A quasi Curtis–Tits–Phan theorem for the symplectic group

We obtain the symplectic group Sp(V) as the universal completion of an amalgam of low rank subgroups akin to Levi components. We let Sp(V) act flag-transitively on the geometry of maximal rank subspaces of V. We show that this geometry and its rank ⩾3 residues are simply connected with few exceptions. The main exceptional residue is described in some detail. The amalgamation result is then obtained by applying Tits' lemma. This provides a new way of recognizing the symplectic groups from a small collection of small subgroups.