A family of transformations on the space of curvesThis note stems from [1] where we study the integrable dynamics of a 1parameter family of correspondences on ideal polygons in the hyperbolic plane and hyperbolic space: two n-gons P = (p 1 , p 2 , . . .) and Q = (q 1 , q 2 , . . .)In the limit n → ∞, a polygon becomes a parameterized curve. The ground field can be either R of C; to fix ideas, choose R. Let us use the following definition of cross-ratio to define our correspondence (other five definitions result in the change of the constant c):We replace polygons by non-degenerate closed curves γ : R → RP 1 with γ ′ (t) > 0; to be concrete, let the period be π: γ(t + π) = γ(t). Also let us assume that the rotation number of the curve γ is 1, that is, γ : R/πZ → RP 1 is a diffeomorphism. Denote the space of such curves by C and let C = C/ PSL(2, R) be the moduli space.Then a continuous version of (1) is γ ′ (t)δ ′ (t) (δ(t) − γ(t)) 2 = c.