Star Products that can not be induced by Drinfel'd Twists
  This is the final version of my master thesis. I prove that there are no twist star products on the 2-sphere and on the higher genus pretzel surfaces deforming a symplectic structure. One of the key arguments is that a connected compact symplectic manifold endowed with a twist star product has to be a homogeneous space. Furthermore, there is a non-degenerate r-matrix on a Lie algebra whose Lie group is acting transitively and effectively on the manifold in this case. I recap the theory of homogeneous spaces, in particular the classification of transitive effective actions on the 2-sphere, and discuss the notion of r-matrices and Drinfel'd twists. Some results of this thesis are available as a preprint with Pierre Bieliavsky, Chiara Esposito and Stefan Waldmann on arxiv.org under the title "Obstructions for Twist Star Products". 
Thomas Weber
math-ph math.MP math.QA math.SG