split off and generalize Kronecker Delta in own file

Signed-off-by: Ali Caglayan <alizter@gmail.com>

Author: Alizter

theories/Algebra/Rings/KroneckerDelta.v

@@ -0,0 +1,148 @@
+Require Import Basics.Overture Basics.Decidable Spaces.Nat.
+Require Import Algebra.Rings.Ring.
+Require Import Classes.interfaces.abstract_algebra.
+
+(** ** Kronecker Delta *)
+
+Section AssumeDecidable.
+  (** Throughout this section, we assume that we have a type [A] with decidable equality. This will be our indexing type and can be thought of as [nat] for reading purposes. *)
+
+  Context {A : Type} `{DecidablePaths A}.
+
+  (** The Kronecker delta function is a function of elements of [A] that is 1 when the two numbers are equal and 0 otherwise. It is useful for working with finite sums of ring elements. *)
+  Definition kronecker_delta {R : Ring} (i j : A) : R
+    := if dec (i = j) then 1 else 0.
+
+  (** Kronecker delta with the same index is 1. *)
+  Definition kronecker_delta_refl {R : Ring} (i : A)
+    : kronecker_delta (R:=R) i i = 1.
+  Proof.
+    unfold kronecker_delta.
+    generalize (dec (i = i)).
+    by rapply decidable_paths_refl.
+  Defined.
+
+  (** Kronecker delta with differing indices is 0. *)
+  Definition kronecker_delta_neq {R : Ring} {i j : A} (p : i <> j)
+    : kronecker_delta (R:=R) i j = 0.
+  Proof.
+    unfold kronecker_delta.
+    by decidable_false (dec (i = j)) p.
+  Defined.
+
+  (** Kronecker delta is symmetric in its arguments. *)
+  Definition kronecker_delta_symm {R : Ring} (i j : A)
+    : kronecker_delta (R:=R) i j = kronecker_delta j i.
+  Proof.
+    unfold kronecker_delta.
+    destruct (dec (i = j)) as [p|q].
+    - by decidable_true (dec (j = i)) p^.
+    - by decidable_false (dec (j = i)) (symmetric_neq q).
+  Defined.
+
+  (** An injective endofunction on [A] preserves the Kronecker delta. *)
+  Definition kronecker_delta_map_inj {R : Ring} (i j : A) (f : A -> A)
+    `{!IsInjective f}
+    : kronecker_delta (R:=R) (f i) (f j) = kronecker_delta i j.
+  Proof.
+    unfold kronecker_delta.
+    destruct (dec (i = j)) as [p|p].
+    - by decidable_true (dec (f i = f j)) (ap f p).
+    - destruct (dec (f i = f j)) as [q|q].
+      + apply (injective f) in q.
+        contradiction.
+      + reflexivity.
+  Defined.
+
+  (** Kronecker delta commutes with any ring element. *)
+  Definition kronecker_delta_comm {R : Ring} (i j : A) (r : R)
+    :  r * kronecker_delta i j = kronecker_delta i j * r.
+  Proof.
+    unfold kronecker_delta.
+    destruct (dec (i = j)).
+    - exact (rng_mult_one_r _ @ (rng_mult_one_l _)^).
+    - exact (rng_mult_zero_r _ @ (rng_mult_zero_l _)^).
+  Defined.
+    
+End AssumeDecidable.
+
+(** The following lemmas are specialised to when the indexing type is [nat]. *)
+
