The On-Line Encyclopedia of Integer Sequences

It is the rare search box that can make a messy scrap of numbers feel like a clue instead of a dead end.

The entries go well beyond a name. A typical result can include formulas, comments, links, references, graphs, history, and related ways to poke at the sequence.

OEIS is useful because it gives math problems a memory. If the sequence has appeared before, this is usually where the trail starts looking less mysterious.