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Extensions to an OEIS sequence entry consist of anything added by anyone at any time other than the original contributor at the original date given in the Author field. All extensions, except the most minor, are acknowledged, if not in the relevant field, in the Extensions field. The most common use of the Extensions field is to acknowledge additional terms, as it would be awkward to give credit in the Data field:

2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47 - more terms from Tom Tomsson, Jan 24 2010, 53, 59, 61, 67 - additional terms from Sally Sallinen, Feb 28 2010

Instead the Extensions field would say something like:

a(13)-a(15) from Tom Tomsson, Jan 24 2010; a(16)-a(19) from Sally Sallinen, Feb 28 2010

Note that additions from the original contributor count as extensions if made after the initial contribution.

The following table explains which fields can have extensions acknowledged within them and which require the use of the Extensions field:

Field Action Notation in Extensions field
Name Simplified or corrected the definition Simplified definition or Corrected definition followed by contributor's name and date
Data Added terms, including an initial term overlooked by the original contributor Something like a(12)-a(14) from followed by contributor's name and date.
Offset Adjusted or corrected offset Adjusted offset from or Corrected offset from followed by old value, new value and contributor's name and date
Comments Added comments None needed, contributor should be acknowledged in Comments field.
References Added references None needed
Links Removed or updated broken links None needed
Example Corrected an example. None needed, but may claim credit in Extensions field if desired.
Maple Added a Maple program None, use Maple comment syntax to claim credit in Maple field.
Mathematica Added a Wolfram Mathematica program None, use Mathematica comment syntax to claim credit in Mathematica field.
Prog Added a program in a programming language or for a computer algebra system other than Maple or Mathematica None, use a comment (if available) in the Prog field.[1]
Crossrefs Added cross-references None needed
Keyword Added or removed keywords List which keywords were added and/or removed, followed by contributor's name and date
Author N/A N/A
Extensions N/A N/A
  1. If the programming language has no comments (e.g., Malbolge), just leave the name and date after a dash.

OEIS sequence entry fields

Name · Data · Offset · Comments · References · Example · Maple · Mathematica · Prog · Crossrefs · Keyword · Author · Extensions