Obstructions for Twist Star Products
  In this short note we point out that not every star product is induced by a Drinfel'd twist by showing that not every Poisson structure is induced by a classical $r$-matrix. Examples include the higher genus symplectic Pretzel surfaces and the symplectic sphere $\mathbb{S}^2$. 
Pierre Bieliavsky, Chiara Esposito, Stefan Waldmann, Thomas Weber
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