Enumerating ways to decompose an integer into the sum of two squares - MathOverflow

Enumerating ways to decompose an integer into the sum of two squares

2010-06-26T22:05:28Z

The well known "Sum of Squares Function" tells you the number of ways you can represent an integer as the sum of two squares. See the link for details, but it is based on counting the factors of the number N into powers of 2, powers of primes = 1 mod 4 and powers of primes = 3 mod 4.

Given such a factorization, it's easy to find the number of ways to decompose N into two squares. But how do you efficiently enumerate the decompositions?

So for example, given N=2*5*5*13*13=8450 , I'd like to generate the four pairs:

13*13+91*91=8450

23*23+89*89=8450

35*35+85*85=8450

47*47+79*79=8450

The obvious algorithm (I used for the above example) is to simply take i=1,2,3,...,$\sqrt{N/2}$ and test if (N-i*i) is a square. But that can be expensive for large N. Is there a way to generate the pairs more efficiently? I already have the factorization of N, which may be useful.

(You can instead iterate between $i=\sqrt{N/2}$ and $\sqrt{N}$ but that's just a constant savings, it's still $O(\sqrt N)$.

Answer by Gerry Myerson for Enumerating ways to decompose an integer into the sum of two squares

2010-06-26T22:41:01Z

The factorization of $N$ is useful, since $$(a^2+b^2)(c^2+d^2)=(ac+bd)^2+(ad-bc)^2$$ There are good algorithms for expressing a prime as a sum of two squares or, what amounts to the same thing, finding a square root of minus one modulo $p$. See, e.g., http://www.emis.de/journals/AMEN/2005/030308-1.pdf

Edit: Perhaps I should add a word about solving $x^2\equiv-1\pmod p$. If $a$ is a quadratic non-residue (mod $p$) then we can take $x\equiv a^{(p-1)/4}\pmod p$. In practice, you can find a quadratic non-residue pretty quickly by just trying small numbers in turn, or trying (pseudo-)random numbers.

Answer by Will Jagy for Enumerating ways to decompose an integer into the sum of two squares

2010-06-27T01:41:32Z

This is the simplest case of the Hardy-Muskat-Williams algorithm. Anyway, here is a link to a 1995 paper by Kenneth S. Williams, http://www.mathstat.carleton.ca/~williams/papers/pdf/202.pdf and to the original HMW paper http://www.ams.org/journals/mcom/1990-55-191/S0025-5718-1990-1023762-3/S0025-5718-1990-1023762-3.pdf .

As I'm not sure you are aware of these details, let me point out that if $$ 4^k \;| \; \; x^2 + y^2$$ then $ 2^k \; | \; x $ and $ 2^k \; | \; y. $ That is, you might as well divide your target by powers of 4 before doing anything difficult. Then after you are finished multiply $x,y$ by the appropriate power of $2.$

This is very similar. If there is a prime $$ q \equiv 3 \pmod 4 $$ and $ q | n,$ then keep dividing the target by powers of $q^2$ until it is no longer divisible by $q^2.$ If the remaining number is divisible by $q$ there is actually no representation at all. But if $$ q^{2k} \;\parallel \; \; x^2 + y^2$$ then $ q^k \; | x $ and $ q^k \; | y. $ The notation $ q^{2k} \;\parallel \; \; x^2 + y^2$ means $ q^{2k} \; | \; \; x^2 + y^2$ but it is not true that $ q^{2k +1} \; | \; \; x^2 + y^2$

Well, that is enough caution. What you really need to know is expressing primes $$ p \equiv 1 \pmod 4 $$ and indeed $ p^m,$ which is not much more difficult. Once you can do that, combine my notes with all possible ways of applying Gerry's multiplication formula (by changing $\pm$ signs and order),

Answer by unknown (yahoo) for Enumerating ways to decompose an integer into the sum of two squares

2012-07-20T21:37:24Z

I wouldn't mind an elaboration on Gerry's hints. For example, N=5*5*13*13*17*17 is going to have 13 representations. What variants of x +/- yi are we multiplying together to come up with those? In Gerry's example, how come we don't look at (2-i) as a term and end up with 8 representations?