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Toshiyasu Arai says in [Arai, A sneak preview of proof theory of ordinals (p.7)] the quote "every multiplicative principal number is ...". Is 2 considered multiplicatively indecomposable, but not multiplicatively principal? C7XWiki (talk) 22:34, 25 January 2021 (UTC)[reply]

I'd like to see a brief section "History" in the article, mentioning who came up with the concept, and linking to a primary source. - Jochen Burghardt (talk) 16:35, 10 August 2021 (UTC)[reply]