fact: Definition differentiable_pt_lim (f : R -> R -> R) (x y : R) (lx ly : R) := forall eps : posreal, locally_2d (fun u v => Rabs (f u v - f x y - (lx * (u - x) + ly * (v - y))) <= eps * Rmax (Rabs (u - x)) (Rabs (v - y))) x y.
imports: Coq(Reals Psatz ssreflect) Rcomplements Hierarchy Continuity Derive