(** This file is part of the Coquelicot formalization of real analysis in Coq: http://coquelicot.saclay.inria.fr/  Copyright (C) 2011-2015 Sylvie Boldo #<br /># Copyright (C) 2011-2015 Catherine Lelay #<br /># Copyright (C) 2011-2015 Guillaume Melquiond  This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 3 of the License, or (at your option) any later version.  This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the COPYING file for more details. *)
(** This file defines complex numbers [C] as [R * R]. Operations are given, and [C] is proved to be a field, a normed module, and a complete space. *)
(** * The set of complex numbers *)
Definition C := (R * R)%type.
Definition RtoC (x : R) : C := (x,0).
Coercion RtoC : R >-> C.
(** ** Constants and usual functions *)
(** 0 and 1 for complex are defined as [RtoC 0] and [RtoC 1] *)
Definition Ci : C := (0,1).
(** *** Arithmetic operations *)
Definition Cplus (x y : C) : C := (fst x + fst y, snd x + snd y).
Definition Copp (x : C) : C := (-fst x, -snd x).
Definition Cminus (x y : C) : C := Cplus x (Copp y).
Definition Cmult (x y : C) : C := (fst x * fst y - snd x * snd y, fst x * snd y + snd x * fst y).
Definition Cinv (x : C) : C := (fst x / (fst x ^ 2 + snd x ^ 2), - snd x / (fst x ^ 2 + snd x ^ 2)).
Definition Cdiv (x y : C) : C := Cmult x (Cinv y).
Delimit Scope C_scope with C.
Local Open Scope C_scope.
Infix "+" := Cplus : C_scope.
Notation "- x" := (Copp x) : C_scope.
Infix "-" := Cminus : C_scope.
Infix "*" := Cmult : C_scope.
Notation "/ x" := (Cinv x) : C_scope.
Infix "/" := Cdiv : C_scope.
(** *** Other usual functions *)
Definition Re (z : C) : R := fst z.
Definition Im (z : C) : R := snd z.
Definition Cmod (x : C) : R := sqrt (fst x ^ 2 + snd x ^ 2).
Definition Cconj (x : C) : C := (fst x, (- snd x)%R).

Lemma Cmod_triangle : forall x y, Cmod (x + y) <= Cmod x + Cmod y.