2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/58785We calculate Tits buildings for certain arithmetic subgroups of Sp(4). These give information about the boundary of the corresponding moduli spaces of abelian surfaces. More pictures (in colour) and a summary of the results (in English) can be found at http://www.bath.ac.uk/~masgks/Buildings/22 pages, 4 figures. LaTeX, epsfig, German. Minor corrections. To appear in Abh. Math. Sem. Univ. HamburgAlgebraic Geometry22E40 (Primary) 14K10, 51E24 (Secondary)text