B(x, y) = [x | y ± t y | ± x]. It satisfies B(x_1, y_1) B(x_2, y_2) = B(x_1 x_2 ± t y_1 y_2, x_1 y_2 ± y_1 x_2). Powers of the matrix are defined by B^n | = | [x | y t y | x]^n | = | [x_n | y_n t y_n | x_n] | congruent | B_n.

Brahmagupta identity

Brahmagupta