Twists versus Modifications

The twist construction is a geometric T-duality that produces new manifolds from old, works well with for example hypercomplex structures and is easily inverted. It tends to destroy properties such as the hyperK"ahler condition. On the other hand modifications preserve the hyperK"ahler property, but do not have an obvious inversion. In this paper we show how elementary deformations provide a link between the two constructions, and use the twist construction to build hyperK"ahler and strong HKT structures. In the process, we provide a full classification of complete hyperK"ahler four-manifolds with tri-Hamiltonian symmetry and study a number singular phenomena in detail.