This site is supported by donations to The OEIS Foundation.

# A-numbers

**A-numbers** are the absolute ID numbers or catalog numbers used for sequences in the OEIS database. The current format (until we reach A999999, which should not happen for at least 25 years) is A?????? where ? is a decimal digit.

*A Handbook of Integer Sequences* assigned sequential N-numbers to sequences in lexicographic order. When *The Encyclopedia of Integer Sequences* was released, in addition to lexicographic M-numbers, sequences were assigned absolute A-numbers which did not depend on this ordering. The first sequence in the resulting list was

- 1, 1, 1, 2, 1, 2, 1, 5, 2, 2, 1, 5, 1, 2, 1, 14, 1, 5, 1, 5, 2, 2, 1, 15, 2, 2, 5, 4, 1, 4, 1, 51, 1, 2, 1, 14, 1, 2, 2, 14, 1, 6, 1, 4, 2, 2, 1, 52, 2, 5, 1, 5, 1, 15, 2, 13, ...

the number of groups of order n, which became sequence A000001.

Some of the most famous A-numbers are (naturally these are among the longest entries in the OEIS)

For other important A-numbers see the entry for "core sequences" in the index of "core sequences" of the OEIS.

For some other especially interesting sequences see the **OEIS Posters**, which can be downloaded from the **OEIS Foundation** web site.

The A-number A000000 is inadmissible (it might have been used for the empty sequence, except that since that sequence has no terms, the lookup programs would not be able to handle it.)