Make mathfont respect the surrounding family

Question

How can one make the math mode change its font according to the surrounding text?

I have to use (pdf)latex. xelatex, luatex etc. are not possible alternatives in this case.

\documentclass{scrartcl}

\usepackage{lmodern}
%\usepackage{sfmath}
%\usepackage{cmbright}

\newcommand{\test}{%
    \sqrt{(a_1+a_2+\gamma)^2}=\sum_{i=1}^2 a_i + \gamma
}

\begin{document}
Text in roman family and math too $\test$
\begin{equation}
    \sqrt{(a_1+a_2+\gamma)^2}=\sum_{i=1}^2 a_i + \gamma
\end{equation}

\sffamily
Text in sans family and math should be in sans too $\test$
\begin{equation}
    \sqrt{(a_1+a_2+\gamma)^2}=\sum_{i=1}^2 a_i + \gamma
\end{equation}

\normalfont
Back to roman $\test$
\end{document}

I found sfmath but it only changes the latin letters and numbers and it works only as a global setting. cmbright contains a more or less matching (and working) math font but it changes the text font also and it works only globally too.

Answer 1

If you use the Latin Modern or Computer Modern fonts, you can switch to CMBright:

\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}

(and Iwona for the large symbols); then a declaration such as

\sffamily\mathversion{sans}

will do; it respects grouping, but also \normalfont\mathversion{normal} will get back to normal.

Answer 2

This is what I compiled form my own ideas and egreg’s answer to make the mathfont automatically change to sans and/or bold font.

\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

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} and re-use the rest of the code from the other answers, or 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 load Libertinus Math Bold with the letters from Libertinus Serif Bold (Italic).

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

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

This is useful if you want to use math mode inside a bold sans-serif heading. You can change the formatting to \sffamily\bfseries\mathversion{boldsans} to have math that matches the text.