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## Homework Statement

Show that if [itex]p[/itex] is an odd prime of the form [itex]4k + 3[/itex] and [itex]a[/itex] is a positive integer such that [itex]1 < a < p - 1[/itex], then [itex]p[/itex] does not divide [itex]a^2 + 1[/itex]

## Homework Equations

If [itex]a[/itex] divides [itex]b[/itex], then there exists an integer [itex]c[/itex] such that [itex]ac = b[/itex].

## The Attempt at a Solution

We have to do this proof by contradiction, so suppose [itex]p[/itex] divides [itex]a^2 + 1[/itex]. Then there exists an integer [itex]c[/itex] such that [itex]pc = a^2 + 1[/itex]. At this point I am stuck. I can't factor anything, and I don't see any other algebraic manipulations that will help. Any ideas?