pub fn normal_pdf(x: f64) -> f64Expand description
Standard normal PDF φ(x). Implementation lives in gam-math; re-exported
to keep crate::probability::normal_pdf resolving for all existing callers.
Standard normal PDF phi(x).
The squared argument is carried exactly (see [square_residual]); without
that, exp(-½·fl(x*x)) degrades like x²·ε/2 and reaches 5.7e-14
relative before φ underflows, against the 3.3e-16 it holds with.