X-Git-Url: http://matita.cs.unibo.it/gitweb/?a=blobdiff_plain;f=matita%2Fmatita%2Flib%2Farithmetics%2Fnat.ma;h=8f7acf2b20003066f80d67cf94af158d20d20ab2;hb=35653f628dc3a3e665fee01acc19c660c9d555e3;hp=266f01ef8c63d072e7c3e9d34657295402a7489c;hpb=e356b6fba9e8fd1e757e7589f01fb9b51979f76c;p=helm.git

diff --git a/matita/matita/lib/arithmetics/nat.ma b/matita/matita/lib/arithmetics/nat.ma
index 266f01ef8..8f7acf2b2 100644
--- a/matita/matita/lib/arithmetics/nat.ma
+++ b/matita/matita/lib/arithmetics/nat.ma
@@ -1,4 +1,4 @@
-  (*
+(*
     ||M||  This file is part of HELM, an Hypertextual, Electronic        
     ||A||  Library of Mathematics, developed at the Computer Science     
     ||T||  Department of the University of Bologna, Italy.                     
@@ -184,6 +184,9 @@ theorem minus_Sn_n: ∀n:nat. S 0 = (S n)-n.
 theorem eq_minus_S_pred: ∀n,m. n - (S m) = pred(n -m).
 @nat_elim2 normalize // qed.
 
+lemma plus_plus_comm_23: ∀x,y,z. x + y + z = x + z + y.
+// qed.
+
 (* Negated equalities *******************************************************)
 
 theorem not_eq_O_S : ∀n:nat. 0 ≠ S n.
@@ -368,6 +371,10 @@ theorem lt_plus_to_minus_r: ∀a,b,c. a + b < c → a < c - b.
 #a #b #c #H @le_plus_to_minus_r //
 qed.
 
+theorem lt_S_S_to_lt: ∀n,m:nat. S n < S m → n < m.
+(* demo *)
+/2/ qed-.
+
 (* not le, lt *)
 
 theorem not_le_Sn_O: ∀ n:nat. S n ≰ 0.
@@ -427,6 +434,16 @@ theorem increasing_to_le2: ∀f:nat → nat. increasing f →
   ]
 qed.
 
+lemma le_inv_plus_l: ∀x,y,z. x + y ≤ z → x ≤ z - y ∧ y ≤ z.
+/3 width=2/ qed-.
+
+lemma lt_inv_plus_l: ∀x,y,z. x + y < z → x < z ∧ y < z - x.
+/3 width=2/ qed-.
+
+lemma lt_or_ge: ∀m,n. m < n ∨ n ≤ m.
+#m #n elim (decidable_lt m n) /2 width=1/ /3 width=1/
+qed-.
+
 (* More general conclusion **************************************************)
 
 theorem lt_O_n_elim: ∀n:nat. 0 < n → 
@@ -519,6 +536,9 @@ pred n - pred m = n - m.
 #n #m #posn #posm @(lt_O_n_elim n posn) @(lt_O_n_elim m posm) //.
 qed.
 
+theorem plus_minus_commutative: ∀x,y,z. z ≤ y → x + (y - z) = x + y - z.
+/2 by plus_minus/ qed.
+
 (* More atomic conclusion ***************************************************)
 
 (* le *)
@@ -561,6 +581,9 @@ theorem increasing_to_le: ∀f:nat → nat.
 @(ex_intro ?? (S a)) /2/
 qed.
 
+lemma le_plus_compatible: ∀x1,x2,y1,y2. x1 ≤ y1 → x2 ≤ y2 → x1 + x2 ≤ y1 + y2.
+/3 by le_minus_to_plus, monotonic_le_plus_r, transitive_le/ qed.
+
 (* lt *)
 
 theorem not_eq_to_le_to_lt: ∀n,m. n≠m → n≤m → n<m.
@@ -630,7 +653,25 @@ theorem minus_minus: ∀n,m,p:nat. p ≤ m → m ≤ n →
 <commutative_plus <plus_minus_m_m //
 qed.
 
+lemma minus_minus_comm: ∀a,b,c. a - b - c = a - c - b.
+/3 by monotonic_le_minus_l, le_to_le_to_eq/ qed.
+
+lemma minus_le_minus_minus_comm: ∀b,c,a. c ≤ b → a - (b - c) = a + c - b.
+#b #c #a #H >(plus_minus_m_m b c) in ⊢ (? ? ?%); //
+qed.
+
+(* Stilll more atomic conclusion ********************************************)
+
+(* le *)
+
+lemma le_fwd_plus_plus_ge: ∀m1,m2. m2 ≤ m1 → ∀n1,n2. m1 + n1 ≤ m2 + n2 → n1 ≤ n2.
+#m1 #m2 #H #n1 #n2 >commutative_plus
+#H elim (le_inv_plus_l … H) -H >commutative_plus <minus_le_minus_minus_comm //
+#H #_ @(transitive_le … H) /2 width=1/
+qed-. 
+
 (*********************** boolean arithmetics ********************) 
+
 include "basics/bool.ma".
 
 let rec eqb n m ≝