Lagrange's four-square theorem explained

Lagrange's four-square theorem, also known as Bachet's conjecture, states that every nonnegative integer can be represented as a sum of four non-negative integer squares. That is, the squares form an additive basis of order four. $p = a^2 + b^2 + c^2 + d^2$ where the four numbers

a,b,c,d

are integers. For illustration, 3, 31, and 310 can be represented as the sum of four squares as follows:

\begin3 & = 1^2+1^2+1^2+0^2 \\[3pt]31 & = 5^2+2^2+1^2+1^2 \\[3pt]310 & = 17^2+4^2+2^2+1^2 \\[3pt]& = 16^2 + 7^2 + 2^2 +1^2 \\[3pt]& = 15^2 + 9^2 + 2^2 +0^2 \\[3pt]& = 12^2 + 11^2 + 6^2 + 3^2.\end

This theorem was proven by Joseph Louis Lagrange in 1770. It is a special case of the Fermat polygonal number theorem.

Historical development

From examples given in the Arithmetica, it is clear that Diophantus was aware of the theorem. This book was translated in 1621 into Latin by Bachet (Claude Gaspard Bachet de Méziriac), who stated the theorem in the notes of his translation. But the theorem was not proved until 1770 by Lagrange.^[1]

Adrien-Marie Legendre extended the theorem in 1797–8 with his three-square theorem, by proving that a positive integer can be expressed as the sum of three squares if and only if it is not of the form

4^k(8m+7)

for integers and . Later, in 1834, Carl Gustav Jakob Jacobi discovered a simple formula for the number of representations of an integer as the sum of four squares with his own four-square theorem.

The formula is also linked to Descartes' theorem of four "kissing circles", which involves the sum of the squares of the curvatures of four circles. This is also linked to Apollonian gaskets, which were more recently related to the Ramanujan–Petersson conjecture.^[2]

Proofs

The classical proof

Several very similar modern versions^[3] ^[4] ^[5] of Lagrange's proof exist. The proof below is a slightly simplified version, in which the cases for which m is even or odd do not require separate arguments.

Proof using the Hurwitz integers

Another way to prove the theorem relies on Hurwitz quaternions, which are the analog of integers for quaternions.^[6]

Generalizations

Lagrange's four-square theorem is a special case of the Fermat polygonal number theorem and Waring's problem. Another possible generalization is the following problem: Given natural numbers

a,b,c,d

, can we solve

$n=ax_1^2+bx_2^2+cx_3^2+dx_4^2$

for all positive integers in integers

x_1,x_2,x_3,x₄

? The case

a=b=c=d=1

is answered in the positive by Lagrange's four-square theorem. The general solution was given by Ramanujan.^[7] He proved that if we assume, without loss of generality, that

a\leqb\leqc\leqd

then there are exactly 54 possible choices for

a,b,c,d

such that the problem is solvable in integers

x_1,x_2,x_3,x₄

for all . (Ramanujan listed a 55th possibility

a=1,b=2,c=5,d=5

, but in this case the problem is not solvable if

n=15

.^[8])

Algorithms

In 1986, Michael O. Rabin and Jeffrey Shallit^[9] proposed randomized polynomial-time algorithms for computing a single representation

	2
n=x
	4

for a given integer, in expected running time

O(log(n)²⁾

. It was further improved to

O(log(n)²log(log(n))^-1)

by Paul Pollack and Enrique Treviño in 2018.^[10]

Number of representations

See main article: Jacobi's four-square theorem. The number of representations of a natural number n as the sum of four squares of integers is denoted by r₄(n). Jacobi's four-square theorem states that this is eight times the sum of the divisors of n if n is odd and 24 times the sum of the odd divisors of n if n is even (see divisor function), i.e.

$r_4(n)=\begin8\sum\limits_m&\textn\text\\[12pt]24\sum\limits_m&\textn\text.\end$

Equivalently, it is eight times the sum of all its divisors which are not divisible by 4, i.e.

$r_4(n)=8\sum_m.$

We may also write this as $r_4(n) = 8 \sigma(n) -32 \sigma(n/4) \,$ where the second term is to be taken as zero if n is not divisible by 4. In particular, for a prime number p we have the explicit formula .^[11]

Some values of r₄(n) occur infinitely often as whenever n is even. The values of r₄(n)/n can be arbitrarily large: indeed, r₄(n)/n is infinitely often larger than 8.

