Great Websites Series - #1: OEIS

I’m starting an occasional series on websites I keep coming back to - not for any practical reason I can always name, just because they’re good. No fixed schedule, no ranking, just things worth pointing at. First up: the On-Line Encyclopedia of Integer Sequences.

What it is

OEIS is a searchable database of integer sequences. You paste in a handful of numbers - e.g., 1, 1, 2, 3, 5, 8, 13 - and it tells you what you’re looking at, in this case the Fibonacci sequence, complete with formulas, generating functions, references back to papers from decades ago, code to generate it in a dozen languages, and links to dozens of related sequences. There are currently well over 380,000 entries.

It started in 1964 as a literal box of index cards kept by Neil Sloane, then a graduate student, because he kept running into sequences he didn’t recognise and wanted a place to look them up. The card catalogue became a book, then two books, then a website in the mid-1990s. The format has barely changed since: a single search box, plain text results, no account, no paywall.

Why it earns a bookmark

I first came across OEIS in a book - I no longer remember which one, probably something about prime numbers or a mathematician’s life. It stuck, though: ever since, whenever I run into a distinctive sequence of integers, my first instinct is to check whether it already has a name.