Pythagorean quadruple

All four primitive Pythagorean quadruples with only single-digit values

A Pythagorean quadruple is a tuple of integers a, b, c and d, such that d > 0 and a² + b² + c² = d². Geometrically, a Pythagorean quadruple (a, b, c, d) defines a cuboid with integer side lengths | a |, | b |, and | c |, whose space diagonal has integer length d. Pythagorean quadruples are thus also called Pythagorean boxes.^[1]

Parametrization of primitive quadruples

A Pythagorean quadruple is called primitive if the greatest common divisor of its entries is 1. Every Pythagorean quadruple is an integer multiple of a primitive quadruple. The set of primitive Pythagorean quadruples for which a is odd is parametrized by

${\displaystyle {\begin{aligned}a&=m^{2}+n^{2}-p^{2}-q^{2},\\b&=2(mq+np),\\c&=2(nq-mp),\\d&=m^{2}+n^{2}+p^{2}+q^{2},\end{aligned}}}$

where m, n, p, q are non-negative integers with greatest common divisor 1 such that m + n + p + q is odd.^[2][3][4] Thus, all primitive Pythagorean quadruples are characterized by the Lebesgue identity

$(m^2 + n^2 + p^2 + q^2)^2 = (2mq + 2np)^2 + (2nq - 2mp)^2 + (m^2 + n^2 - p^2 - q^2)^2.$

Alternate parametrization

All Pythagorean quadruples (including non-primitives, and with repetition, though a, b and c do not appear in all possible orders) can be generated from two positive integers a and b as follows:

If a and b have different parity, let p be any factor of a² + b² such that p² < a² + b². Then c = a² + b² − p²/2p and d = a² + b² + p²/2p. Note that p = d − c.

A similar method exists^[5] for generating all Pythagorean quadruples for which a and b are both even. Let l = a/2 and m = b/2 and let n be a factor of l² + m² such that n² < l² + m². Then c = l² + m² − n²/n and d = l² + m² + n²/n. This method generates all Pythagorean quadruples exactly once each when l and m run through all pairs of natural numbers and n runs through all permissible values for each pair.

No such method exists if both a and b are odd, in which case no solutions exist as can be seen by the parametrization in the previous section.

Properties

The largest number that always divides the product abcd is 12.^[6] The quadruple with the minimal product is (1, 2, 2, 3).

Relationship with quaternions and rational orthogonal matrices

A primitive Pythagorean quadruple (a, b, c, d) parametrized by (m,n,p,q) corresponds to the first column of the matrix representation E(α) of conjugation α(⋅)α by the Hurwitz quaternion α = m + ni + pj + qk restricted to the subspace of ℍ spanned by i, j, k, which is given by

${\displaystyle E(\alpha )={\begin{pmatrix}m^{2}+n^{2}-p^{2}-q^{2}&2np-2mq&2mp+2nq\\2mq+2np&m^{2}-n^{2}+p^{2}-q^{2}&2pq-2mn\\2nq-2mp&2mn+2pq&m^{2}-n^{2}-p^{2}+q^{2}\\\end{pmatrix}},}$

where the columns are pairwise orthogonal and each has norm d. Furthermore, we have 1/dE(α) ∈ SO(3,ℚ), and, in fact, all 3 × 3 orthogonal matrices with rational coefficients arise in this manner.^[7]

Primitive Pythagorean quadruples with small norm

There are 31 primitive Pythagorean quadruples in which all entries are less than 30.

(	1	,	2	,	2	,	3	)	(	2	,	10	,	11	,	15	)	(	4	,	13	,	16	,	21	)	(	2	,	10	,	25	,	27	)
(	2	,	3	,	6	,	7	)	(	1	,	12	,	12	,	17	)	(	8	,	11	,	16	,	21	)	(	2	,	14	,	23	,	27	)
(	1	,	4	,	8	,	9	)	(	8	,	9	,	12	,	17	)	(	3	,	6	,	22	,	23	)	(	7	,	14	,	22	,	27	)
(	4	,	4	,	7	,	9	)	(	1	,	6	,	18	,	19	)	(	3	,	14	,	18	,	23	)	(	10	,	10	,	23	,	27	)
(	2	,	6	,	9	,	11	)	(	6	,	6	,	17	,	19	)	(	6	,	13	,	18	,	23	)	(	3	,	16	,	24	,	29	)
(	6	,	6	,	7	,	11	)	(	6	,	10	,	15	,	19	)	(	9	,	12	,	20	,	25	)	(	11	,	12	,	24	,	29	)
(	3	,	4	,	12	,	13	)	(	4	,	5	,	20	,	21	)	(	12	,	15	,	16	,	25	)	(	12	,	16	,	21	,	29	)
(	2	,	5	,	14	,	15	)	(	4	,	8	,	19	,	21	)	(	2	,	7	,	26	,	27	)

See also

• Beal's conjecture
• Euler brick
• Euler's sum of powers conjecture
• Fermat cubic
• Jacobi–Madden equation
• Prouhet–Tarry–Escott problem
• Pythagorean triple
• Quaternions and spatial rotation
• Euler-Rodrigues formula for 3D rotations
• Taxicab number

References

1. R.A. Beauregard and E. R. Suryanarayan, Pythagorean boxes, Math. Magazine 74 (2001), 222–227.
2. R.D. Carmichael, Diophantine Analysis, New York: John Wiley & Sons, 1915.
3. L.E. Dickson, Some relations between the theory of numbers and other branches of mathematics, in Villat (Henri), ed., Conférence générale, Comptes rendus du Congrès international des mathématiciens, Strasbourg, Toulouse, 1921, pp. 41–56; reprint Nendeln/Liechtenstein: Kraus Reprint Limited, 1967; Collected Works 2, pp. 579–594.
4. R. Spira, The diophantine equation x² + y² + z² = m², Amer. Math. Monthly Vol. 69 (1962), No. 5, 360–365.
5. Sierpiński, Wacław, Pythagorean Triangles, Dover, 2003 (orig. 1962), p.102–103.
6. MacHale, Des, and van den Bosch, Christian, "Generalising a result about Pythagorean triples", Mathematical Gazette 96, March 2012, pp. 91-96.
7. J. Cremona, Letter to the Editor, Amer. Math. Monthly 94 (1987), 757–758.

External links

• Weisstein, Eric W. "Pythagorean Quadruple". MathWorld.

• Weisstein, Eric W. "Lebesgue's Identity". MathWorld.

• Carmichael. Diophantine Analysis at Project Gutenberg
• The complete parametrization derived using a Minkowskian Clifford Algebra