Presentation by conjugation for A1-type extended affine Weyl groups

Abstract

There is a well-known presentation for finite and affine Weyl groups called the presentation by conjugation. Recently, it has been proved that this presentation holds for certain sub-classes of extended affine Weyl groups, the Weyl groups of extended affine root systems. In particular, it is shown that if nullity is ≤2, an A1-type extended affine Weyl group has the presentation by conjugation. We set up a general framework for the study of simply laced extended affine Weyl groups. As a result, we obtain certain necessary and sufficient conditions for an A1-type extended affine Weyl group of arbitrary nullity to have the presentation by conjugation. This gives an affirmative answer to a conjecture that there are extended affine Weyl groups which are not presented by "presentation by conjugation.". © 2007 Elsevier Inc. All rights reserved.