pub fn normal_logcdf(x: f64) -> f64Expand description
Numerically stable ln Φ(x) for the standard normal CDF. Implementation lives
in gam-math; re-exported to keep crate::probability::normal_logcdf resolving
for all existing callers.
Numerically stable ln Φ(x) for the standard normal CDF. For x ≥ 0,
evaluates ln(1 - 0.5 erfc(x/sqrt(2))) with ln_1p, retaining the small
negative result after Φ(x) itself rounds to one. For x < 0, rewrites
ln Φ(x) = −u² + ln(½·erfcx(u)), u = −x/√2,
which preserves digits throughout the representable left tail without a
probability floor. Returns the corresponding IEEE limit at infinities and
propagates NaN.