Function sph_ik_impl_n 

pub fn sph_ik_impl_n<P, E, V, const N: usize, const SCALED: bool>(
    x: V,
    far_threshold: E,
) -> (V, V, V, V)
where
    E: FloatElement,
    V: FloatVector<Element = E> + TranscendentalMathWithPolicy,
    P: Policy,

$(i_{n-1},\; i_n,\; k_{n-1},\; k_n)$, scaled by $(e^{-|x|}, e^{x})$ when SCALED. $i_n$ is folded by parity onto the whole line. $k_n$ is NaN for $x < 0$.

The modified spherical pair, $i_n(x) = \sqrt{\pi/2x}\,I_{n+1/2}(x)$ and likewise for $k$. SciPy ships both as spherical_in / spherical_kn. Boost ships neither.

Seeds are $i_{-1} = \cosh x / x$, $i_0 = \sinh x / x$ and $k_{-1} = k_0 = \tfrac{\pi}{2}e^{-x}/x$, the last two equal because $K$ is even in order. As with the oscillating pair, the work is done in the spherical normalization so no square root is formed only to be cancelled.

far_threshold is where the unscaled $i$ halves its exponential. See [unscale_i_pair].