Where is 'erf' defined and can we include 'erfcx'?

[Søren Hauberg]

Hi All

We had an inquiry on the Octave-Forge list regarding having a proper
implementation of the 'erfcx' function. This functions computes

  exp(x^2)*erfc(x)

in a numerically stable way. The poster mentioned that other people seem
to use

  http://www.netlib.org/specfun/erf

for evaluating this. I was wondering if we were already using the Netlib
code for evaluating 'erf'? (I couldn't find the definition of 'erf' in
the source code; where can I find this?) If we already use Netlib, then
I guess the easiest solution would be to also include 'erfcx' in Octave.
If not, well then we'll create something for Octave-Forge.

Soren