Function bessel_i_ratio_deriv 

pub fn bessel_i_ratio_deriv<P, E, V>(a: V, x: V, nu: V) -> V
where
    P: Policy,
    E: FloatElement,
    V: FloatVector<Element = E>,

A' = 1 - A^2 - (2 nu - 1) A / x from A itself.