On the unramified spherical automorphic spectrum

This thesis contains two results on harmonic analysis of reductive groups. First, let G be connected and defined over a number field F, A be the ring of adèles and K be a maximal compact subgroup of G(A). We studied the decomposition of the space of square-integrable functions on the quotient G(F)\G(A)/K, as a module for a global Hecke algebra. Similar results than the ones obtained here have been established by various authors for many special cases of reductive groups. The main feature of the present approach is the fact that it is uniform. Such approach was greatly inspired by results of G. Heckman and E. Opdam in treating spectral problems for graded affine Hecke algebras. In the proof, we need a result by M. Reeder on the weight spaces of the (anti)spherical discrete series representations of affine Hecke algebras, as well as we are faced with the problem of computing certain rational constants factors involved in the global spectral measure in terms of local Plancherel measures which are known only in the affine Hecke algebra context.
As for the second result, we show that a Coxeter complex and a Euclidean building can be endowed with piecewise linear Morse functions that allows one to write down explicit contractions of the underlying cell complexes. Such approach via PL Morse theory to study buildings was heavily inspired by ideas from G. Savin and M. Bestvina in the specific case of the building of SL(n). We conjecture that these contractions have nice bounds on their coefficients and thus can be used to compute Ext groups between tempered representations in an analogous way as was done by M. Solleveld and E. Opdam.