# Fermat’s right triangle theorem

How many ways are there to prove the Pythagorean theorem? – Betty Fei
How many ways are there to prove the Pythagorean theorem? – Betty Fei

Fermat’s right triangle theorem

Fermat’s right triangle theorem is a non-existence proof in number theory, published in 1670 among the works of Pierre de Fermat, soon after his death. It is the only complete proof given by Fermat.[1] It has several equivalent formulations, one of which was stated (but not proved) in 1225 by Fibonacci. In its geometric forms, it states:

• A right triangle in the Euclidean plane for which all three side lengths are rational numbers cannot have an area that is the square of a rational number. The area of a rational-sided right triangle is called a congruent number, so no congruent number can be square.
• A right triangle and a square with equal areas cannot have all sides commensurate with each other.
• There do not exist two integer-sided right triangles in which the two legs of one triangle are the leg and hypotenuse of the other triangle.

More abstractly, as a result about Diophantine equations (integer or rational-number solutions to polynomial equations), it is equivalent to the statements that:

• If three square numbers form an arithmetic progression, then the gap between consecutive numbers in the progression (called a congruum) cannot itself be square.
• The only rational points on the elliptic curve are the three trivial points with and .
• The quartic equation has no nonzero integer solution.

An immediate consequence of the last of these formulations is that Fermat’s Last Theorem is true in the special case that its exponent is 4.

## Formulation

### Squares in arithmetic progression

In 1225, Emperor Frederick II challenged the mathematician Fibonacci to take part in a mathematical contest against several other mathematicians, with three problems set by his court philosopher John of Palermo. The first of these problems asked for three rational numbers whose squares were equally spaced five units apart, solved by Fibonacci with the three numbers , , and . In The Book of Squares, published later the same year by Fibonacci, he solved the more general problem of finding triples of square numbers that are equally spaced from each other, forming an arithmetic progression. Fibonacci called the gap between these numbers a congruum.[2] One way of describing Fibonacci’s solution is that the numbers to be squared are the difference of legs, hypotenuse, and sum of legs of a Pythagorean triangle, and that the congruum is four times the area of the same triangle.[3] Fibonacci observed that it is impossible for a congruum to be a square number itself, but did not present a satisfactory proof of this fact.[4]

If three squares , , and could form an arithmetic progression whose congruum was also a square , then these numbers would satisfy the Diophantine equations

### Areas of right triangles

Because the congrua are exactly the numbers that are four times the area of a Pythagorean triangle, and multiplication by four does not change whether a number is square, the existence of a square congruum is equivalent to the existence of a Pythagorean triangle with a square area. It is this variant of the problem that Fermat’s proof concerns: he shows that there is no such triangle. In considering this problem, Fermat was inspired not by Fibonacci but by an edition of Arithmetica by Diophantus, published in a translation into French in 1621 by Claude Gaspar Bachet de Méziriac.[6] This book described various special right triangles whose areas had forms related to squares, but did not consider the case of areas that were themselves square.[7]

By rearranging the equations for the two Pythagorean triangles above, and then multiplying them together, one obtains the single Diophantine equation

Another equivalent formulation of the same problem involves congruent numbers, the numbers that are areas of right triangles whose three sides are all rational numbers. By multiplying the sides by a common denominator, any congruent number may be transformed into the area of a Pythagorean triangle, from which it follows that the congruent numbers are exactly the numbers formed by multiplying a congruum by the square of a rational number.[8] Therefore, the existence of a square congruum is equivalent to the statement that the number 1 is not a congruent number.[9] Another more geometric way of stating this formulation is that it is impossible for a square (the geometric shape) and a right triangle to have both equal areas and all sides commensurate with each other.[10]

### Elliptic curve

Yet another equivalent form of Fermat’s theorem involves the elliptic curve consisting of the points whose Cartesian coordinates satisfy the equation

## Fermat’s proof

During his lifetime, Fermat challenged several other mathematicians to prove the non-existence of a Pythagorean triangle with square area, but did not publish the proof himself. However, he wrote a proof in his copy of Diophantus’s Arithmetica, the same copy in which he wrote that he could prove Fermat’s Last Theorem. Fermat’s son Clement-Samuel published an edition of this book, including Fermat’s marginal notes with the proof of the right triangle theorem, in 1670.[12]

Fermat’s proof is a proof by infinite descent. It shows that, from any example of a Pythagorean triangle with square area, one can derive a smaller example. Since Pythagorean triangles have positive integer areas, and there does not exist an infinite descending sequence of positive integers, there also cannot exist a Pythagorean triangle with square area.[13]

