Veech surface
In mathematics, a Veech surface is a translation surface (X,ω) (a Riemann surface X with a holomorphic 1-form ω) whose group SL(X,ω) of affine diffeomorphisms is a lattice in SL2(R) (a discrete subgroup of cofinite volume). The image of SL(X,ω) in PSL2(R) is called the Veech group.