From e5a87f12545cd97dea383040590d62aff068b8ad Mon Sep 17 00:00:00 2001
From: Cyril Cohen
Date: Tue, 1 Sep 2020 15:47:49 +0200
Subject: Submatrix theory

---
 mathcomp/ssreflect/fintype.v | 11 +++++++++++
 1 file changed, 11 insertions(+)

(limited to 'mathcomp/ssreflect')

diff --git a/mathcomp/ssreflect/fintype.v b/mathcomp/ssreflect/fintype.v
index dca9a48..7d65f9e 100644
--- a/mathcomp/ssreflect/fintype.v
+++ b/mathcomp/ssreflect/fintype.v
@@ -1281,6 +1281,9 @@ apply: (iffP eqP) => [eqfA |]; last exact: card_in_image.
 by apply/dinjectiveP; apply/card_uniqP; rewrite size_map -cardE.
 Qed.
 
+Lemma leq_card_in A : {in A &, injective f} -> #|A| <= #|T'|.
+Proof. by move=> /card_in_image <-; rewrite max_card. Qed.
+
 Hypothesis injf : injective f.
 
 Lemma card_image A : #|image f A| = #|A|.
@@ -1295,6 +1298,8 @@ rewrite -card_image /=; apply: eq_card => y.
 by rewrite [y \in _]image_pre !inE andbC.
 Qed.
 
+Lemma leq_card : #|T| <= #|T'|. Proof. exact: (leq_card_in (in2W _)). Qed.
+
 Hypothesis card_range : #|T| = #|T'|.
 
 Lemma inj_card_onto y : y \in codom f.
@@ -1308,6 +1313,8 @@ Qed.
 End CardFunImage.
 
 Arguments image_injP {T T' f A}.
+Arguments leq_card_in [T T'] f.
+Arguments leq_card [T T'] f.
 
 Section FinCancel.
 
@@ -1866,6 +1873,10 @@ Qed.
 Lemma rev_ord_inj {n} : injective (@rev_ord n).
 Proof. exact: inv_inj rev_ordK. Qed.
 
+Lemma inj_leq m n (f : 'I_m -> 'I_n) : injective f -> m <= n.
+Proof. by move=> /leq_card; rewrite !card_ord. Qed.
+Arguments inj_leq [m n] f _.
+
 (* bijection between any finType T and the Ordinal finType of its cardinal *)
 Section EnumRank.
 
-- 
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