I noticed that some of the more frequently used MO tags are formatted according to a certain pattern, e.g.:

  • nt.number-theory
  • pr.probability
  • lo.logic
  • fa.functional-analysis

Why are the tags formatted like this? Are they shortcuts for user convenience?


1 Answer 1


The tags with a two-letter prefix are exactly those tags that correspond to a math subject class on arXiv. They serve as top-level tags. As a rule, each question should have at least one of these top-level tags.

The formatting is mainly in place to distinguish them from usual tags; that it can be convenient is a side aspect, in particular, other tags of this form must not be created 'for convenience', they will be deleted as soon as they are noted.

A complete list of these tags (copied from François G. Dorais)


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