Berggren tree

The Berggren tree is a rooted tree of primitive Pythagorean triples, with three children at every node. Fix the convention $a$ odd, $b$ even and $c>0$. Starting from the column vector $(3,4,5{)}^{\mathsf{T}}$, multiply by any ofThe first children are $(5,12,13)$, $(21,20,29)$ and $(15,8,17)$. These unimodular matrices preserve the quadratic form $a^2+b^2-c^2$, integer parity and greatest common divisor. Every primitive Pythagorean triple occurs exactly once. F. J. M. Barning's report gives this matrix-generation theorem with the even leg first.^[ref]

An infinite descent argument explains completeness. Except at the root, the unique parent isIts hypotenuse is positive and smaller than $c$, since $c<a+b\le \sqrt{2}c$. The first two unmodified entries cannot both be negative; their signs select the inverse of $A$, $B$ or $C$. The odd entry cannot vanish; the even entry vanishes only at $(3,4,5)$. Repeated descent therefore reaches that root. Multiplying all three coordinates by a positive integer supplies every nonprimitive Pythagorean triple.