On the Strict Endoscopic Part of Modular Siegel Threefolds : 9783639666496 : 14 Oct 2014 : In this book, we study the non-holomorphic strict ndoscopic parts of inner cohomology spaces of a modular Siegel threefold respect to local systems. First we show that there is a non-zero subspace of the strict endoscopic part such that it is constructed by global theta lift of automorphic froms of a pair of genus one cuspidal forms. Secondly, we present an explicit analytic calculation of levels of lifted forms into GSp(4), based on the paramodular representations theory for GSp(4;F). Finally, we prove the conjecture, by C. Faber and G. van der Geer, that gives a description of the strict endoscopic part for Betti cohomology and (real) Hodge structures in the category of mixed Hodge structures, in which the modular Siegel threefold has level structure one.