On-Line Encyclopedia of Integer Sequences

Valuable tool when solving problems at Project Euler.

From the Welcome-page:

Most people use the OEIS to get information about a particular number sequence.

Citing Wikipedia:

The On-Line Encyclopedia of Integer Sequences (OEIS) is an online database of integer sequences. It was created and maintained by Neil Sloane while researching at AT&T Labs. He transferred the intellectual property and hosting of the OEIS to the OEIS Foundation in 2009.[4] Sloane is the chairman of the OEIS Foundation.

OEIS records information on integer sequences of interest to both professional and amateur mathematicians, and is widely cited. As of April 2023, it contains over 360,000 sequences,[5] making it the largest database of its kind.[citation needed]

Each entry contains the leading terms of the sequence, keywords, mathematical motivations, literature links, and more, including the option to generate a graph or play a musical representation of the sequence. The database is searchable by keyword, by subsequence, or by any of 16 fields.

https://oeis.org/