About

This is a notebook for small pieces of arithmetic that I find satisfying: tricks for doing sums in your head, quick ways to check an answer, and old puzzles worked through slowly enough to see why they come out the way they do.

None of it is new. Most of these ideas are centuries old and have been explained better elsewhere. Writing them down is how I make sure I actually understand them.

New notes appear irregularly, usually every month or two. There is an RSS feed if you want to follow along.

The name is a reminder that even the simplest sum is worth checking.