[Rd] erf calculation (PR#13271)

[Jerry.Lewis at biogenidec.com]

Full_Name: Jerry W. Lewis
Version: 2.8.0
OS: Windows XP Professional
Submission from: (NULL) (71.184.139.210)


On p.1202 of the Reference manual, calculating erf(x) is given as an example
using the code

erf <- function(x) 2 * pnorm(x * sqrt(2)) - 1

A numerically better (avoiding cancellation for x near 0) formula is

erf <- function(x){
  ret <- pgamma(x^2,0.5,1)
  ret[x<0] <- -ret[x<0]
  ret
}

It would also be convenient if erf and erfc functions were provided in R, as
they are in S-PLUS.