I've been working on some code using GSL and wondering why it didn't match the results I was getting from Mathematica and other programs. Well, one (of perhaps many) differences is that GSL defines the sinc function as
for , but Mathematica defines it as
The latter is the definition I was using in my math, and I didn't realize it didn't match the one in GSL until I broke the code down to individual mathematical terms.
So remember to make sure that any code you use is actually doing what you think it's doing!