Definition Bessel1 (n : nat) (x : R) :=
  (x/2)^n * PSeries (Bessel1_seq n) ((x/2)^2).

Lemma Bessel1_equality_2 (n : nat) (x : R) : (0 < n)%nat -> x<>0 -> Bessel1 (S n)%nat x + Bessel1 (pred n)%nat x = (2*INR n)/x * Bessel1 n x.