Section PS_incr.
Context {K : AbsRing} {V : NormedModule K}.
Definition PS_incr_1 (a : nat -> V) (n : nat) : V :=
match n with
| 0 => zero
| S n => a n
end.
Fixpoint PS_incr_n (a : nat -> V) (n k : nat) : V :=
match n with
| O => a k
| S n => PS_incr_1 (PS_incr_n a n) k
end.
Definition PS_decr_1 (a : nat -> V) (n : nat) : V := a (S n).
Definition PS_decr_n (a : nat -> V) (n k : nat) : V := a (n + k)%nat.
End PS_incr.
Definition PS_mult (a b : nat -> R) n :=
sum_f_R0 (fun k => a k * b (n - k)%nat) n.
(** Sums on even and odd *)

Lemma is_pseries_odd_even (a : nat -> R) (x l1 l2 : R) :
is_pseries (fun n => a (2*n)%nat) (x^2) l1 -> is_pseries (fun n => a (2*n+1)%nat) (x^2) l2
-> is_pseries a x (l1 + x * l2).