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If you’ve worked with modular arithmetic, this might be feeling sort of familiar.

For example, in modular arithmetic, also called clock arithmetic, you have a modulus—say, 12—and when you add or multiply two numbers, you look at the remainder when you divide by the modulus.

If you start playing with it a little bit, you’ll find the mod 12 system familiar: modular arithmetic is sometimes called clock arithmetic because 10:00 plus 1 hour is the same number on the clock as 10:00 plus 13 hours or 10:00 plus 25 hours.

The icing on the cake was spending part of last Saturday morning with my spouse and a tough little modular arithmetic problem.

We started the course with modular arithmetic, the foundation of number theory.