Despite being released about 4 years after the TI-82, the TI-86 does not include median-median regression, which can be a useful method of linear regression for data with extreme outliers. Fortunately it's a simple enough algorithm to implement in a small program.
Here's a small TI-86 program for twiddling Linux/UNIX permission bits and seeing both the symbolic and numeric results. The simple ones (750, 640, 755, etc.) are easy enough to do in one's head, but it can start to get a bit messy when setuid, setgid, and sticky get involved.