Let p be a prime number with p>3. Prove that the sum of the quadratic residues modulo p is divisible by p.

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- November 27th 2012, 08:53 AMbabygillQuadratic Residues
Let p be a prime number with p>3. Prove that the sum of the quadratic residues modulo p is divisible by p.

- November 27th 2012, 02:04 PMtopsquarkRe: Quadratic Residues
- November 27th 2012, 02:31 PMbabygillRe: Quadratic Residues
I don't know where to start

- November 27th 2012, 05:25 PMtopsquarkRe: Quadratic Residues
Okay. I've refreshed my memory of this topic, but I am in no way a professional.

First, I am thinking that your problem statement is incorrect. (Which may mean that I'm incorrect, but anyway...)

For example, take a look at the multiplicative group mod 7: .

Squaring each of these elements gives the quadratic residues by inspection: q = 1, 4. The sum of these is obviously not divisible by 7. I think what you are looking for is the sum of the square of all members of the group. This can be easily shown by looking at the summation:

This turns out to be divisible by 7 as required. (And thus the sum of the residues is also divisible by 7. Prove this!) I leave it to you to look at the general case.

-Dan