Fundamental group of homotopy colimits. (Q1427015)

The author formulates a generalised van Kampen theorem in the following way: the fundamental groupoid functor preserves homotopy colimits. In order to do this, he develops a theory of homotopy colimits for diagrams of groupoids. He gives applications to the Bass-Serre theory of groups acting on trees, and to the fundamental groups of orbit spaces.