quadratic non residue

**scubasteve123** · November 12th 2009, 10:28 PM

Show that the smallest quadratic non residue of an odd prime p is itself prime. Hint: Assume to the contrary and use Legendre Symbols

**chiph588@** · November 13th 2009, 09:36 AM

Assume not so let $n$ be the smallest QNR such that $n=ab$ . Then $-1=\left(\frac{n}{p}\right)=\left(\frac{ab}{p}\righ t) = \left(\frac{a}{p}\right)\left(\frac{b}{p}\right) = (1)(1) = 1$ since $a,b < n$ . Hence a contradiction.

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