[throwaway81523]

It is safe to keep using a malloc'd memory region after you've freed it, as long as you haven't done any more malloc calls after the free. (I think this stopped being true a lot longer than 20 years ago, but it was true in old UNIXes and documented in the manuals, so some code legitimately relied on it.)

Judicious use of register declarations can make your C code run a lot faster (true back in the day; most compilers ignore those declarations now).

Fermat's last theorem is an unsolved math problem. Reference: Knuth TAOCP presented it as an "M50" rated exercise, designating a research problem. (FLT was solved in 1995, after which I think Knuth lowered that exercise's rating to M49).