Pythagorean quadruple

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 is odd can be generated by the formulas \begin{align} 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{align} where , , , are non-negative integers with greatest common divisor 1 such that is odd. Thus, all primitive Pythagorean quadruples are characterized by the 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 , , and do not appear in all possible orders) can be generated from two positive integers and as follows: If and have different parity, let be any factor of such that . Then and . Note that . A similar method exists for generating all Pythagorean quadruples for which and are both even. Let and and let be a factor of such that . Then and . This method generates all Pythagorean quadruples exactly once each when and run through all pairs of natural numbers and runs through all permissible values for each pair. No such method exists if both and 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 is 12. The quadruple with the minimal product is (1, 2, 2, 3). Given a Pythagorean quadruple (a,b,c,d) where d^2=a^2+b^2+c^2 then d can be defined as the norm of the quadruple in that d = \sqrt{a^2+b^2+c^2} and is analogous to the hypotenuse of a Pythagorean triple. Of the 3 terms within a primitive Pythagorean quadruple that are not the norm, exactly one will always be odd and the other two will be even. Consequently, the norm has to be odd. Every odd positive number other than 1 and 5 can be the norm of a primitive Pythagorean quadruple d^2=a^2+b^2+c^2 such that a, b, c are greater than zero and are coprime. All primitive Pythagorean quadruples with the odd numbers as norms up to 29 except 1 and 5 are given in the table below. Similar to a Pythagorean triple which generates a distinct right triangle, a Pythagorean quadruple will generate a distinct Heronian triangle. If is a Pythagorean quadruple with a^2 + b^2 + c^2 = d^2 it will generate a Heronian triangle with sides as follows: \begin{align} x &= d^2 - a^2 \\ y &= d^2 - b^2 \\ z &= d^2 - c^2 \end{align} It will have a semiperimeter s = d^2, an area A = abcd and an inradius r = abc/d. The exradii will be: \begin{align} r_x &= bcd/a, \\ r_y &= acd/b, \\ r_z &= abd/c. \end{align} The circumradius will be: R = \frac{(d^2 - a^2)(d^2 - b^2)(d^2 - c^2)}{4abcd} = \frac{abcd(1/a^2 + 1/b^2 + 1/c^2 -1/d^2)}{4} The ordered sequence of areas of this class of Heronian triangles can be found at . ==Relationship with quaternions and rational orthogonal matrices==

A primitive Pythagorean quadruple parametrized by corresponds to the first column of the matrix representation of conjugation by the Hurwitz quaternion restricted to the subspace of quaternions spanned by , , , which is given by 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 . Furthermore, we have that belongs to the orthogonal group SO(3,\mathbb{Q}), and, in fact, all 3 × 3 orthogonal matrices with rational coefficients arise in this manner. ==Primitive Pythagorean quadruples with small norm==

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