Definition PS_scal (c : K) (a : nat -> V) (n : nat) : V := scal c (a n).

Lemma is_pseries_scal (c : K) (a : nat -> V) (x : K) (l : V) : mult x c = mult c x -> is_pseries a x l -> is_pseries (PS_scal c a) x (scal c l).