We probably all agree that eventually reducing a difficult problem to a "nice" situation is at the heart of mathematics. - Beno Eckmann

Still another important area is Poincaré duality for groups, invented by Robert Bieri and myself. They behave like manifolds: homology, cohomology, you see, in complementary dimensions, but with another dualizing module. Many groups that are interesting in algebraic geometry, group theory or other areas are such duality groups.

Together with many other people and after a long development I could prove that a Poincaré duality group of cohomological dimension 2 is the group of a Riemann surface. That was actually a conjecture of Jean-Pierre Serre. "You have to prove it!" he had always insisted.