(** 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).