Associative algebra deformations of the Connes-Moscovici's Hopf algebra   $\mathcal{H}_1$
  We compute the second Hochschild cohomology space $HH^2(\mathcal{H}_1)$ of Connes-Moscovici's Hopf algebra $\mathcal{H}_1$, giving the infinitesimal deformations (up to equivalence) of the associative structure.   $HH^2(\mathcal{H}_1)$ is shown to be one dimensional, and thus Connes-Moscovici's formal deformation of $\mathcal{H}_1$ using Rankin-Cohen brackets is unique up to equivalence. 
Alice Fialowski (Eotvos Lorand University Budapest, Hungary),   Friedrich Wagemann (Universite Nantes, France)