I saw this symbol: $\underline{\big|6} $ in a question bank for a chapter of permutations and combinations. I have included the question from the book to provide more context: image link.
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6
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1$\begingroup$ Unless it's just a typo, I have no idea. $\endgroup$– luluDec 14, 2018 at 19:20
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2$\begingroup$ The symbol means $6!$ $\endgroup$– Shubham JohriDec 14, 2018 at 19:21
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4$\begingroup$ That's an old symbol for the factorial of a number. It was mostly phased out by the early 1900s, but you can find it in most 1800s algebra textbooks. $\endgroup$– Dave L. RenfroDec 14, 2018 at 19:21
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$\begingroup$ Use of this symbol is referenced in this answer and the comments indicate a usage by Hilbert. $\endgroup$– MasonDec 14, 2018 at 19:26
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1$\begingroup$ @DaveL.Renfro Make an answer out of your comment :) $\endgroup$– mrtaurhoDec 14, 2018 at 19:28
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1 Answer
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This an old symbol for the factorial of a number. It was mostly phased out by the early 1900s, but you can find it in most 1800s literature, especially mathematicians in Great Britain. A useful historical survey of various notations for factorial is given in
Florian Cajori, History of symbols for $\underline{n}=$ factorial, Isis 3 #3 (Summer 1921), 414-418.
answered Dec 14, 2018 at 19:51