A Beurling-Helson type theorem for modulation spaces
  We prove a Beurling-Helson type theorem on modulation spaces. More precisely, we show that the only $\mathcal{C}^{1}$ changes of variables that leave invariant the modulation spaces $\M{p,q}(\rd)$ are affine functions on $\rd$. A special case of our result involving the Sj\"ostrand algebra was considered earlier by A. Boulkhemair. 
Kasso A Okoudjou