erf(x) returns the error function of its argument.  erfc(x) returns 1.0-erf(x).

erf(x) is defined by ${2 over sqrt(pi)} int from 0 to x e sup {-t sup 2} dt$

erfc() is provided because of the extreme loss of relative accuracy if erf(x) is
called for large x and the result subtracted from 1. (e.g. for x=10, 12 places
are lost).