# Why can't I use 𝔽?

I can't figure out why LaTeX won't let me use the 𝔽 symbol (U+1D53D, mathematical double-stuck capital F). When I try, I get the error Package inputenc Error: Unicode char \u8:𝔽 not set up for use with LaTeX.

The input file is UTF-8 encoded. Here is a minimal working (or rather, non-working) example to demonstrate the issue.

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{mathtools}
\begin{document}
% \newcommand{\eff}[1]{F\text{#1}}
\newcommand{\eff}[1]{𝔽\text{#1}}
$\eff{some}$
\end{document}


When using the first variant of \eff, everything works (the output is rendered as "Fsome" with an italic F, just as expected). However, if I comment that one out and instead uncomment the second variant, I get the error on line 8 (where I actually use the freshly defined command).

I'm using pdfLaTeX 3.1415926-1.40.10-2.2 from and on Debian Squeeze (package texlive-latex-base version 2009-11+squeeze1) with no special options at all (just pdflatex mwe.tex) to try to render the document.

I find it hard to believe that nobody would have wanted to use this particular symbol in a LaTeX document, so surely it can be done. What am I missing?

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You can use \mathbb{F} from the package amssymb, see texblog.org/2007/08/27/… – N.N. Sep 29 '12 at 8:02
I would assume that not every symbol defined in Unicode is actually mapped to a glyph that is available in the fonts you are using. This is also going to create portability problems for your source code. – Christian Lindig Sep 29 '12 at 8:03
@N.N. \mathbb{F} worked perfectly, thank you! If you make that an answer, I'll accept it. – Michael Kjörling Sep 29 '12 at 8:06
@ChristianLindig As for source code portability, this is a personal document so portability is not a great concern for me. (And on that note it's worth noting that the particular symbol happens to be mapped to a glyph in the window title bar font I use, but not the font Firefox uses for active only tab title text. Go figure. :)) – Michael Kjörling Sep 29 '12 at 8:06
@MichaelKjörling The fonts used for window decoration by your operating system are not necessarily the ones used by LaTeX to render your document to PDF or PostScript. – Christian Lindig Sep 29 '12 at 9:06
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As explained at LaTeX Matters you can use the package amssymb or amsfonts together with the macro \mathbb and F as an argument. Note that you have to use \mathbb in math mode.

\documentclass{article}

\usepackage{amssymb}            % or \usepackage{amsfonts}

\begin{document}

$$\mathbb{F}$$

\end{document}


If you want to define your own macro like you do in your example this is possible with \mathbb{F} too.

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This worked perfectly. Now, in my actual document it's claiming that I'm not in math mode, but that's a completely separate issue. Thank you! – Michael Kjörling Sep 29 '12 at 8:12
@MichaelKjörling You have to use \mathbb inside math mode, e.g. as $$\mathbb{F}$$. – N.N. Sep 29 '12 at 8:13
Yes, I had a mix of math mode and text mode, and had got the ordering wrong. That was a trivial fix once I saw the error (which was easy to spot from the differences between my actual document and the MWE). – Michael Kjörling Sep 29 '12 at 8:17

If you want to use 𝔽 inside your document you can use:

Edit:

As egreg pointed out you can use the package newunicodechar which makes it easier:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath,amssymb}
\usepackage{newunicodechar}
\newunicodechar{𝔽}{\ensuremath{\mathbb{F}}}
\begin{document}
𝔽
\end{document}


Original:

% !TEX program = pdflatex
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath,amssymb}

\expandafter\def\csname u8:\detokenize{𝔽}\endcsname{\ensuremath{\mathbb{F}}}
\begin{document}
𝔽
\end{document}


The command \ensuremath is testing whether you are in math mode or not and sets the argument in math mode.

Frank Mittelbach wrote a great explanation about inputenc:

How can I get utf8 character

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That's a neat trick. Thank you! – Michael Kjörling Sep 29 '12 at 8:18
\usepackage{newunicodechar} and then \newunicodechar{𝔽}{\mathbb{F}} is simpler. :) – egreg Sep 29 '12 at 8:46
@egreg: Oh a new package ;-) -- However I use \ensuremath ;-) – Marco Daniel Sep 29 '12 at 8:47
It's not new (2011/02/18); don't use \ensuremath: that's a math symbol and ought to be used only in math. I added an answer to Frank's. – egreg Sep 29 '12 at 8:57
Package selinput v1.3 (requires rempreamblecmds v1.0) provides \DeclareInputUnicodeChar and \DeclareInputChar that also support utf8x.def from package ucs that has a different syntax for \DeclareUnicodeCharacter. Also these commands can be used after \begin{document}. (The PDF files above also contain the DTX files as attachments that are unpacked using plain TeX, release to CTAN later.) – Heiko Oberdiek Sep 29 '12 at 12:30
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