pub fn bessel_polynomial(n: usize) -> Polynomial {
    let mut prev = Polynomial::new(vec![1f64]); // 1
    let mut curr = Polynomial::new(vec![1f64, 1f64]); // x + 1

    match n {
        0 => prev,
        1 => curr,
        _ => {
            for idx in 1..n {
                let k = idx as f64;
                std::mem::swap(&mut prev, &mut curr);
                curr = (2.0 * k + 1.0) * poly(vec![1f64, 0f64]) * prev.clone() + curr;
            }
            curr
        }
    }
}
Bessel Polynomial

# Description
Generate `n`-th order Bessel polynomial. Definition according to
Krall and Fink (1949).
src/structure/polynomial.rs