The automorphism group of the non-split Cartan modular curve of level 11
  We derive equations for the modular curve $X_{ns}(11)$ associated to a non-split Cartan subgroup of $\,\mathrm{GL}_2(\mathbf{F}_{11})$. This allows us to compute the automorphism group of the curve and show that it is isomorphic to Klein's four group. 
Valerio Dose, Julio Fern\'andez, Josep Gonz\'alez, Ren\'e Schoof
math.NT