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# A-numbers

### From OeisWiki

**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.

When N. J. A. Sloane's initial collection of sequences reached a few hundred, in the 1960's, he sorted them in lexicographic order, using the same ordering that is still in use today. That is, discard any initial terms that are 0, 1 or -1, and then sort into lexicographic order by increasing magnitude. 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 in the main OEIS (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.) (The OEIS wiki has a page A000000 for the empty sequence.)