The Challenge

Let's get down to business. You hate me, /prog/, and I hate you. Being a Calculus, I am unsatisfied with my programming skill and seek improvement. I challenge any of you who take pride in your skill with C to engage in the following game:

- Put down a challenge. A criteria for a program that both of us shall implement with C.
- Wait for me to accept your challenge. I don't accept those stupid challenges you find on that project euler website. Math doesn't interest me, "such that there exists"-boys. The exercises in K&R2 and, perhaps simple Unix programs, are good examples of the kinds of programs that interest me. You know, useful ones.
- Post your code.
- Wait for me to post my code.

If your program is fancier than my own and yields little criticism from one Lambda A. Calculus, I shall leave /prog/. I'm open to other stakes as well.

I await your challenges and, if any stupid stack boys want to challenge me, go for it.

>>50
I forgot to say that it must be at least as accurate as my code (assume the taylor series contains enough terms to approximate to the long double precision between 0.5 and -0.5)
tgamma or tgammal is very inaccurate.