Using Singer polygons, we construct locally finite affine buildings of types Ã2 and C2 which admit uniform lattices acting regularly on panels. This construction produces very explicit descriptions of these buildings as well as very short presentations of the lattices. All but one of the C2 -buildings are necessarily exotic. To the knowledge of the author, these are the first presentations of lattices in buildings of type C2 . Integral and rational group homology for the lattices is also calculated.
Associated to every group with a weak spherical Tits system of rank n + 1 with an appropriate rank n subgroup, we construct a relative spectral sequence involving group homology of Levi subgroups of both groups. Using the fact that such Levi subgroups frequently split as semidirect products of smaller groups, we prove homological stability results for unitary groups over division rings with infinite centre as well as for special linear and special orthogonal groups over infinite fields.
IntroductionHomological stability is the following question: Given an infinite series of groups G n , such as the general linear groups GL n , we consider the sequence of inclusionsThen, if we apply group homology of a fixed degree, does the corresponding sequence of homology modules stabilise eventually? This is an old question and there are many interesting results for various series of classical groups, usually over rings of finite stable rank. An overview of results in this area can be found in [Knu01, Chapter 2] and we will also provide references to the best known results for specific series of groups. Although the method of proof is usually based on a common idea, the action of the larger group on a highly connected simplicial complex, all proofs known to the author are tailored to specific series of groups.In this paper, we present a general method to prove homological stability, valid for all groups with weak spherical Tits systems, that is, groups acting strongly transitively on possibly weak spherical buildings. We then use this method to prove homological stability for various series of classical groups over division rings, usually improving the stability range previously known from work with larger classes of rings.The method is based on the observation that the simplicial complexes used by Charney in [Cha80]
Wagoner complexes are simplicial complexes associated to groups of Kac-Moody type. They admit interesting homotopy groups which are related to integral group homology if the root datum is of 2-spherical type. We give a general definition of Wagoner complexes, exhibit some simple properties and calculate low dimensional homotopy groups. In addition, we give a definition of affine Wagoner complexes related to groups admitting a root datum with valuation.
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