Central extensions of the Ptolemy-Thompson group and quantized   Teichmuller theory
  The central extension of the Thompson group $T$ that arises in the quantized Teichm\"uller theory is 12 times the Euler class. This extension is obtained by taking a (partial) abelianization of the so-called braided Ptolemy-Thompson group introduced and studied in \cite{FK2}. We describe then the cyclic central extensions of $T$ by means of explicit presentations. 
Louis Funar (IF), Vlad Sergiescu (IF)