GMP gives wrong (negative) results

[V. Nicolai Friedhoff]

Dear GMP Mailinglist,

I am using GMP to calculate the value of a convergent series f(x) which
includes large numbers. That function f(x) is a Cauchy product of two other
convergent series, the exponential function and the 2F2 Hypergeometric
function.
For small values of x<29 the results seem to be correct, so I am sure that
my maths and my implementation is correct. But for larger x>29 the results
are wrong and even negative (the function should be positive for positive
x).

What is wrong when GMP gives me negative results for a function that should
be positive once some numbers become large?

More details can be found in this stack overflow question i created
yesterday:

http://stackoverflow.com/questions/43121211/gmp-gives-wrong-result-of-a-series-expansion


BTW: Something similar happens on Mathematica. For approximately x>120 the
function also becomes abruptly negative.

Thank you a lot, Best,
Nicolai