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`erf', `erff', `erfc', `erfcf'--error function

     #include <math.h>
     double erf(double X);
     float erff(float X);
     double erfc(double X);
     float erfcf(float X);
`erf' calculates an approximation to the "error function", which
estimates the probability that an observation will fall within X
standard deviations of the mean (assuming a normal distribution).
   `erfc' calculates the complementary probability; that is, `erfc(X)'
is `1 - erf(X)'.  `erfc' is computed directly, so that you can use it
to avoid the loss of precision that would result from subtracting large
probabilities (on large X) from 1.
   `erff' and `erfcf' differ from `erf' and `erfc' only in the argument
and result types.
For positive arguments, `erf' and all its variants return a
probability--a number between 0 and 1.
None of the variants of `erf' are ANSI C.