The On-Line Encyclopedia of Integer Sequences (OEIS) is a searchable database of integer sequences accessible via the internet. It is regarded as one of the essential resources in mathematics.

The Encyclopedia is a database that contains information on integer sequences of mathematical relevance. The database has approximately 100,000 sequences, with each entry containing the sequence’s leading terms, keywords, mathematical justifications, literature connections, and other information about the sequence. The database is entirely searchable by keyword and a subset of phrases in a given sequence.

The Open Educational Information System (OEIS) is one of the most important online tools in mathematics for both professionals and amateurs. Thousands of hits are received each day, making it the world’s most comprehensive database of its sort in terms of size. The fact that the database is still free is unquestionably a contributing factor to its success.