Nats smaller than n
Fixpoint natsLess n : list nat :=
match n with
0 => []
| S n => n :: natsLess n
end.
Lemma natsLess_in_iff n m:
n el natsLess m <-> n < m.
Proof.
induction m in n|-*;cbn. omega.
split.
-intuition. destruct n;intuition. apply IHm in H0. omega.
-intros. decide (m=n). intuition. right. apply IHm. omega.
Qed.
Lemma natsLess_S n :
natsLess (S n) = map S (natsLess n)++[0].
Proof.
induction n;cbn in *;congruence.
Qed.
Sum
Fixpoint sumn (A:list nat) :=
match A with
[] => 0
| a::A => a + sumn A
end.
Lemma sumn_app A B : sumn (A++B) = sumn A + sumn B.
Proof.
induction A;cbn;omega.
Qed.
Hint Rewrite sumn_app : list.
Lemma length_concat X (A : list (list X)) :
length (concat A) = sumn (map (@length _) A).
induction A;cbn. reflexivity. autorewrite with list in *. omega.
Qed.
Lemma sumn_rev A :
sumn A = sumn (rev A).
Proof.
enough (H:forall B, sumn A + sumn B = sumn (rev A++B)).
{specialize (H []). cbn in H. autorewrite with list in H. cbn in H. omega. }
induction A as [|a A];intros B. reflexivity.
cbn in *. specialize (IHA (a::B)). autorewrite with list in *. cbn in *. omega.
Qed.
Lemma sumn_map_natsLess f n :
sumn (map f (natsLess n)) = sumn (map (fun i => f (n - (1 + i))) (natsLess n)).
Proof.
rewrite sumn_rev. f_equal.
rewrite <- map_rev.
rewrite <- map_map with (g:=f) (f:= fun i => (n - (1+i))).
f_equal.
induction n;intros;autorewrite with list in *. reflexivity.
rewrite natsLess_S at 2. cbn. rewrite map_app. cbn.
rewrite map_map. cbn in IHn.
rewrite IHn. rewrite <- minus_n_O. reflexivity.
Qed.
Lemma sumn_map_add X f g (l:list X) :
sumn (map (fun x => f x + g x) l) = sumn (map f l) + sumn (map g l).
Proof.
induction l;cbn;nia.
Qed.
Lemma sumn_map_mult_c_r X f c (l:list X) :
sumn (map (fun x => f x *c) l) = sumn (map f l)*c.
Proof.
induction l;cbn;nia.
Qed.
Lemma sumn_map_c X c (l:list X) :
sumn (map (fun _ => c) l) = length l * c.
Proof.
induction l;cbn;nia.
Qed.
Definition maxl := fold_right max 0.
Lemma maxl_leq n l: n el l -> n <= maxl l.
Proof.
induction l;cbn.
-easy.
-intros [->|]. all:apply Nat.max_case_strong;try intuition Lia.lia.
Qed.
Lemma maxl_leq_l c l :
(forall n, n el l -> n <= c) -> maxl l <= c.
Proof.
induction l;cbn. Lia.lia.
intros H. eapply Nat.max_lub_iff;split. all:eauto.
Qed.
Lemma maxl_app l l': maxl (l++l') = max (maxl l) (maxl l').
Proof.
induction l;cbn;Lia.lia.
Qed.
Lemma maxl_rev l: maxl (rev l) = maxl l.
Proof.
unfold maxl. rewrite fold_left_rev_right. rewrite fold_symmetric. 2,3:now intros;Lia.lia.
induction l;cbn;try Lia.lia.
Qed.