arithmetics for λδ

[helm.git] / matita / matita / contribs / lambdadelta / ground / arith / nat_iter.ma

diff --git a/matita/matita/contribs/lambdadelta/ground/arith/nat_iter.ma b/matita/matita/contribs/lambdadelta/ground/arith/nat_iter.ma
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+(**************************************************************************)
+(*       ___                                                              *)
+(*      ||M||                                                             *)
+(*      ||A||       A project by Andrea Asperti                           *)
+(*      ||T||                                                             *)
+(*      ||I||       Developers:                                           *)
+(*      ||T||         The HELM team.                                      *)
+(*      ||A||         http://helm.cs.unibo.it                             *)
+(*      \   /                                                             *)
+(*       \ /        This file is distributed under the terms of the       *)
+(*        v         GNU General Public License Version 2                  *)
+(*                                                                        *)
+(**************************************************************************)
+
+include "ground/arith/pnat_iter.ma".
+include "ground/arith/nat.ma".
+
+(* NON-NEGATIVE INTEGERS ****************************************************)
+
+definition niter (n:nat) (A:Type[0]) (f:A→A) (a:A) ≝
+match n with
+[ nzero  ⇒ a
+| ninj p ⇒ f^{A}p a
+]
+.
+
+interpretation
+  "iterated function (non-negative integers)"
+  'Exp A f n = (niter n A f).
+
+(* Basic rewrites ***********************************************************)
+
+lemma niter_zero (A) (f) (a): a = (f^{A}𝟎) a.
+// qed.
+
+lemma niter_inj (A) (f) (p) (a): f^p a = f^{A}(ninj p) a.
+// qed.
+
+(* Advanced rewrites ********************************************************)
+
+lemma niter_appl (A) (f) (n) (a): f (f^n a) = f^{A}n (f a).
+#A #f * //
+#p #a <niter_inj <niter_inj <piter_appl //
+qed.