## Integer sequences every computer scientist should know?

The On-Line Encyclopedia of Integer Sequences is quite a nifty website. Suppose that you’re solving a problem, and you come up with the following sequence of integers: `0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2...` and you wonder to yourself: “huh, what’s that sequence?” Well, just type it in and the answer comes back: A007814, along with all sorts of tasty tidbits like constructions, closed forms, mathematical properties, and more. Even simple sequences like powers of two have a bazillion alternate interpretations and generators.

This got me wondering: what are integer sequences that every computer scientist should know? That is, the ones they should be able to see the first few terms of and think, “Oh, I know that sequence!” and then rack their brains for a little bit trying to remember the construction, closed form or some crucial property. For example, almost anyone with basic math background will recognize the sequences 1, 2, 3, 4, 5; 0, 1, 4, 9, 16; or 1, 1, 2, 3, 5. The very first sequence I cited in this article holds a special place in my heart because I accidentally derived it while working on my article Adventures in Three Monads for The Monad.Reader. Maybe a little less familiar might be 1, 1, 2, 5, 14, 42 or 3, 7, 31, 127, 8191, 131071, 524287, 2147483647, but they are still quite important for computer scientists.

So, what integer sequences every computer scientist should know? (Alternatively, what’s your favorite integer sequence?)