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I think it happened dozens of times that I corrected expressions like $$\huge\mathbb{F_5[x]}$$ to $$\huge\mathbb F_5[x].$$

I wonder why this particular misspelling — putting the whole expression as an argument into \mathbb — happens so often.

EDIT For an ordinary typo, this error happens way to often, in my opinion. For example, an auto-completion of missing closing brackets in the end of an expression could be a kind of an explanation. (But I never saw this happening).

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    $\begingroup$ Well, maybe because people are not paying enough attention to where the second bracket should be placed? $\endgroup$
    – user89712
    Commented Nov 17, 2013 at 19:56
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    $\begingroup$ Mis-typeset maybe? $\endgroup$
    – copper.hat
    Commented Nov 20, 2013 at 8:36
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    $\begingroup$ I guess that many people do not know that bracket is not needed if a parameter of the macro is only one character. $\endgroup$
    – Martin Sleziak
    Commented Nov 21, 2013 at 7:39

2 Answers 2

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The fact that MathJax renders $\mathbb{F_5[x]}$ as $\mathbb{F_5[x]}$ is just a quirk. If you enter the code into LaTeX, you will get something like this as a result: $$\mathbb{F}_{\ntrianglelefteq}[\curvearrowleft]$$ This is because the LaTeX mathbb command (which is an AMS extended command) has the effect of changing to a different font that has blackboard bold in the letter positions, and other symbols in other positions. For example, the 'x' position of this font appears to hold the symbol $\curvearrowleft$, which makes sense because that is another symbol in the AMS extended symbols set.

It seems MathJax handles the font changes differently, with output that is not so strange. So this is an error that only happens when people use MathJax but not LaTeX.

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Just guessing, but it might be that people consider the whole expression the name of a field, and they are used to writing the names of sets and similar objects in blackboard bold. After all, $\mathbb F$ all by itself usually does not denote a set, but the whole expression does. This does not mean that I agree with this use, only that I find it understandable.

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