How to switch easily between normal font and sans-serif font for maths?

Question

I would like to switch easily to sans-serif font, even for the mathematical symbols. Is it possible?

So far is what I have tried :

\documentclass{article}

\def\mySfFamily{\fontfamily{cmbr}\selectfont\sffamily}
\def\N{\mathbf{N}}
\def\R{\mathbf{R}}
\def\Q{\mathbf{Q}}
\def\Z{\mathbf{Z}}
\def\C{\mathbf{C}}


\begin{document}

\begingroup
\mySfFamily
This should be sans-serif : $\R, \Q, \N, \Z, \C$ are sets.
\endgroup

\bigskip

This should be serif : $\R, \Q, \N, \Z, \C$ are sets.

\end{document}

But as you see, it doesn't work for maths. I have chosen the font \fontfamily{cmbr} because of this post.

Answer 1

The sansmath package provides a command (\sansmath) and environment (sansmath) for exactly this purpose:

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{sansmath}

\def\mySfFamily{\fontfamily{cmbr}\selectfont\sffamily\sansmath}
\def\N{\mathbf{N}}
\def\R{\mathbf{R}}
\def\Q{\mathbf{Q}}
\def\Z{\mathbf{Z}}
\def\C{\mathbf{C}}


\begin{document}

\begingroup
\mySfFamily
This should be sans-serif : $\R, \Q, \N, \Z, \C$ are sets.
\endgroup

\bigskip

This should be serif : $\R, \Q, \N, \Z, \C$ are sets.

\end{document}

To apply this "automagically", I would hack it into the \sffamily and \rmmfamily commands. (I prefer \appto from the etoolbox package for this, but one could as well use the usual \let\OLDmacro\macro and \renewcommand\macro{\OLDmacro <extension>} idiom.):

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{sansmath,etoolbox}

\renewcommand{\sfdefault}{cmbr}

\appto{\sffamily}{\sansmath}
\appto{\rmfamily}{\unsansmath}


\def\N{\mathbf{N}}
\def\R{\mathbf{R}}
\def\Q{\mathbf{Q}}
\def\Z{\mathbf{Z}}
\def\C{\mathbf{C}}


\begin{document}


\sffamily

This should be sans-serif : $\R, \Q, \N, \Z, \C$ are sets.

\rmfamily

This should be serif : $\R, \Q, \N, \Z, \C$ are sets.


\end{document}

Answer 2

Some time ago I asked nearly the same question, the answer was to define a new math alphabet and hack it into the definition of \sffamily. So I’d suggest to close this question as duplicate but it can’ be done until the bounty is closed.

---

This is the code a build upon the answer of egreg.

\documentclass[parskip=half]{scrartcl}

\usepackage[T1]{fontenc}
\usepackage{lmodern}

\DeclareMathVersion{sans}
\SetSymbolFont{operators}{sans}{OT1}{cmbr}{m}{n}
\SetSymbolFont{letters}{sans}{OML}{cmbrm}{m}{it}
\SetSymbolFont{symbols}{sans}{OMS}{cmbrs}{m}{n}
\SetMathAlphabet{\mathit}{sans}{OT1}{cmbr}{m}{sl}
\SetMathAlphabet{\mathbf}{sans}{OT1}{cmbr}{bx}{n}
\SetMathAlphabet{\mathtt}{sans}{OT1}{cmtl}{m}{n}
\SetSymbolFont{largesymbols}{sans}{OMX}{iwona}{m}{n}

\DeclareMathVersion{boldsans}
\SetSymbolFont{operators}{boldsans}{OT1}{cmbr}{b}{n}
\SetSymbolFont{letters}{boldsans}{OML}{cmbrm}{b}{it}
\SetSymbolFont{symbols}{boldsans}{OMS}{cmbrs}{b}{n}
\SetMathAlphabet{\mathit}{boldsans}{OT1}{cmbr}{b}{sl}
\SetMathAlphabet{\mathbf}{boldsans}{OT1}{cmbr}{bx}{n}
\SetMathAlphabet{\mathtt}{boldsans}{OT1}{cmtl}{b}{n}
\SetSymbolFont{largesymbols}{boldsans}{OMX}{iwona}{bx}{n}

