The TI-89 calculator is a powerful workhorse. In addition to its significant library of pre-installed functions and programs, you can write your own functions and programs. This collection of functions and programs was developed during an undergraduate course of study in Mathematics. You are free to copy/use them as you see fit, just indicate where they came from.
If you are interested in learning how to write programs for the TI-89, please consult the manual that came with your TI-89 or check out the manual for the TI-89 Titanium (the section on programming starts on p. 572).