diff --git a/Modules/mathmodule.c b/Modules/mathmodule.c
index 6b3e1396b79..7bbcf58f542 100644
--- a/Modules/mathmodule.c
+++ b/Modules/mathmodule.c
@@ -400,9 +400,8 @@ m_lgamma(double x)
    Implementations of the error function erf(x) and the complementary error
    function erfc(x).
 
-   Method: following 'Numerical Recipes' by Flannery, Press et. al. (2nd ed.,
-   Cambridge University Press), we use a series approximation for erf for
-   small x, and a continued fraction approximation for erfc(x) for larger x;
+   Method: we use a series approximation for erf for small x, and a continued
+   fraction approximation for erfc(x) for larger x;
    combined with the relations erf(-x) = -erf(x) and erfc(x) = 1.0 - erf(x),
    this gives us erf(x) and erfc(x) for all x.