Uniqueness

The sequence of positive integers which have only one representation as a sum of four squares of non-negative integers (up to order) is:

1, 2, 3, 5, 6, 7, 8, 11, 14, 15, 23, 24, 32, 56, 96, 128, 224, 384, 512, 896 ... .

These integers consist of the seven odd numbers 1, 3, 5, 7, 11, 15, 23 and all numbers of the form

2(4^k),6(4^k)

14(4^k)

The sequence of positive integers which cannot be represented as a sum of four non-zero squares is:

1, 2, 3, 5, 6, 8, 9, 11, 14, 17, 24, 29, 32, 41, 56, 96, 128, 224, 384, 512, 896 ... .

These integers consist of the eight odd numbers 1, 3, 5, 9, 11, 17, 29, 41 and all numbers of the form

2(4^k),6(4^k)

14(4^k)

Further refinements

Lagrange's four-square theorem can be refined in various ways. For example, Zhi-Wei Sun^[12] proved that each natural number can be written as a sum of four squares with some requirements on the choice of these four numbers.

One may also wonder whether it is necessary to use the entire set of square integers to write each natural as the sum of four squares. Eduard Wirsing proved that there exists a set of squares with

|S|=O(n^1/4log^1/4n)

such that every positive integer smaller than or equal to can be written as a sum of at most 4 elements of .

References

Book: Hardy . G. H. . G. H. Hardy . Wright . E. M. . E. M. Wright . Heath-Brown . D. R. . Roger Heath-Brown . Silverman . J. H. . Joseph H. Silverman . Wiles . Andrew . Andrew Wiles . An Introduction to the Theory of Numbers . 6th . Oxford University Press . 2008 . 1938 . 978-0-19-921985-8 .
Book: Ireland . Kenneth . Rosen . Michael . A Classical Introduction to Modern Number Theory . Springer . 1990 . 2nd . 978-1-4419-3094-1 . 10.1007/978-1-4757-2103-4 .
Book: Landau , Edmund . Edmund Landau . 2nd . Goodman . Jacob E. . Elementary Number Theory . AMS Chelsea Publishing . 1958 . 1927 . 125 .
Book: Niven . Ivan . Zuckerman . Herbert S. . An introduction to the theory of numbers . . 1960 .
Oh . Byeong-Kweon . Representations of Binary Forms by Quinary Quadratic Forms . Trends in Mathematics . 2000 . 3 . 1 . 102–107 .
Rabin . M. O. . Michael O. Rabin . Shallit . J. O. . Jeffrey Shallit . Randomized Algorithms in Number Theory . Communications on Pure and Applied Mathematics . 39 . 1986 . S1 . S239–S256 . 10.1002/cpa.3160390713 .
Ramanujan . S. . Srinivasa Ramanujan . On the expression of a number in the form ax² + by² + cz² + du² . . 1916 . 19 . 11–21.
Web site: Sarnak . Peter . Peter Sarnak . The Ramanujan Conjecture and some Diophantine Equations . . 2013 . Lecture at Tata Institute of Fundamental Research . ICTS Lecture Series . Bangalore, India .
Book: Stillwell , John . John Stillwell . Elements of Number Theory . Springer . 2003 . 978-0-387-95587-2 . 1112.11002 . 10.1007/978-0-387-21735-2 . Undergraduate Texts in Mathematics .
Sun . Z.-W. . Zhi-Wei Sun . Refining Lagrange's four-square theorem . J. Number Theory . 175 . 2017 . 167–190 . 10.1016/j.jnt.2016.11.008 . 1604.06723 . 119597024 .
Book: Williams , Kenneth S. . Number theory in the spirit of Liouville . London Mathematical Society Student Texts . 76 . . 2011 . 978-0-521-17562-3 . 1227.11002 .
Book: Spencer , Joel . 1996 . 295–297 . Springer US . en . 10.1007/978-1-4612-2418-1_22 . 9780387948263. Number Theory: New York Seminar 1991–1995 . Four Squares with Few Squares .
P. . Pollack . E. . Treviño . Finding the four squares in Lagrange's theorem . Integers . 18A . 2018 . A15 .