Hard hexagon model

In mathematics, a pseudoreflection is a transformation of the form

$\left[\begin{array}{ccc}1& \cdots & 0\\ ⋮& \ddots & ⋮\\ 0& \cdots & -1\end{array}\right]$

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.