∀r[a ≢ r 2
(mod n)]}
(mod n)]}
Example:
since
QR 17 = {1, 2, 4, 8, 9, 13, 15, 16}
QNR 17 = {3, 5, 6, 7, 10, 11, 12, 14}
{1 2 , 2 2 , 3 2 , 4 2 , 5 2 , 6 2 , 7 2 , 8 2 , 9 2 , 10 2 , 11 2 , 12 2 , 13 2 , 14 2 , 15 2 , 16 2 }≡
{1, 2, 4, 8, 9, 13, 15, 16} (mod 17).
Theorem 1.2 Let p be an odd prime number
#QR p =#QNR p =(p − 1)/2.