Evaluation of the upper incomplete gamma function

This function evaluates the upper incomplete gamma function

GAMMA_INCOMPLETE evaluates the upper incomplete gamma function
(incomplete gamma function of the second kind) $\Gamma(a,x)$
at non-negative values of the argument. This function extends the
MATLAB function gammainc to negative values of the parameter a.

SYNOPSIS: fun=gamma_incomplete(x,a)

INPUT x : function argument
a : parameter

OUTPUT fun : a vector of the same length as x; it contains NaN values
at places where elements of x are negative.

REMARKS 1. This function extends the MATLAB function gammainc
to negative values of the parameter a. 2. For positive a this function is equivalent
to the call fun=gamma(a).*gammainc(x,a,'upper');

EXAMPLES

x=0.01:0.01:8;
f=gamma_incomplete(x,1);
plot(x,f);

x=0.001:0.001:.1;
f=gamma_incomplete(x,-2.3);
plot(x,f);

x=0.001:0.001:.2;
f=gamma_incomplete(x,-1);
plot(x,f);