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(*         *      The Rocq Prover / The Rocq Development Team           *)
(*  v      *         Copyright INRIA, CNRS and contributors             *)
(* <O___,, * (see version control and CREDITS file for authors & dates) *)
(*   \VV/  **************************************************************)
(*    //   *    This file is distributed under the terms of the         *)
(*         *     GNU Lesser General Public License Version 2.1          *)
(*         *     (see LICENSE file for the text of the license)         *)
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Local Open Scope R_scope.
(** TT Ak; 0<=k<=N *)
Fixpoint prod_f_R0 (f:nat -> R) (N:nat) : R :=
match N with
| O => f O
| S p => prod_f_R0 f p * f (S p)
end.
Notation prod_f_SO := (fun An N => prod_f_R0 (fun n => An (S n)) N).

(* Specifications: *)
(**********)
Lemma prod_SO_split :
forall (An:nat -> R) (n k:nat),
(k < n)%nat ->
prod_f_R0 An n =
prod_f_R0 An k * prod_f_R0 (fun l:nat => An (k +1+l)%nat) (n - k -1).
(**********)
Lemma prod_SO_pos :
forall (An:nat -> R) (N:nat),
(forall n:nat, (n <= N)%nat -> 0 <= An n) -> 0 <= prod_f_R0 An N.
(**********)
Lemma prod_SO_Rle :
forall (An Bn:nat -> R) (N:nat),
(forall n:nat, (n <= N)%nat -> 0 <= An n <= Bn n) ->
prod_f_R0 An N <= prod_f_R0 Bn N.
(** Application to factorial *)
Lemma fact_prodSO :
forall n:nat, INR (fact n) = prod_f_R0 (fun k:nat =>
(match (eq_nat_dec k 0) with
| left   _ => 1%R
| right _ => INR k
end)) n.
Lemma le_n_2n : forall n:nat, (n <= 2 * n)%nat.
(** We prove that (N!)^2<=(2N-k)!*k! forall k in [|O;2N|] *)
Lemma RfactN_fact2N_factk :
forall N k:nat,
(k <= 2 * N)%nat ->
Rsqr (INR (fact N)) <= INR (fact (2 * N - k)) * INR (fact k).
(**********)
Lemma INR_fact_lt_0 : forall n:nat, 0 < INR (fact n).
(** We have the following inequality : (C 2N k) <= (C 2N N) forall k in [|O;2N|] *)
Lemma C_maj : forall N k:nat, (k <= 2 * N)%nat -> C (2 * N) k <= C (2 * N) N.