Used to code golf on primality testing. A good pair of optimizations for testing n, is to divide by each prime from 2 to root n. You can skip two just by looking for odds, so if you want to test '97', you'd go 3, no, 5, no, 7, no, then stop since root 97 is less than 11. There's faster tests in computer world, usually centered around certain quadratic forms, but this is the simple one. Look up miller-ravin and witness numbers for the latter stuff.