# Fermat's theorem on sums of two squares

In additive number theory, Fermat's theorem on sums of two squares states that an odd prime *p* can be expressed as:

with *x* and *y* integers, if and only if

The prime numbers for which this is true are called Pythagorean primes. For example, the primes 5, 13, 17, 29, 37 and 41 are all congruent to 1 modulo 4, and they can be expressed as sums of two squares in the following ways:

On the other hand, the primes 3, 7, 11, 19, 23 and 31 are all congruent to 3 modulo 4, and none of them can be expressed as the sum of two squares. This is the easier part of the theorem, and follows immediately from the observation that all squares are congruent to 0 or 1 modulo 4.

Since the Diophantus identity implies that the product of two integers each of which can be written as the sum of two squares is itself expressible as the sum of two squares, by applying Fermat's theorem to the prime factorization of any positive integer *n*, we see that if all the prime factors of *n* congruent to 3 modulo 4 occur to an even exponent, then *n* is expressible as a sum of two squares. The converse also holds.^{[1]} This generalization of Fermat's theorem is known as the sum of two squares theorem.

## History[edit]

Albert Girard was the first to make the observation, describing all positive integer numbers (not necessarily primes) expressible as the sum of two squares of positive integers; this was published in 1625.^{[2]}^{[3]} The statement that every prime *p* of the form *4n+1* is the sum of two squares is sometimes called *Girard's theorem*.^{[4]} For his part, Fermat wrote an elaborate version of the statement (in which he also gave the number of possible expressions of the powers of *p* as a sum of two squares) in a letter to Marin Mersenne dated December 25, 1640: for this reason this version of the theorem is sometimes called *Fermat's Christmas theorem.*

## Gaussian primes[edit]

Fermat's theorem on sums of two squares is strongly related with the theory of Gaussian primes.

A Gaussian integer is a complex number such that a and b are integers. The *norm* of a Gaussian integer is an integer equal to the square of the absolute value of the Gaussian integer. The norm of a product of Gaussian integers is the product of their norm. This is Diophantus identity, which results immediately from the similar property of the absolute value.

Gaussian integers form a principal ideal domain. This implies that Gaussian primes can be defined similarly as primes numbers, that is as those Gaussian integers that are not the product of two non-units (here the units are 1, −1, *i* and −*i*).

The multiplicative property of the norm implies that a prime number p is either a Gaussian prime or the norm of a Gaussian prime. Fermat's theorem asserts that the first case occurs when and that the second case occurs when and The last case is not considered in Fermat's statement, but is trivial, as

## Related results[edit]

Above point of view on Fermat's theorem is a special case of the theory of factorization of ideals in rings of quadratic integers. In summary, if is the ring of algebraic integers in the quadratic field, then an odd prime number p, not dividing d, is either a prime element in or the ideal norm of an ideal of which is necessarily prime. Moreover, the law of quadratic reciprocity allows distinguishing the two cases in terms of congruences. If is a principal ideal domain, then p is an ideal norm if and only

with a and b both integers.

In a letter to Blaise Pascal dated September 25, 1654 Fermat announced the following two results that are essentially the special cases and If p is an odd prime, then

Fermat wrote also:

*If two primes which end in 3 or 7 and surpass by 3 a multiple of 4 are multiplied, then their product will be composed of a square and the quintuple of another square.*

In other words, if *p, q* are of the form 20*k* + 3 or 20*k* + 7, then *pq* = *x*^{2} + 5*y*^{2}. Euler later extended this to the conjecture that

Both Fermat's assertion and Euler's conjecture were established by Joseph-Louis Lagrange. This more complicate formulation relies on the fact that is not a principal ideal domain, contrarily to and

## Algorithm[edit]

There is a trivial algorithm for decomposing a prime of the form into a sum of two squares: For all n such , test whether the square root of is an integer. If this the case, one has got the decomposition.

However the input size of the algorithm is the number of digits of p (up to a constant factor that depends on the numeral base). The number of needed tests is of the order of and thus exponential in the input size. So the computational complexity of this algorithm is exponential.

An algorithm with a polynomial complexity has been described by
Stan Wagon in 1990, based on work by Serret and Hermite (1848), and Cornacchia (1908).^{[5]}

### Description[edit]

Given an odd prime in the form , first find such that . This can be done by finding a Quadratic non-residue modulo , say , and letting .