In more detail, suppose that , , and are the integer sides of a right triangle with square area. By dividing by any common factors, one can assume that this triangle is primitive[10] and from the known form of all primitive Pythagorean triples, one can set , , and , by which the problem is transformed into finding relatively prime integers and (one of which is even) such that the area is square. For this number to be a square, its four linear factors , , , and (which are relatively prime) must themselves be squares; let and . Both and must be odd since exactly one of or is even and the other is odd. Therefore, both and are even, and one of them is divisible by 4. Dividing them by two produces two more integers and , one of which is even by the previous sentence. Because is a square, and are the legs of another primitive Pythagorean triangle whose area is . Since is itself a square and since is even, is a square. Thus, any Pythagorean triangle with square area leads to a smaller Pythagorean triangle with square area, completing the proof.[14]

## Notes

1. ^ Edwards (2000). Many subsequent mathematicians published proofs, including Gottfried Wilhelm Leibniz (1678), Leonhard Euler (1747), and Bernard Frenicle de Bessy (before 1765); see Dickson (1920) and Goldstein (1995).
3. ^ Beiler (1964).
4. ^ Ore (2012); Dickson (1920).
5. ^ The fact that there can be no two right triangles that share two of their sides, and the connection between this problem and the problem of squares in arithmetic progression, is described as “well known” by Cooper & Poirel (2008)
6. ^ Edwards (2000).
7. ^ a b Stillwell (1998).
8. ^ Conrad (2008); Koblitz (1993, p. 3).
9. ^ Conrad (2008), Theorem 2; Koblitz (1993), Exercise 3, p. 5.
10. ^ a b Dickson (1920).
11. ^ Koblitz (1993), Proposition 19, pp. 46–47; Kato & Saitō (2000).
12. ^ Edwards (2000); Dickson (1920). For other proofs, see Grant & Perella (1999) and Barbara (2007).
13. ^ Edwards (2000); Dickson (1920).
14. ^ Edwards (2000); Dickson (1920); Stillwell (1998).

## References

• Barbara, Roy (July 2007), “91.33 Fermat’s last theorem in the case “, Notes, The Mathematical Gazette, 91 (521): 260–262, doi:10.1017/S002555720018163X, JSTOR 40378352, S2CID 125255403
• Beiler, Albert H. (1964), Recreations in the Theory of Numbers: The Queen of Mathematics Entertains, Dover Books, p. 153, ISBN 978-0-486-21096-4
• Bradley, Michael John (2006), The Birth of Mathematics: Ancient Times to 1300, Infobase Publishing, p. 124, ISBN 978-0-8160-5423-7
• Conrad, Keith (Fall 2008), “The congruent number problem” (PDF), Harvard College Mathematical Review, 2 (2): 58–73, archived from the original (PDF) on 2013-01-20
• Cooper, Joshua; Poirel, Chris (2008), Pythagorean partition-regularity and ordered triple systems with the sum property, arXiv:0809.3478
• Dickson, Leonard Eugene (1920), “Sum or difference of two biquadrates never a square; area of a rational right triangle never a square”, History of the Theory of Numbers, Volume II: Diophantine Analysis, Carnegie Institution of Washington, pp. 615–620
• Edwards, Harold M. (2000), “1.6 Fermat’s one proof”, Fermat’s Last Theorem: A Genetic Introduction to Algebraic Number Theory, Graduate Texts in Mathematics, vol. 50, Springer, pp. 10–14, ISBN 978-0-387-95002-0
• Goldstein, Catherine (1995), Un théorème de Fermat et ses lecteurs, Saint-Denis: Presses Universaires de Vincennes
• Grant, Mike; Perella, Malcolm (July 1999), “83.25 Descending to the irrational”, Notes, The Mathematical Gazette, 83 (497): 263–267, doi:10.2307/3619054, JSTOR 3619054, S2CID 125167994
• Kato, Kazuya; Saitō, Takeshi (2000), Number Theory: Fermat’s dream, Translations of mathematical monographs, translated by Nobushige Kurokawa, American Mathematical Society, p. 17, ISBN 978-0-8218-0863-4
• Koblitz, Neal (1993), Introduction to Elliptic Curves and Modular Forms, Graduate Texts in Mathematics, vol. 97 (2nd ed.), Springer-Verlag, ISBN 0-387-97966-2
• Ore, Øystein (2012), Number Theory and Its History, Dover Books, pp. 202–203, ISBN 978-0-486-13643-1
• Stillwell, John (1998), “4.7 The area of rational right triangles”, Numbers and Geometry, Undergraduate Texts in Mathematics, Springer, pp. 131–133, ISBN 978-0-387-98289-2

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