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In a LaTeX document, I found the definition of a structure as {\cal_A}_0 = (bbbn; 0; s; =) (written as literals appearing in the compiled document, not as source code and / or in math mode).
What could the '\bbbn' mean? From the context, something like $\mathhbb{N}$ would be possible, but I am not sure and it could be everything else from the subject computability theory. I cannot ask the author.

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    The relation does not seem to make sense, there is no ` in front of bbbn, so it could well mean b times b times b times n` and use \mathcal{A} instead of {\cal A}. – user121799 Oct 12 '18 at 16:24
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    There is an obsolete amsmath macro \Bbb which was replaced by \mathbb, so \Bbbn would yield \mathbb{n}. It's still in amssymb. But the semicolons here make me wonder if this is some CSS from somewhere. Is this Mathjax source? – Alan Munn Oct 12 '18 at 19:04
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    See e.g. \BbbN vs \mathbb{N} – Alan Munn Oct 12 '18 at 19:09
  • If you post the two comments as one answer, I will grant you the best solution. Thanks! The semikolon is context specific, it is printed as-is and serves as a separator for the different entries of a structure – user7427029 Oct 12 '18 at 20:50
  • @AlanMunn please see above – Johannes_B Oct 13 '18 at 12:04
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There is an obsolete amsmath command \Bbb, which was replaced by \mathbb and \Bbbn would therefore be equivalent to \Bbb{n}, i.e., \mathbb{n}. The symbol still exists in amssymb for backwards compatibility purposes. It also still exists in MathJax source.

See: \BbbN vs \mathbb{N}

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