Function imhof_sf 

pub fn imhof_sf(q: f64, weights: &[f64]) -> f64

Imhof’s (1961) exact inversion for P(Σ λ_i χ²₁ > q) (central case): P = 1/2 + (1/π) ∫₀^∞ sin θ(u) / (u ρ(u)) du with θ(u) = ½ Σ atan(λ_i u) − ½ q u, ρ(u) = Π (1 + λ_i² u²)^{1/4}.