Such an will satisfy the condition since quadratic non-residues satisfy .

Once is determined, one can apply the Euclidean algorithm with and . Denote the first two remainders that are less than the square root of as and . Then it will be the case that .

### Example[edit]

Take . A possible quadratic non-residue for 97 is 13, since . so we let . The Euclidean algorithm applied to 97 and 22 yields:

## Proofs[edit]

Fermat usually did not write down proofs of his claims, and he did not provide a proof of this statement. The first proof was found by Euler after much effort and is based on infinite descent. He announced it in two letters to Goldbach, on May 6, 1747 and on April 12, 1749; he published the detailed proof in two articles (between 1752 and 1755).^{[6]}^{[7]} Lagrange gave a proof in 1775 that was based on his study of quadratic forms. This proof was simplified by Gauss in his *Disquisitiones Arithmeticae* (art. 182). Dedekind gave at least two proofs based on the arithmetic of the Gaussian integers. There is an elegant proof using Minkowski's theorem about convex sets. Simplifying an earlier short proof due to Heath-Brown (who was inspired by Liouville's idea), Zagier presented a non-constructive one-sentence proof in 1990.^{[8]}
And more recently Christopher gave a partition-theoretic proof.^{[9]}

### Euler's proof by infinite descent[edit]

Euler succeeded in proving Fermat's theorem on sums of two squares in 1749, when he was forty-two years old. He communicated this in a letter to Goldbach dated 12 April 1749.^{[10]} The proof relies on infinite descent, and is only briefly sketched in the letter. The full proof consists in five steps and is published in two papers. The first four steps are Propositions 1 to 4 of the first paper^{[11]} and do not correspond exactly to the four steps below. The fifth step below is from the second paper.^{[12]}^{[13]}

For the avoidance of ambiguity, zero will always be a valid possible constituent of "sums of two squares", so for example every square of an integer is trivially expressible as the sum of two squares by setting one of them to be zero.

1. *The product of two numbers, each of which is a sum of two squares, is itself a sum of two squares.*

- This is a well-known property, based on the identity

- This is a well-known property, based on the identity

- due to Diophantus.

2. *If a number which is a sum of two squares is divisible by a prime which is a sum of two squares, then the quotient is a sum of two squares.*
(This is Euler's first Proposition).

- Indeed, suppose for example that is divisible by and that this latter is a prime. Then divides

- Indeed, suppose for example that is divisible by and that this latter is a prime. Then divides

- Since is a prime, it divides one of the two factors. Suppose that it divides . Since

- Since is a prime, it divides one of the two factors. Suppose that it divides . Since

- (Diophantus's identity) it follows that must divide . So the equation can be divided by the square of . Dividing the expression by yields:

- (Diophantus's identity) it follows that must divide . So the equation can be divided by the square of . Dividing the expression by yields:

- and thus expresses the quotient as a sum of two squares, as claimed.

- On the other hand if divides , a similar argument holds by using the following variant of Diophantus's identity:

- On the other hand if divides , a similar argument holds by using the following variant of Diophantus's identity:

3. *If a number which can be written as a sum of two squares is divisible by a number which is not a sum of two squares, then the quotient has a factor which is not a sum of two squares.* (This is Euler's second Proposition).

- Suppose is a number not expressible as a sum of two squares, which divides . Write the quotient, factored into its (possibly repeated) prime factors, as so that . If all factors can be written as sums of two squares, then we can divide successively by , , etc., and applying step (2.) above we deduce that each successive, smaller, quotient is a sum of two squares. If we get all the way down to then itself would have to be equal to the sum of two squares, which is a contradiction. So at least one of the primes is not the sum of two squares.

4. *If and are relatively prime positive integers then every factor of is a sum of two squares.*
(This is the step that uses step (3.) to produce an 'infinite descent' and was Euler's Proposition 4. The proof sketched below also includes the proof of his Proposition 3).

- Let be relatively prime positive integers: without loss of generality is not itself prime, otherwise there is nothing to prove. Let therefore be a
*proper*factor of , not necessarily prime: we wish to show that is a sum of two squares. Again, we lose nothing by assuming since the case is obvious.

