# Euler brick

In mathematics, an Euler brick, named after Leonhard Euler, is a rectangular cuboid whose edges and face diagonals all have integer lengths. A primitive Euler brick is an Euler brick whose edge lengths are relatively prime. A perfect Euler brick is one whose space diagonal is also an integer but such a brick has not yet been found.

Euler brick with edges a, b, c and face diagonals d, e, f

## Definition

The definition of an Euler brick in geometric terms is equivalent to a solution to the following system of Diophantine equations:

${\displaystyle {\begin{cases}a^{2}+b^{2}=d^{2}\\a^{2}+c^{2}=e^{2}\\b^{2}+c^{2}=f^{2}\end{cases}}}$

where a, b, c are the edges and d, e, f are the diagonals.

## Properties

• If (a, b, c) is a solution, then (ka, kb, kc) is also a solution for any k. Consequently, the solutions in rational numbers are all rescalings of integer solutions. Given an Euler brick with edge-lengths (a, b, c), the triple (bc, ac, ab) constitutes an Euler brick as well.[1]: p. 106
• Exactly one edge and two face diagonals of a primitive Euler brick are odd.
• At least two edges of an Euler brick are divisible by 3.[1]: p. 106
• At least two edges of an Euler brick are divisible by 4.[1]: p. 106
• At least one edge of an Euler brick is divisible by 11.[1]: p. 106

## Examples

The smallest Euler brick, discovered by Paul Halcke in 1719, has edges (a, b, c) = (44, 117, 240) and face diagonals (d, e, f ) = (125, 244, 267).[2] Some other small primitive solutions, given as edges (a, b, c) — face diagonals (d, e, f), are below:

All five primitive Euler bricks with dimensions under 1000
 ( 85, 132, 720 ) — ( 157, 725, 732 ) ( 140, 480, 693 ) — ( 500, 707, 843 ) ( 160, 231, 792 ) — ( 281, 808, 825 ) ( 187, 1020, 1584 ) — ( 1037, 1595, 1884 ) ( 195, 748, 6336 ) — ( 773, 6339, 6380 ) ( 240, 252, 275 ) — ( 348, 365, 373 ) ( 429, 880, 2340 ) — ( 979, 2379, 2500 ) ( 495, 4888, 8160 ) — ( 4913, 8175, 9512 ) ( 528, 5796, 6325 ) — ( 5820, 6347, 8579 )

## Generating formula

Euler found at least two parametric solutions to the problem, but neither gives all solutions.[3]

An infinitude of Euler bricks can be generated with Saunderson's[4] parametric formula. Let (u, v, w) be a Pythagorean triple (that is, u2 + v2 = w2.) Then[1]: 105  the edges

${\displaystyle a=u|4v^{2}-w^{2}|,\quad b=v|4u^{2}-w^{2}|,\quad c=4uvw}$

give face diagonals

${\displaystyle d=w^{3},\quad e=u(4v^{2}+w^{2}),\quad f=v(4u^{2}+w^{2}).}$

There are many Euler bricks which are not parametrized as above, for instance the Euler brick with edges (a, b, c) = (240, 252, 275) and face diagonals (d, e, f ) = (348, 365, 373).

## Perfect cuboid

Unsolved problem in mathematics:

Does a perfect cuboid exist?

A perfect cuboid (also called a perfect Euler brick or perfect box) is an Euler brick whose space diagonal also has integer length. In other words, the following equation is added to the system of Diophantine equations defining an Euler brick:

${\displaystyle a^{2}+b^{2}+c^{2}=g^{2},}$

where g is the space diagonal. As of September 2020, no example of a perfect cuboid had been found and no one has proven that none exist.[5]

Euler brick with edges a, b, c and face diagonals d, e, f

Exhaustive computer searches show that, if a perfect cuboid exists,

• the odd edge must be greater than 2.5 × 1013,[5]
• the smallest edge must be greater than 5×1011.[5]
• the space diagonal must be greater than 9 × 1015.[6]

Some facts are known about properties that must be satisfied by a primitive perfect cuboid, if one exists, based on modular arithmetic:[7]

• One edge, two face diagonals and the space diagonal must be odd, one edge and the remaining face diagonal must be divisible by 4, and the remaining edge must be divisible by 16.
• Two edges must have length divisible by 3 and at least one of those edges must have length divisible by 9.
• One edge must have length divisible by 5.
• One edge must have length divisible by 7.
• One edge must have length divisible by 11.
• One edge must have length divisible by 19.
• One edge or space diagonal must be divisible by 13.
• One edge, face diagonal or space diagonal must be divisible by 17.
• One edge, face diagonal or space diagonal must be divisible by 29.
• One edge, face diagonal or space diagonal must be divisible by 37.

