Hard hexagon model

In mathematics, a pseudoreflection is a transformation of the form

${\begin{bmatrix}1&\cdots &0\\\vdots &\ddots &\vdots \\0&\cdots &-1\end{bmatrix}}$

with all but one of the diagonal elements 1. If the space on which this transformation acts has a symmetric bilinear form defined on it, so that vectors can be orthogonal, then the transformation is a true reflection.