\newif\IfInSansMode
\let\oldsf\sffamily
\renewcommand*{\sffamily}{\oldsf\mathversion{sans}\InSansModetrue}
\let\oldmd\mdseries
\renewcommand*{\mdseries}{\oldmd\IfInSansMode\mathversion{sans}\fi\relax}
\let\oldbf\bfseries
\renewcommand*{\bfseries}{\oldbf\IfInSansMode\mathversion{boldsans}\else%
   \mathversion{bold}\fi\relax}
\let\oldnorm\normalfont
\renewcommand*{\normalfont}{\oldnorm\InSansModefalse\mathversion{normal}}
\let\oldrm\rmfamily
\renewcommand*{\rmfamily}{\oldrm\InSansModefalse\mathversion{normal}}

\usepackage[textwidth=9cm]{geometry}

\begin{document}
\section{Inline-$m\alpha \tau h$}
Normal: A $Ax\alpha+\chi b$

\sffamily Sans: A $Ax\alpha+\chi b$

\bfseries Bold-Sans: A $ Ax\alpha+\chi b$

\normalfont Normal: A $ Ax\alpha+\chi b$

\bfseries  Bold: A $ Ax\alpha+\chi b$

\normalfont Normal: A $ Ax\alpha+\chi b$

\section{Display-$\mu\alpha\tau\eta$}
Text in roman family
\begin{equation}
     \sqrt{(a_1+a_2+\gamma)^2}=\sum_{i=1}^2 a_i + \gamma
\end{equation}

\sffamily
Text in sans family
\begin{equation}
    \sqrt{(a_1+a_2+\gamma)^2}=\sum_{i=1}^2 a_i + \gamma
\end{equation}

\sffamily\bfseries
Text in sans family, bold version
\begin{equation}
    \sqrt{(a_1+a_2+\gamma)^2}=\sum_{i=1}^2 a_i + \gamma
\end{equation}

\rmfamily\bfseries
Text in roman family, bold version
\begin{equation}
    \sqrt{(a_1+a_2+\gamma)^2}=\sum_{i=1}^2 a_i + \gamma
\end{equation}
\end{document}

Answer 3

For comparison, this is how one can switch from sans to serif math and back in ConTeXt.

\def\N{{\bf N}}
\def\R{{\bf R}}
\def\Q{{\bf Q}}
\def\Z{{\bf Z}}
\def\C{{\bf C}}

\startbuffer
Some random text: $\R, \Q, \N, \Z, \C$ are sets.
\stopbuffer

% To format the examples
\setuphead[subject][style=bold, after=]

\starttext

\subject{Default} \getbuffer

\subject{Sans math}

\switchtobodyfont[ss] 
\appendtoks \ss \to \everymathematics
\getbuffer

\subject{Serif math}
\switchtobodyfont[rm]
\appendtoks \rm \to \everymathematics
\getbuffer


\stoptext

You can also use group {...} to limit the effect of font changes.

Answer 4

With unicode-math, you can declare a new math version as

\defaultfontfeatures{Scale = MatchLowercase}
\setmathfont[version=sans]{Fira Math}

You can then switch to it with \mathversion{sans}. If you want, you can define commands corresponding to \boldmath and \boldsymbol.

If you also want a bold sans-serif version, there is no OpenType math font that works out of the box. As of June 2020, the only sans-serif math font is Fira Math, but the only bold math fonts are XITS, Libertinus and Minion. Furthermore, range= and version= do not work together, so you cannot yet load a math version that mixes Libertinus Math Bold with the letters from Libertinus Serif Bold (Italic) or the sf alphabets from Libertinus Math.

For the time being, you can apply FakeBold to Fira Math:

 \setmathfont[version=boldsans, FakeBold=1.2]{Fira Math}

You could then change the formatting of headings to \sffamily\bfseries\mathversion{boldsans} to get bold sans-serif math symbols.