Function even_series 

fn even_series(
    power: u32,
    leading: f64,
    z: Complex,
    factor: impl Fn(f64) -> f64,
) -> RadialJet

The even power series sum_q t_q with t_0 = leading z^power and t_q = factor(q) z^2 t_(q-1), differentiated term by term: every term reuses its predecessor’s powers.