If a perfect cuboid exists and ${\displaystyle a,b,c}$ are its edges, ${\displaystyle d,e,f}$ — the corresponding face diagonals and the space diagonal ${\displaystyle g}$, then

• The triangle with the side lengths ${\displaystyle (d^{2},e^{2},f^{2})}$ is a Heronian triangle an area ${\displaystyle abcg}$ with rational angle bisectors.[10]
• The acute triangle with the side lengths ${\displaystyle (af,be,cd)}$, the obtuse triangles with the side lengths ${\displaystyle (bf,ae,gd),(ad,cf,ge),(ce,bd,gf)}$ are Heronian triangles an equal area ${\displaystyle {\frac {abcg}{2}}\in \mathbb {N} }$.

### Cuboid conjectures

Three cuboid conjectures are three mathematical propositions claiming irreducibility of three univariate polynomials with integer coefficients depending on several integer parameters. The conjectures are related to the perfect cuboid problem.[11][12] Though they are not equivalent to the perfect cuboid problem, if all of these three conjectures are valid, then no perfect cuboids exist. They are neither proved nor disproved.

Cuboid conjecture 1. For any two positive coprime integer numbers ${\displaystyle \displaystyle a\neq u}$ the eighth degree polynomial

${\displaystyle P_{au}(t)=t^{8}+6\,(u^{2}-a^{2})\,t^{6}+(a^{4}-4\,a^{2}\,u^{2}+u^{4})\,t^{4}-6\,a^{2}\,u^{2}\,(u^{2}-a^{2})\,t^{2}+u^{4}\,a^{4}}$

(1)

is irreducible over the ring of integers ${\displaystyle \displaystyle \mathbb {Z} }$.

Cuboid conjecture 2. For any two positive coprime integer numbers ${\displaystyle \displaystyle p\neq q}$ the tenth-degree polynomial

{\displaystyle {\begin{aligned}Q_{pq}(t)={}&t^{10}+(2q^{2}+p^{2})(3q^{2}-2p^{2})t^{8}\\[4pt]&{}+(q^{8}+10p^{2}q^{6}+4p^{4}q^{4}-14p^{6}q^{2}+p^{8})t^{6}\\[4pt]&{}-p^{2}q^{2}(q^{8}-14p^{2}q^{6}+4p^{4}q^{4}+10p^{6}\,q^{2}+p^{8})t^{4}\\[4pt]&{}-p^{6}\,q^{6}\,(q^{2}+2\,p^{2})\,(-2\,q^{2}+3\,p^{2})\,t^{2}\\[4pt]&{}-q^{10}\,p^{10}\end{aligned}}}

(2)

is irreducible over the ring of integers ${\displaystyle \displaystyle \mathbb {Z} }$.

Cuboid conjecture 3. For any three positive coprime integer numbers ${\displaystyle \displaystyle a}$, ${\displaystyle \displaystyle b}$, ${\displaystyle \displaystyle u}$ such that none of the conditions

${\displaystyle {\begin{array}{lcr}{\text{1)}}\qquad a=b;\qquad \qquad &{\text{3)}}\qquad b\,u=a^{2};\qquad \qquad &{\text{5)}}\qquad a=u;\\{\text{2)}}\qquad a=b=u;\qquad \qquad &{\text{4)}}\qquad a\,u=b^{2};\qquad \qquad &{\text{6)}}\qquad b=u\end{array}}}$

(3)

are fulfilled, the twelfth-degree polynomial

{\displaystyle {\begin{aligned}P_{abu}(t)={}&t^{12}+(6u^{2}-2a^{2}-2b^{2})t^{10}\\&{}+(u^{4}+b^{4}+a^{4}+4a^{2}u^{2}+4b^{2}u^{2}-12b^{2}a^{2})t^{8}\\&{}+(6a^{4}u^{2}+6u^{2}b^{4}-8a^{2}b^{2}u^{2}-2u^{4}a^{2}-2u^{4}b^{2}-2a^{4}b^{2}-2b^{4}a^{2})t^{6}\\&{}+(4u^{2}b^{4}a^{2}+4a^{4}u^{2}b^{2}-12u^{4}a^{2}b^{2}+u^{4}a^{4}+u^{4}b^{4}+a^{4}b^{4})t^{4}\\&{}+(6a^{4}u^{2}b^{4}-2u^{4}a^{4}b^{2}-2u^{4}a^{2}b^{4})t^{2}+u^{4}a^{4}b^{4}.\end{aligned}}}

(4)

is irreducible over the ring of integers ${\displaystyle \displaystyle \mathbb {Z} }$.

## Almost-perfect cuboids

An almost-perfect cuboid has 6 out of the 7 lengths as rational. Such cuboids can be sorted into three types, called body, edge, and face cuboids.[13] In the case of the body cuboid, the body (space) diagonal g is irrational. For the edge cuboid, one of the edges a, b, c is irrational. The face cuboid has one of the face diagonals d, e, f irrational.