- Let be relatively prime positive integers: without loss of generality is not itself prime, otherwise there is nothing to prove. Let therefore be a

- Let be non-negative integers such that are the closest multiples of (in absolute value) to respectively. Notice that the differences and are integers of absolute value strictly less than : indeed, when is even, gcd; otherwise since gcd, we would also have gcd.

- Multiplying out we obtain
- uniquely defining a non-negative integer . Since divides both ends of this equation sequence it follows that must also be divisible by : say . Let be the gcd of and which by the co-primeness of is relatively prime to . Thus divides , so writing , and , we obtain the expression for relatively prime and , and with , since

- Multiplying out we obtain

- Now finally, the
*descent*step: if is not the sum of two squares, then by step (3.) there must be a factor say of which is not the sum of two squares. But and so repeating these steps (initially with in place of , and so on*ad infinitum*) we shall be able to find a strictly decreasing infinite sequence of positive integers which are not themselves the sums of two squares but which divide into a sum of two relatively prime squares. Since such an**infinite descent**is impossible, we conclude that must be expressible as a sum of two squares, as claimed.

- Now finally, the

5. *Every prime of the form is a sum of two squares.*
(This is the main result of Euler's second paper).

- If , then by Fermat's Little Theorem each of the numbers is congruent to one modulo . The differences are therefore all divisible by . Each of these differences can be factored as
- Since is prime, it must divide one of the two factors. If in any of the cases it divides the first factor, then by the previous step we conclude that is itself a sum of two squares (since and differ by , they are relatively prime). So it is enough to show that cannot always divide the second factor. If it divides all differences , then it would divide all differences of successive terms, all differences of the differences, and so forth. Since the th differences of the sequence are all equal to (Finite difference), the th differences would all be constant and equal to , which is certainly not divisible by . Therefore, cannot divide all the second factors which proves that is indeed the sum of two squares.

- If , then by Fermat's Little Theorem each of the numbers is congruent to one modulo . The differences are therefore all divisible by . Each of these differences can be factored as

### Lagrange's proof through quadratic forms[edit]

Lagrange completed a proof in 1775^{[14]} based on his general theory of integral quadratic forms. The following presentation incorporates a slight simplification of his argument, due to Gauss, which appears in article 182 of the Disquisitiones Arithmeticae.

An (integral binary) quadratic form is an expression of the form with integers. A number is said to be *represented by the form* if there exist integers such that . Fermat's theorem on sums of two squares is then equivalent to the statement that a prime is represented by the form (i.e., , ) exactly when is congruent to modulo .

The discriminant of the quadratic form is defined to be . The discriminant of is then equal to .

Two forms and are *equivalent* if and only if there exist substitutions with integer coefficients

with such that, when substituted into the first form, yield the second. Equivalent forms are readily seen to have the same discriminant, and hence also the same parity for the middle coefficient , which coincides with the parity of the discriminant. Moreover, it is clear that equivalent forms will represent exactly the same integers, because these kind of substitutions can be reversed by substitutions of the same kind.

Lagrange proved that all positive definite forms of discriminant −4 are equivalent. Thus, to prove Fermat's theorem it is enough to find *any* positive definite form of discriminant −4 that represents . For example, one can use a form

where the first coefficient *a* = was chosen so that the form represents by setting *x* = 1, and *y* = 0, the coefficient *b* = 2*m* is an arbitrary even number (as it must be, to get an even discriminant), and finally is chosen so that the discriminant is equal to −4, which guarantees that the form is indeed equivalent to . Of course, the coefficient must be an integer, so the problem is reduced to finding some integer *m* such that divides : or in other words, a * 'square root of -1 modulo ' *.

We claim such a square root of is given by . Firstly it follows from Euclid's Fundamental Theorem of Arithmetic that . Consequently, : that is, are their own inverses modulo and this property is unique to them. It then follows from the validity of Euclidean division in the integers, and the fact that is prime, that for every the gcd of and may be expressed via the Euclidean algorithm yielding a unique and *distinct* inverse of modulo . In particular therefore the product of *all* non-zero residues modulo is . Let : from what has just been observed, . But by definition, since each term in may be paired with its negative in , , which since is odd shows that , as required.

