Monoids generated by projections

Abstract

We define and explore semireflection monoids on a finite-dimensional vector space. These are monoids generated by semireflections: linear maps fixing a subspace of codimension 1. We mostly focus on the case of projection monoids (where the generating semireflections are non-invertible). After exploring some general theory, we give some important examples, and give classification results for projection monoids on $\mathbb C^2$ and $\mathbb R^3$. We then briefly introduce affine projection monoids.