+(** Kronecker delta where the first index is strictly less than the second is 0. *)
+Definition kronecker_delta_lt {R : Ring} {i j : nat} (p : (i < j)%nat)
+  : kronecker_delta (R:=R) i j = 0.
+Proof.
+  apply kronecker_delta_neq.
+  intros q; destruct q.
+  by apply not_lt_n_n in p.
+Defined.
+
+(** Kronecker delta where the first index is strictly greater than the second is 0. *)
+Definition kronecker_delta_gt {R : Ring} {i j : nat} (p : (j < i)%nat)
+  : kronecker_delta (R:=R) i j = 0.
+Proof.
+  apply kronecker_delta_neq.
+  intros q; destruct q.
+  by apply not_lt_n_n in p.
+Defined.
+
+(** Kronecker delta can be used to extract a single term from a finite sum. *)
+Definition rng_sum_kronecker_delta_l {R : Ring} (n i : nat) (Hi : (i < n)%nat)
+  (f : forall k, (k < n)%nat -> R)
+  : rng_sum n (fun j Hj => kronecker_delta i j * f j Hj) = f i Hi.
+Proof.
+  induction n as [|n IHn] in i, Hi, f |- *.
+  1: destruct (not_leq_Sn_0 _ Hi).
+  destruct (dec (i = n)) as [p|p].
+  - destruct p; simpl.
+    rewrite kronecker_delta_refl.
+    rewrite rng_mult_one_l.
+    rewrite <- rng_plus_zero_r.
+    f_ap; [f_ap; rapply path_ishprop|].
+    nrapply rng_sum_zero.
+    intros k Hk.
+    rewrite (kronecker_delta_gt Hk).
+    apply rng_mult_zero_l.
+  - simpl; lhs nrapply ap.
+    + nrapply IHn.
+      apply diseq_implies_lt in p.
+      destruct p; [assumption|].
+      contradiction (lt_implies_not_geq Hi).
+    + rewrite (kronecker_delta_neq p).
+      rewrite rng_mult_zero_l.
+      rewrite rng_plus_zero_l.
+      f_ap; apply path_ishprop.
+Defined.
+
+(** Variant of [rng_sum_kronecker_delta_l] where the indexing is swapped. *)
+Definition rng_sum_kronecker_delta_l' {R : Ring} (n i : nat) (Hi : (i < n)%nat)
+  (f : forall k, (k < n)%nat -> R)
+  : rng_sum n (fun j Hj => kronecker_delta j i * f j Hj) = f i Hi.
+Proof.
+  lhs nrapply path_rng_sum.
+  2: nrapply rng_sum_kronecker_delta_l.
+  intros k Hk.
+  cbn; f_ap; apply kronecker_delta_symm.
+Defined.
+
+(** Variant of [rng_sum_kronecker_delta_l] where the Kronecker delta appears on the right. *)
+Definition rng_sum_kronecker_delta_r {R : Ring} (n i : nat) (Hi : (i < n)%nat)
+  (f : forall k, (k < n)%nat -> R)
+  : rng_sum n (fun j Hj => f j Hj * kronecker_delta i j) = f i Hi.
+Proof.
+  lhs nrapply path_rng_sum.
+  2: nrapply rng_sum_kronecker_delta_l.
+  intros k Hk.
+  apply kronecker_delta_comm.
+Defined.
+
+(** Variant of [rng_sum_kronecker_delta_r] where the indexing is swapped. *)
+Definition rng_sum_kronecker_delta_r' {R : Ring} (n i : nat) (Hi : (i < n)%nat)
+  (f : forall k, (k < n)%nat -> R)
+  : rng_sum n (fun j Hj => f j Hj * kronecker_delta j i) = f i Hi.
+Proof.
+  lhs nrapply path_rng_sum.
+  2: nrapply rng_sum_kronecker_delta_l'.
+  intros k Hk.
+  apply kronecker_delta_comm.
+Defined.

