SPFNS2 is a special function file for the HP-41 written by
Sherman Lowell. Please refer to SPFNS1T for additional
information.

The SPFNS2 file contains several mathematical functions useful in
probability and statistics. They all relate to normal (or
Gaussian) probability distributions. LBL"ZX" returns the Normal
Probability Density function, Z(x). LBL"PX" returns the Normal
Probability function, P(x), the integral from minus infinity to x
of Z(x). LBL"QX" returns the Complementary Normal Probability
function, Q(x)=1-P(x). LBL"AX" returns the Symmetric Normal
Probability function, A(x)= 2P(x)-1. LBL"ERFX" returns the Error
function, erf(x). LBL"ERFCX" returns the Complementary Error
function, erfc(x)=1-erf(x). All of the above functions use only
the stack registers, no numbered storage registers. They expect x
in the X-register, return the function value in X-register and x
in LASTX. In this respect they behave like the built-in
functions. No additional modules of any kind are required. The
maximum error is believed to be < 5.E-9 for all x. An additional
probability function, the Chi Square function, is a part of the
SPFNS1 file. X<>Y