Computes the normalized incomplete Beta function
I_x(a,b)=B_x(a,b)/B(a,b)
where B_x(a,b) = \int_0^x t^(a-1) (1-t)^(b-1) dt for 0 \le x \le 1.
For a > 0, b > 0 the value is computed using a continued fraction expansion. For all other values it is computed using the relation I_x(a,b,x) = (1/a) x^a 2F1(a,1-b,a+1,x)/B(a,b).
err contains an estimate of the absolute error in the value z.
This function is from the GNU Scientific Library, see http://www.gnu.org/software/gsl/ for documentation.
Package: gsl