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

Lemma Bessel1_equality_1 (n : nat) (x : R) : x <> 0 -> Bessel1 (S n)%nat x = INR n * Bessel1 n x / x - Derive (Bessel1 n) x.