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/// 1 / √(2π). pub const FRAC_1_SQRT_2PI: f64 = std::f64::consts::FRAC_2_SQRT_PI * 0.5 * std::f64::consts::FRAC_1_SQRT_2; /// Standard normal PDF: φ(x) = exp(−x²/2) / √(2π). #[must_use] pub fn norm_pdf(x: f64) -> f64 { (-0.5 * x * x).exp() * FRAC_1_SQRT_2PI }