# Conformal Groups and Vahlen Matrices

**Jacques Helmstetter**, Universite Grenoble I, France

This article recalls some facts about the conformal group
Conf(V,Q) of a quadratic space (V,Q); in particular there is
a surjective group morphism O(V',Q')$\rightarrow$Conf(V,Q), where
(V',Q') is the orthogonal sum of (V,Q) and a hyperbolic
plane; its kernel is a group of order two. Then it explains
how the elements of O(V',Q') and Conf(V,Q) can be represented
by Vahlen matrices. And finally, it recalls some properties
of Vahlen matrices.