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What is the motivation behind naming the category O appearing in the theory of Lie algebras? Does O stand for something? Here is a question Why the BGG category O? that further confuses me. It seems like there is a notion of when a category is O, is it?

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    $\begingroup$ @VítTuček, isn't that because of something like "O for olomorfe"? $\endgroup$
    – LSpice
    Commented Jun 11, 2020 at 5:43
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    $\begingroup$ I think that the $\mathcal O$ notation comes from the notation for rings of integers in number fields, probably standing for "order" ("Ordnung" in German). $\endgroup$
    – Angelo
    Commented Jun 11, 2020 at 7:25
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    $\begingroup$ Four comments, four different answers... I guess that proves this question is worthy of an authoritative answer! $\endgroup$
    – Wojowu
    Commented Jun 11, 2020 at 9:31
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    $\begingroup$ the paper by Bernstein, Gelfand & Gelfand that introduces category O just says: "We shall call this category of $g$ modules the category O." $\endgroup$ Commented Jun 11, 2020 at 18:31
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    $\begingroup$ Someone (Mirkovic? I'm not sure) told me that when they discovered it, they said "Oh, that's the right category!" This was probably a joke, but is my preferred explanation. $\endgroup$
    – Ben Webster
    Commented Feb 4, 2021 at 2:35

1 Answer 1

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From [Humphreys: Representations of semisimple Lie algebras in the BGG category O], notes for Chapter 1:

The letter chosen to label the category is the first letter of a Russian word meaning “basic”

which is основной.

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