In mathematics, an Euler brick, named after Leonhard Euler, is a cuboid whose edges and face diagonals all have integer lengths. A primitive Euler brick is an Euler brick whose edge lengths are relatively prime.
The definition of an Euler brick in geometric terms is equivalent to a solution to the following system of Diophantine equations:
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.:p. 106
At least two edges of an Euler brick are divisible by 3.:p. 106
At least two edges of an Euler brick are divisible by 4.:p. 106
At least one edge of an Euler brick is divisible by 11.:p. 106
give face diagonals
The smallest Euler brick, discovered by Paul Halcke in 1719, has edges and face diagonals 125, 244, and 267.
Some other small primitive solutions, given as edges (a, b, c) — face diagonals (d, e, f), are below:
- (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) ;
Does a perfect cuboid exist?
A perfect cuboid (also called a 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:
where g is the space diagonal. Thus (a, b, c, g) must be a Pythagorean quadruple. As of May 2015[update], no example of a perfect cuboid had been found and no one has proven that none exist. Exhaustive computer searches show that, if a perfect cuboid exists, one of its edges must be greater than 3·1012. Furthermore, its smallest edge must be longer than 1010.
Some facts are known about properties that must be satisfied by a primitive perfect cuboid, if one exists, based on modular arithmetic:
- One edge, two face diagonals and the body 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.
- The space diagonal cannot be a power of 2 or 5 times a power of 2.:p. 101
Solutions have been found where the space diagonal and two of the three face diagonals are integers, such as:
Solutions are also known where all four diagonals but only two of the three edges are integers, such as:
There is no cuboid with integer space diagonal and successive integers for edges.:p.99
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, answering an open question of Richard Guy. Some of these have two rectangular faces.
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