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.

Lemma ex_pseries_incr_n (a : nat -> V) (n : nat) (x : K) :
ex_pseries a x -> ex_pseries (PS_incr_n a n) x.