Order question

From: Peter Fairbrother (zenadsl6186_at_zen.co.uk)
Date: 04/29/04 

Date: Thu, 29 Apr 2004 20:33:45 +0100

In general, the exponents of a prime p contains a unique subgroup of order
x, where x is a divisor of phi(p).

A safe prime p=2q+1 has a unique subgroup of order q. This is either the
unique subgroup of quadratic residues or the subgroup of non-residues, but
which one?

Take 11 = 2*5+1 as an example. 1,3,4,5,and 9 are quadratic residues, and
2,6,7,8 and 10 are non-residues. Both groups have five members.

Is 0 a member of the first or second set? Is 0 a QR or not?

Or is the generalisation not true for x=2 ?