Functions and differentials on the non-split Cartan modular curve of   level 11
  The genus $4$ modular curve $X_{ns}(11)$ attached to a non-split Cartan group of level $11$ admits a model defined over $\mathbb{Q}$. We compute generators for its function field in terms of Siegel modular functions. We also show that its Jacobian is isomorphic over $\mathbb{Q}$ to the new part of the Jacobian of the classical modular curve $X_0(121)$. 
Julio Fern\'andez and Josep Gonz\'alez
math.NT