theories/Algebra/Rings/Ring.v

@@ -561,139 +561,3 @@ Proof.
   1: apply rng_mult_zero_l.
   lhs nrapply rng_dist_r; simpl; f_ap.
 Defined.
-
-(** ** Kronecker Delta *)
-
-(** The Kronecker delta function is a function of two natural numbers that is 1 when the two numbers are equal and 0 otherwise. It is useful for working with finite sums of ring elements. *)
-Definition kronecker_delta {R : Ring} (i j : nat) : R
-  := if dec (i = j) then 1 else 0.
-
-(** Kronecker delta with the same index is 1. This holds definitionally for a given natural number, but not definitionally for an arbitrary one. *)
-Definition kronecker_delta_refl {R : Ring} (i : nat)
-  : kronecker_delta (R:=R) i i = 1.
-Proof.
-  unfold kronecker_delta.
-  generalize (dec (i = i)).
-  by rapply decidable_paths_refl.
-Defined.
-
-(** Kronecker delta with different indices is 0. *)
-Definition kronecker_delta_neq {R : Ring} {i j : nat} (p : i <> j)
-  : kronecker_delta (R:=R) i j = 0.
-Proof.
-  unfold kronecker_delta.
-  by decidable_false (dec (i = j)) p.
-Defined.
-
-(** Kronecker delta is symmetric in its arguments. *)
-Definition kronecker_delta_symm {R : Ring} (i j : nat)
-  : kronecker_delta (R:=R) i j = kronecker_delta j i.
-Proof.
-  unfold kronecker_delta.
-  destruct (dec (i = j)) as [p|q].
-  - by decidable_true (dec (j = i)) p^.
-  - by decidable_false (dec (j = i)) (symmetric_neq q).
-Defined.
-
-(** Kronecker delta where the first index is strictly less than the second is 0. *)
-Definition kronecker_delta_lt {R : Ring} {i j : nat} (p : (i < j)%nat)
-  : kronecker_delta (R:=R) i j = 0.
-Proof.
-  apply kronecker_delta_neq.
-  intros q; destruct q.
-  by apply not_lt_n_n in p.
-Defined.
-
-(** Kronecker delta where the first index is strictly greater than the second is 0. *)
-Definition kronecker_delta_gt {R : Ring} {i j : nat} (p : (j < i)%nat)
-  : kronecker_delta (R:=R) i j = 0.
-Proof.
-  apply kronecker_delta_neq.
-  intros q; destruct q.
-  by apply not_lt_n_n in p.
-Defined.
-
-(** An injective endofunction on nat preserves the Kronecker delta. *)
-Definition kronecker_delta_map_inj {R : Ring} (i j : nat) (f : nat -> nat)
-  `{!IsInjective f}
-  : kronecker_delta (R:=R) (f i) (f j) = kronecker_delta i j.
-Proof.
-  unfold kronecker_delta.
-  destruct (dec (i = j)) as [p|p].
-  - by decidable_true (dec (f i = f j)) (ap f p).
-  - destruct (dec (f i = f j)) as [q|q].
-    + apply (injective f) in q.
-      contradiction.
-    + reflexivity.
-Defined.
-
-(** Kronecker delta commutes with any ring element. *)
-Definition kronecker_delta_comm {R : Ring} (i j : nat) (r : R)
-  :  r * kronecker_delta i j = kronecker_delta i j * r.
-Proof.
-  unfold kronecker_delta.
-  destruct (dec (i = j)).
-  - exact (rng_mult_one_r _ @ (rng_mult_one_l _)^).
-  - exact (rng_mult_zero_r _ @ (rng_mult_zero_l _)^).
-Defined.
-  
-(** Kronecker delta can be used to extract a single term from a finite sum. *)
-Definition rng_sum_kronecker_delta_l {R : Ring} (n i : nat) (Hi : (i < n)%nat)
-  (f : forall k, (k < n)%nat -> R)
-  : rng_sum n (fun j Hj => kronecker_delta i j * f j Hj) = f i Hi.
-Proof.
-  induction n as [|n IHn] in i, Hi, f |- *.
-  1: destruct (not_leq_Sn_0 _ Hi).
-  destruct (dec (i = n)) as [p|p].
-  - destruct p; simpl.
-    rewrite kronecker_delta_refl.
-    rewrite rng_mult_one_l.
-    rewrite <- rng_plus_zero_r.
-    f_ap; [f_ap; rapply path_ishprop|].
-    nrapply rng_sum_zero.
-    intros k Hk.
-    rewrite (kronecker_delta_gt Hk).
-    apply rng_mult_zero_l.
-  - simpl; lhs nrapply ap.
-    + nrapply IHn.
-      apply diseq_implies_lt in p.
-      destruct p; [assumption|].
-      contradiction (lt_implies_not_geq Hi).
-    + rewrite (kronecker_delta_neq p).
-      rewrite rng_mult_zero_l.
-      rewrite rng_plus_zero_l.
-      f_ap; apply path_ishprop.
-Defined.
-
-(** Variant of [rng_sum_kronecker_delta_l] where the indexing is swapped. *)
-Definition rng_sum_kronecker_delta_l' {R : Ring} (n i : nat) (Hi : (i < n)%nat)
-  (f : forall k, (k < n)%nat -> R)
-  : rng_sum n (fun j Hj => kronecker_delta j i * f j Hj) = f i Hi.
-Proof.
-  lhs nrapply path_rng_sum.
-  2: nrapply rng_sum_kronecker_delta_l.
-  intros k Hk.
-  cbn; f_ap; apply kronecker_delta_symm.
-Defined.
-
-(** Variant of [rng_sum_kronecker_delta_l] where the Kronecker delta appears on the right. *)
-Definition rng_sum_kronecker_delta_r {R : Ring} (n i : nat) (Hi : (i < n)%nat)
-  (f : forall k, (k < n)%nat -> R)
-  : rng_sum n (fun j Hj => f j Hj * kronecker_delta i j) = f i Hi.
-Proof.
-  lhs nrapply path_rng_sum.
-  2: nrapply rng_sum_kronecker_delta_l.
-  intros k Hk.
-  apply kronecker_delta_comm.
-Defined.
-
-(** Variant of [rng_sum_kronecker_delta_r] where the indexing is swapped. *)
-Definition rng_sum_kronecker_delta_r' {R : Ring} (n i : nat) (Hi : (i < n)%nat)
-  (f : forall k, (k < n)%nat -> R)
-  : rng_sum n (fun j Hj => f j Hj * kronecker_delta j i) = f i Hi.
-Proof.
-  lhs nrapply path_rng_sum.
-  2: nrapply rng_sum_kronecker_delta_l'.
-  intros k Hk.
-  apply kronecker_delta_comm.
-Defined.