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Quadratic Residues

Date: 03/19/2004 at 01:02:09
From: Jeanne
Subject: quadratic residues

I don't fully understand the concept of quadratic residues.  Here's 
what I do understand.  Given x^2==a(mod p), 'a' is a quadratic 
residue if there exists a solution to the congruence.  If 'a' does not 
give a solution, a is a non-residue.  

Furthermore, given the theorem: Let p be an odd prime.  There are
exactly (p - 1)/2 incongruent quadratic residues of p and exactly 
(p - 1)/2 quadratic nonresidues of p.  Can you provide an example that
helps explain this concept?  My book doesn't give any examples.

Date: 03/19/2004 at 02:57:18
From: Doctor Jacques
Subject: Re: quadratic residues

Hi Jeanne,

The quadratic residues modulo n are simply the squares modulo n. 
(Note: this is a concept of modular arithmetic, so I'll not write 
"mod n" or "mod p" after each line--consider this as implicit).

For example, for n = 15 (n need not be a prime), the elements of Z_15 
(the residue classes modulo 15) are 0, 1, ... 14.  If we look at the 
squares of these elements, we get the table:

  x  |  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14
 x^2 |  0  1  4  9  1 10  6  4  4  6 10  1  9  4  1

(For example, in the 6th column, we have 5^2 = 25 = 10 (mod 15)).

The elements in the second row are the squares modulo 15: {0, 1, 4, 
6, 9, and 10}; these numbers are called quadratic residues (mod 15), 
because, if q is one of these numbers, the equation:

  x^2 = q (mod 15)

has at least one solution (to find solutions, look for q in the second 
row; the solutions are the corresponding numbers in the first row).  
For example, q = 4 is a quadratic residue, since 4 appears in the 
second row of the table.  The solutions of the equation x^4 = 4 are 
the numbers in the first row above the 4's in the second row, in this 
case 2, 7, 8, and 13 (note that this second-degree equation has more 
than 2 solutions--this is because 15 is not prime).

On the other hand, 7 is not a quadratic residue modulo 15, since it 
does not appear in the second row of the table; there is no number x 
such that x^2 = 7 (mod 15).

Modulo 10, the quadratic residues are {0, 1, 4, 5, 6, 9}.  These are 
the only possible unit digits for a perfect square.

As (-x)^2 = x^2, the table has left-to-right symmetry; the residues 
in the right half are the same as those in the left half.  This 
already shows that there cannot be more than n/2 residues.

If n = p is prime, however, we can say more. Let us look at a specific 
example, p = 7:

  x   |  0  1  2  3  4  5  6
  x^2 |  0  1  4  2  2  4  1

We see that the residues are 0, 1, 2, and 4 (0 is sometimes excluded 
from the list as a special case). If p is prime, the equation:

  x^2 = a   (mod p)

cannot have more than 2 solutions.  Indeed, if x0 is one solution, we 

               x^2 = x0^2     (mod p)
      (x^2 - x0^2) = 0        (mod p)
  (x - x0)(x + x0) = 0        (mod p)

The last line shows that p divides the product (x - x0)(x + x0).  *As
p is prime*, it must divide one of the factors, i.e., we must have
either p | (x - x0) - which means x = x0, or p | (x + x0), which means
x = -x0.  This shows that we can have at most two solutions, x0 and
-x0.  If p > 2 and x is not 0, x0 and -x0 are different (mod p).  In
that case, either we have no solution at all (a is a non-residue), or
we have exactly two solutions (a is a residue).

Note also that x^2 = 0 (mod p) has exactly one solution x = 0 (this 
would not be the case if p were not prime, for example, x^2 = 0 has 3 
solutions modulo 9: 0, 3, 6).

To summarize, if we exclude 0, there are p - 1 columns in the table.
In the second row, each entry is a number between 1 and p - 1, and
appears twice (in symmetrical positions).  This means that half of the 
available numbers [(p - 1)/2] do appear as quadratic residues, and the 
other half do not appear in the second row and are non-residues.

Does this help?  Write back if you'd like to talk about this some 
more, or if you have any other questions.

- Doctor Jacques, The Math Forum

Date: 03/21/2004 at 19:07:27
From: Jeanne
Subject: quadratic residues

This really helped me!  After days of reading my book and trying to 
figure it out, it finally makes sense to me.  Thank you so very, very 
much!!!  I am going to continue in my book with the Law of Quadratic 
Reciprocity, which I'm hoping now will make sense to me.
Associated Topics:
College Number Theory

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