Sum of squares function

The sum of squares function is an arithmetic function that gives the number of representations for a given positive integer $n$ as the sum of $k$ squares, where representations that differ only in the order of the summands or in the signs of the square roots are counted as different, and is denoted by $r k (n)$ .

Definition

The function is defined as

r_{k}(n)=|\{(a_{1},a_{2},\ldots ,a_{k})\in \mathbb {Z} ^{k}\ :\ n=a_{1}^{2}+a_{2}^{2}+\cdots +a_{k}^{2}\}|

where |.| denotes the cardinality of the set. In other words, $r k (n)$ is the number of ways $n$ can be written as a sum of $k$ squares.

For example, $r_{2}(1)=4$ , since $1=0^{2}+(\pm 1)^{2}=(\pm 1)^{2}+0^{2}$ , where every sum has 2 sign combinations, and also $r_{2}(2)=4$ , since $2=(\pm 1)^{2}+(\pm 1)^{2}$ with 4 sign combinations. On the other hand is $r_{2}(3)=0$ , because there exists no way to represent 3 as a sum of two squares.

The first 30 values for $r_{k}(n),\;k=1,\dots ,8$

n	=	r₁(n)	r₂(n)	r₃(n)	r₄(n)	r₅(n)	r₆(n)	r₇(n)	r₈(n)
0	0	1	1	1	1	1	1	1	1
1	1	2	4	6	8	10	12	14	16
2	2	0	4	12	24	40	60	84	112
3	3	0	0	8	32	80	160	280	448
4	2²	2	4	6	24	90	252	574	1136
5	5	0	8	24	48	112	312	840	2016
6	2‧3	0	0	24	96	240	544	1288	3136
7	7	0	0	0	64	320	960	2368	5504
8	2³	0	4	12	24	200	1020	3444	9328
9	3²	2	4	30	104	250	876	3542	12112
10	2‧5	0	8	24	144	560	1560	4424	14112
11	11	0	0	24	96	560	2400	7560	21312
12	2²‧3	0	0	8	96	400	2080	9240	31808
13	13	0	8	24	112	560	2040	8456	35168
14	2‧7	0	0	48	192	800	3264	11088	38528
15	3‧5	0	0	0	192	960	4160	16576	56448
16	2⁴	2	4	6	24	730	4092	18494	74864
17	17	0	8	48	144	480	3480	17808	78624
18	2‧3²	0	4	36	312	1240	4380	19740	84784
19	19	0	0	24	160	1520	7200	27720	109760
20	2²‧5	0	8	24	144	752	6552	34440	143136
21	3‧7	0	0	48	256	1120	4608	29456	154112
22	2‧11	0	0	24	288	1840	8160	31304	149184
23	23	0	0	0	192	1600	10560	49728	194688
24	2³‧3	0	0	24	96	1200	8224	52808	261184
25	5²	2	12	30	248	1210	7812	43414	252016
26	2‧13	0	8	72	336	2000	10200	52248	246176
27	3³	0	0	32	320	2240	13120	68320	327040
28	2²‧7	0	0	0	192	1600	12480	74048	390784
29	29	0	8	72	240	1680	10104	68376	390240
30	2‧3‧5	0	0	48	576	2720	14144	71120	395136

Particular cases

The number of ways to write a natural number as sum of two squares is given by $r 2 (n)$ . It is given explicitly by

r_{2}(n)=4(d_{1}(n)-d_{3}(n))

where $d 1 (n)$ is the number of divisors of $n$ which are congruent with 1 modulo 4 and $d 3 (n)$ is the number of divisors of $n$ which are congruent with 3 modulo 4. Using sums, the expression can be written as:

r_{2}(n)=4\sum _{d\mid n \atop d\equiv 1,3{\pmod {4}}}(-1)^{(d-1)/2}

The prime factorization $n=2^{g}p_{1}^{f_{1}}p_{2}^{f_{2}}\cdots q_{1}^{h_{1}}q_{2}^{h_{2}}\cdots$ , where $p_{i}$ are the prime factors of the form $p_{i}\equiv 1{\pmod {4}},$ and $q_{i}$ are the prime factors of the form $q_{i}\equiv 3{\pmod {4}}$ gives another formula

r_{2}(n)=4(f_{1}+1)(f_{2}+1)\cdots

, if all exponents

h_{1},h_{2},\cdots

are even. If one or more

h_{i}

are odd, then

r_{2}(n)=0

.

The number of ways to represent $n$ as the sum of four squares was due to Carl Gustav Jakob Jacobi and it is eight times the sum of all its divisors which are not divisible by 4, i.e.

r_{4}(n)=8\sum _{d\mid n;4\nmid d}d

Jacobi also found an explicit formula for the case $k =8$ :

r_{8}(n)=16\sum _{d\mid n}(-1)^{n+d}d^{3}

The generating function of the sequence $r_{k}(n)$ for fixed $k$ can be expressed in terms of the Jacobi theta function:[1]

\vartheta (0;q)^{k}=\vartheta _{3}^{k}(q)=\sum _{n=0}^{\infty }r_{k}(n)q^{n}

where

\vartheta (0;q)=\sum _{n=-\infty }^{\infty }q^{n^{2}}=1+2q+2q^{4}+2q^{9}+2q^{16}+\cdots

References

Milne, Stephen C. (2002). "Introduction". Infinite Families of Exact Sums of Squares Formulas, Jacobi Elliptic Functions, Continued Fractions, and Schur Functions. Springer Science & Business Media. p. 9. ISBN 1402004915.

External links

Weisstein, Eric W. "Sum of Squares Function". MathWorld.

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[1] Milne, Stephen C. (2002). "Introduction". Infinite Families of Exact Sums of Squares Formulas, Jacobi Elliptic Functions, Continued Fractions, and Schur Functions. Springer Science & Business Media. p. 9. ISBN 1402004915.