### Dedekind's two proofs using Gaussian integers[edit]

Richard Dedekind gave at least two proofs of Fermat's theorem on sums of two squares, both using the arithmetical properties of the Gaussian integers, which are numbers of the form *a* + *bi*, where *a* and *b* are integers, and *i* is the square root of −1. One appears in section 27 of his exposition of ideals published in 1877; the second appeared in Supplement XI to Peter Gustav Lejeune Dirichlet's *Vorlesungen über Zahlentheorie*, and was published in 1894.

**1. First proof.** If is an odd prime number, then we have in the Gaussian integers. Consequently, writing a Gaussian integer ω = *x* + *iy* with *x,y* ∈ **Z** and applying the Frobenius automorphism in **Z**[*i*]/(*p*), one finds

since the automorphism fixes the elements of **Z**/(*p*). In the current case, for some integer n, and so in the above expression for ω^{p}, the exponent (p-1)/2 of -1 is even. Hence the right hand side equals ω, so in this case the Frobenius endomorphism of **Z**[*i*]/(*p*) is the identity.

Kummer had already established that if *f* ∈ {1,2} is the order of the Frobenius automorphism of **Z**[*i*]/(*p*), then the ideal in **Z**[*i*] would be a product of 2/*f* distinct prime ideals. (In fact, Kummer had established a much more general result for any extension of **Z** obtained by adjoining a primitive *m*-th root of unity, where *m* was any positive integer; this is the case *m* = 4 of that result.) Therefore, the ideal (*p*) is the product of two different prime ideals in **Z**[*i*]. Since the Gaussian integers are a Euclidean domain for the norm function , every ideal is principal and generated by a nonzero element of the ideal of minimal norm. Since the norm is multiplicative, the norm of a generator of one of the ideal factors of (*p*) must be a strict divisor of , so that we must have , which gives Fermat's theorem.

**2. Second proof.** This proof builds on Lagrange's result that if is a prime number, then there must be an integer *m* such that is divisible by *p* (we can also see this by Euler's criterion); it also uses the fact that the Gaussian integers are a unique factorization domain (because they are a Euclidean domain). Since *p* ∈ **Z** does not divide either of the Gaussian integers and (as it does not divide their imaginary parts), but it does divide their product , it follows that cannot be a prime element in the Gaussian integers. We must therefore have a nontrivial factorization of *p* in the Gaussian integers, which in view of the norm can have only two factors (since the norm is multiplicative, and , there can only be up to two factors of p), so it must be of the form for some integers and . This immediately yields that .

### Proof by Minkowski's Theorem[edit]

For congruent to mod a prime, is a quadratic residue mod by Euler's criterion. Therefore, there exists an integer such that divides . Let be the standard basis elements for the vector space and set and . Consider the lattice . If then . Thus divides for any .

The area of the fundamental parallelogram of the lattice is . The area of the open disk, , of radius centered around the origin is . Furthermore, is convex and symmetrical about the origin. Therefore, by Minkowski's theorem there exists a nonzero vector such that . Both and so . Hence is the sum of the squares of the components of .

### Zagier's "one-sentence proof"[edit]

Let be prime, let denote the natural numbers (with or without zero), and consider the finite set of triples of numbers. Then has two involutions: an obvious one whose fixed points correspond to representations of as a sum of two squares, and a more complicated one,

which has exactly one fixed point . Two involutions over the same finite set must have sets of fixed points with the same parity, and since the second involution has an odd number of fixed points, so does the first. Zero is even, so the first involution has a nonzero number of fixed points, any one of which gives a representation of as a sum of two squares.

This proof, due to Zagier, is a simplification of an earlier proof by Heath-Brown, which in turn was inspired by a proof of Liouville. The technique of the proof is a combinatorial analogue of the topological principle that the Euler characteristics of a topological space with an involution and of its fixed-point set have the same parity and is reminiscent of the use of *sign-reversing involutions* in the proofs of combinatorial bijections.

This proof is equivalent to a geometric or "visual" proof using "windmill" figures, given by Alexander Spivak in 2006 and described in this MathOverflow post and this Mathologer YouTube video Why was this visual proof missed for 400 years? (Fermat's two square theorem) on YouTube.

