The scipy.special module contains numerically stable definitions of useful functions. Most often, the straightforward evaluation of a function at a single value is not very efficient. For instance, we would rather use a Horner scheme (http://en.wikipedia.org/wiki/Horner%27s_method) to find the value of a polynomial at a point than use the raw formula. The NumPy and SciPy modules ensure that this optimization is always guaranteed with the definition of all its functions, whether by means of Horner schemes or with more advanced techniques.