wreaths
endoscopy for gsp 4 and the cohomology of siegel modular threefolds
p the geometry of modular curves and the structure of their cohomology groups have been a rich source for various number theoretical applications over the last decades . similar applications may be expected from the arithmetic of higher dimensional modular varieties . for siegel modular threefolds some basic results on their cohomology groups are derived in this book from considering topological trace formulas .