### Proof with partition theory[edit]

In 2016, A. David Christopher gave a partition-theoretic proof by considering partitions of the odd prime having exactly two sizes , each occurring exactly times, and by showing that at least one such partition exists if is congruent to 1 modulo 4.^{[15]}

## See also[edit]

- Legendre's three-square theorem
- Lagrange's four-square theorem
- Landau–Ramanujan constant
- Thue's lemma

## References[edit]

- D. A. Cox (1989).
*Primes of the Form x*. Wiley-Interscience. ISBN 0-471-50654-0.*Richard Dedekind,^{2}+ ny^{2}*The theory of algebraic integers*. - L. E. Dickson.
*History of the Theory of Numbers*Vol. 2. Chelsea Publishing Co., New York 1920 - Harold M. Edwards,
*Fermat's Last Theorem. A genetic introduction to algebraic number theory*. Graduate Texts in Mathematics no. 50, Springer-Verlag, NY, 1977. - C. F. Gauss,
*Disquisitiones Arithmeticae*(English Edition). Transl. by Arthur A. Clarke. Springer-Verlag, 1986. - Goldman, Jay R. (1998),
*The Queen of Mathematics: A historically motivated guide to Number Theory*, A K Peters, ISBN 1-56881-006-7 - D. R. Heath-Brown,
*Fermat's two squares theorem*. Invariant, 11 (1984) pp. 3–5. - John Stillwell, Introduction to
*Theory of Algebraic Integers*by Richard Dedekind. Cambridge Mathematical Library, Cambridge University Press, 1996. ISBN 0-521-56518-9 - Don Zagier,
*A one-sentence proof that every prime p ≡ 1 mod 4 is a sum of two squares*. Amer. Math. Monthly 97 (1990), no. 2, 144, doi:10.2307/2323918

## Notes[edit]

**^**For a proof of the converse see for instance 20.1, Theorems 367 and 368, in: G.H. Hardy and E.M. Wright. An introduction to the theory of numbers, Oxford 1938.**^**Simon Stevin.*l'Arithmétique de Simon Stevin de Bruges*, annotated by Albert Girard, Leyde 1625, p. 622.**^**L. E. Dickson, History of the Theory of Numbers, Vol. II, Ch. VI, p. 227. "A. Girard ... had already made a determination of the numbers expressible as a sum of two integral squares: every square, every prime 4n+1, a product formed of such numbers, and the double of the foregoing"**^**L. E. Dickson, History of the Theory of Numbers, Vol. II, Ch. VI, p. 228.**^**Wagon, Stan (1990), "Editor's Corner: The Euclidean Algorithm Strikes Again",*American Mathematical Monthly*,**97**(2): 125, doi:10.2307/2323912, MR 1041889.**^**De numerus qui sunt aggregata quorum quadratorum. (Novi commentarii academiae scientiarum Petropolitanae 4 (1752/3), 1758, 3-40)**^**Demonstratio theorematis FERMATIANI omnem numerum primum formae 4n+1 esse summam duorum quadratorum. (Novi commentarii academiae scientiarum Petropolitanae 5 (1754/5), 1760, 3-13)**^**Zagier, D. (1990), "A one-sentence proof that every prime*p*≡ 1 (mod 4) is a sum of two squares",*American Mathematical Monthly*,**97**(2): 144, doi:10.2307/2323918, MR 1041893.**^**A. David Christopher. "A partition-theoretic proof of Fermat's Two Squares Theorem",*Discrete Mathematics***339**:4:1410–1411 (6 April 2016) doi:10.1016/j.disc.2015.12.002**^**Euler à Goldbach, lettre CXXV**^**De numerus qui sunt aggregata duorum quadratorum. (Novi commentarii academiae scientiarum Petropolitanae 4 (1752/3), 1758, 3-40) [1]**^**Demonstratio theorematis FERMATIANI omnem numerum primum formae 4n+1 esse summam duorum quadratorum. (Novi commentarii academiae scientiarum Petropolitanae 5 (1754/5), 1760, 3-13) [2]**^**The summary is based on Edwards book, pages 45-48.**^**Nouv. Mém. Acad. Berlin, année 1771, 125; ibid. année 1773, 275; ibid année 1775, 351.**^**A. David Christopher, A partition-theoretic proof of Fermat's Two Squares Theorem", Discrete Mathematics, 339 (2016) 1410–1411.

## External links[edit]

- Two more proofs at PlanetMath.org
- "A one-sentence proof of the theorem". Archived from the original on 5 February 2012.CS1 maint: unfit URL (link)
- Fermat's two squares theorem, D. R. Heath-Brown, 1984.