Sgmentation fault from hypergeometric function

[Dan, Ho-Jin]

  I don't have the book by Abramowitz & Stegun refered in the gsl manual,
But I found the confluent hypergeometrc function from the scratch
using the method of frobenius at x=0 (a regular singular point)
x y'' + (b-x) y' - a y = 0
one of sol'ns 1F1(a,b,x)=M(a,b,x)
M(a,b,x) = sum_{k=0}[ (a)_k / (b)_k ] x^k / k!
where, (a)_k = a*(a+1)...*(a+k-1) with (a)_0 = 1
and the other sol'n is expressed,
x^{1-b} M(a-b+1, 2-b, x)
and with a=1, b=0.5, x=1.0 is included the circle of convergence of the 
series M

is it right? If the above sol'ns is right and generally used notation,
then the gsl_sf_hyperg_1F1function seems to be abnormal.
but, I'm sorry that I can't assure of this conclusion.

In the future, I hope for the definition of the special function to be 
included

Regards,
Dan, Ho-Jin