proposition: Lemma ex_derive_inv (f : R -> R) (x : R) : ex_derive f x -> f x <> 0 -> ex_derive (fun y : R => / f y) x.
proof: Proof. case => l Hf Hl. exists (-l/(f x)^2). by apply is_derive_inv. Qed.
imports: Coq(Reals Psatz ssreflect) Rcomplements Rbar Lim_seq Iter Hierarchy Continuity Equiv