fn kambe_below_base(n: i32, z: Complex, eta: Complex) -> (Complex, f64)Expand description
Orders below the base pair from the series, which cancels by about e^(|c| - Re c)
for complex eta, or the downward recurrence from the base pair (as in treams),
accurate while |1/eta^2| exceeds about |n| and |z|: the smaller error bound
wins. A series whose predicted loss stays within RECURRENCE_GROWTH is kept
outright; otherwise odd orders sum their base pair in the same pass, and even orders
keep a recurrence that grows by at most RECURRENCE_GROWTH and try the series only
where its predicted loss is below RECURRENCE_GROWTH times the recurrence’s growth.