Let be an odd prime, be a positive number such that (i.e., does not divide), and let be one of the numbers 1, 2, 3, ..., . Then there is a unique , called the associate of , such that
with (Hardy and Wright 1979, p. 67). If , then is called a quadratic residue of .
be an odd prime, 
be a positive number such that 
(i.e., 
does not divide 
), and let 
be one of the numbers 1, 2, 3, ..., 
. Then there is a unique 
, called the associate of 
, such that 
(Hardy and Wright 1979, p. 67). If 
, then 
is called a quadratic residue of 
. 