The body cuboid is commonly referred to as the Euler cuboid in honor of Leonhard Euler, who discussed this type of cuboid.[14] He was also aware of face cuboids, and provided the (104, 153, 672) example.[15] The three integer cuboid edge lengths and three integer diagonal lengths of a face cuboid can also be interpreted as the edge lengths of a Heronian tetrahedron that is also a Schläfli orthoscheme. There are infinitely many face cuboids, and infinitely many Heronian orthoschemes.[16]

The smallest solutions for each type of almost-perfect cuboids, given as edges, face diagonals and the space diagonal (a, b, c, d, e, f, g), are as follows:

• Body cuboid: (44, 117, 240, 125, 244, 267, 73225)
• Edge cuboid: (520, 576, 618849, 776, 943, 975, 1105)
• Face cuboid: (104, 153, 672, 185, 680, 474993, 697)

As of July 2020, there are 167,043 found cuboids with the smallest integer edge less than 200,000,000,027: 61,042 are Euler (body) cuboids, 16,612 are edge cuboids with a complex number edge length, 32,286 were edge cuboids, and 57,103 were face cuboids.[17]

As of December 2017, an exhaustive search counted all edge and face cuboids with the smallest integer space diagonal less than 1,125,899,906,842,624: 194,652 were edge cuboids, 350,778 were face cuboids.[6]

## Perfect parallelepiped

A perfect parallelepiped is a parallelepiped with integer-length edges, face diagonals, and body diagonals, but not necessarily with all right angles; a perfect cuboid is a special case of a perfect parallelepiped. In 2009, dozens of perfect parallelepipeds were shown to exist,[18] answering an open question of Richard Guy. Some of these perfect parallelepipeds have two rectangular faces. The smallest perfect parallelepiped has edges 271, 106, and 103; short face diagonals 101, 266, and 255; long face diagonals 183, 312, and 323; and body diagonals 374, 300, 278, and 272.

## Notes

1. Wacław Sierpiński, Pythagorean Triangles, Dover Publications, 2003 (orig. ed. 1962).
2. ^ Visions of Infinity: The Great Mathematical Problems By Ian Stewart, Chapter 17
3. ^ Weisstein, Eric W. "Euler Brick". MathWorld.
4. ^ Knill, Oliver (February 24, 2009). "Treasure Hunting Perfect Euler bricks" (PDF). Math table. Harvard University.
5. ^ a b c Matson, Robert D. "Results of a Computer Search for a Perfect Cuboid" (PDF). unsolvedproblems.org. Retrieved February 24, 2020.
6. ^ a b Alexander Belogourov, Distributed search for a perfect cuboid, https://www.academia.edu/39920706/Distributed_search_for_a_perfect_cuboid
7. ^ M. Kraitchik, On certain Rational Cuboids, Scripta Mathematica, volume 11 (1945).
8. ^ a b I. Korec, Lower bounds for Perfect Rational Cuboids, Math. Slovaca, 42 (1992), No. 5, p. 565-582.
9. ^ Ronald van Luijk, On Perfect Cuboids, June 2000
10. ^ Florian Luca (2000) "Perfect Cuboids and Perfect Square Triangles", Mathematics Magazine, 73:5, p. 400-401
11. ^ Sharipov R.A. (2012). "Perfect cuboids and irreducible polynomials". Ufa Math Journal. 4 (1): 153–160. arXiv:1108.5348. Bibcode:2011arXiv1108.5348S.
12. ^ Sharipov R.A. (2015). "Asymptotic approach to the perfect cuboid problem". Ufa Math Journal. 7 (3): 100–113.
13. ^ Rathbun R. L., Granlund Т., The integer cuboid table with body, edge, and face type of solutions // Math. Comp., 1994, Vol. 62, P. 441-442.
14. ^ Euler, Leonhard, Vollst¨andige Anleitung zur Algebra, Kayserliche Akademie der Wissenschaften, St. Petersburg, 1771
15. ^ Euler, Leonhard, Vollst¨andige Anleitung zur Algebra, 2, Part II, 236, English translation: Euler, Elements of Algebra, Springer-Verlag 1984
16. ^ "Problem 930" (PDF), Solutions, Crux Mathematicorum, 11 (5): 162–166, May 1985
17. ^ Rathbun, Randall L. (14 Jul 2020). "The Integer Cuboid Table". arXiv:1705.05929v4 [math.NT].
18. ^ Sawyer, Jorge F.; Reiter, Clifford A. (2011). "Perfect parallelepipeds exist". Mathematics of Computation. 80 (274): 1037–1040. arXiv:0907.0220. doi:10.1090/s0025-5718-2010-02400-7..

## References

• Leech, John (1977). "The Rational Cuboid Revisited". American Mathematical Monthly. 84 (7): 518–533. doi:10.2307/2320014. JSTOR 2320014.
• Shaffer, Sherrill (1987). "Necessary Divisors of Perfect Integer Cuboids". Abstracts of the American Mathematical Society. 8 (6): 440.
• Guy, Richard K. (2004). Unsolved Problems in Number Theory. Springer-Verlag. pp. 275–283. ISBN 0-387-20860-7.
• Kraitchik, M. (1945). "On certain rational cuboids". Scripta Mathematica. 11: 317–326.
• Roberts, Tim (2010). "Some constraints on the existence of a perfect cuboid". Australian Mathematical Society Gazette. 37: 29–31. ISSN 1326-2297.