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Reduction theory using semistability. (English) Zbl 0564.20027
This paper gives a fresh approach in some cases to the Borel-Serre compactification of locally symmetric spaces of arithmetic type. The method begins from a construction of Stuhler’s of a canonical filtration of a given lattice L in Euclidean space. The elements in the filtration are sublattices K that give vertices of the convex hull of the set of points (dim K, log volume K) as K runs through all sublattices of L. This set is bounded in the (x,y) plane. Functions which measure the distance from L to a given ”cusp” at infinity are given in terms of these canonical filtrations. Deleting all L’s closer than a certain fixed amount to any cusp, we get a contractible manifold M whose boundary has the homotopy type of the appropriate Tits building.
If A is the ring of integers in a number field F and P a finitely generated projective A-module, we take for our L’s the group P equipped with an inner product on $$P\otimes F_ v$$ for every infinite place v. Then GL(P) will act properly discontinuously on M with compact quotient and all the results of A. Borel and J. P. Serre [in Comment. Math. Helv. 48, 436-491 (1973; Zbl 0274.22011)] are reproven for the arithmetic groups commensurate with GL(P). In the last section, the methods and results are extended to orthogonal and symplectic groups. This paper is reasonably self-contained.
Reviewer: A.Ash

##### MSC:
 20H05 Unimodular groups, congruence subgroups (group-theoretic aspects) 11E04 Quadratic forms over general fields 20G10 Cohomology theory for linear algebraic groups
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