fact: Fixpoint ex_diff_n f n x y := continuity_2d_pt f x y /\ match n with | O => True | S n => ex_derive (fun z => f z y) x /\ ex_derive (fun z => f x z) y /\ ex_diff_n (fun u v => Derive (fun z => f z v) u) n x y /\ ex_diff_n (fun u v => Derive (fun z => f u z) v) n x y end.
imports: Coq(Reals Psatz ssreflect) Rcomplements Hierarchy Continuity Derive