\mathsf{} prints digits in serif font

Question

I am using mathsf{} in a command to mark special kinds of variables in a formula.

I have:

\newcommand{\mycmd}[1]{\mathsf{#1}}

In formulas I usually have

$\mycmd{specialvariable_1} = yada yada yada$

Now I noticed that the digits in the subscript are still printed in the serif font. This also happens for non-subscript numbers. A minimal example is:

$\mathsf{a5\,b_3}$

here the digits are printed in a serif font, the letters are printed in a sans-serif font.

How to force LaTeX to also print the digits in a sans-serif font?

(Maybe of interest: my command is often nested in other commands that occasionally are themselves nested in other commands...)

[Update] As requested I made a minimal working example. It seems the package sansmath is the problem. Without the package, the digits are printed in sans-serif, with the package, the digits are printed with serifs. Nevertheless this seems a bit counterintuitve...

\documentclass[11pt]{scrbook}

\usepackage{sansmath}

\begin{document}
 $\mathsf{a5\,b_3}$
\end{document}

Answer 1

The sansmath package redefines how numbers are handled, making them \mathord instead of \mathalpha. It does this because it takes the letters from the slanted sans serif font for math characters under \sansmath, and usually you do not want the numbers to be the slanted version, but it is incorrectly affecting the normal math version. Either drop the sansmath package or define your command as follows:

\newcommand{\mycmd}[1]{\mbox{\sansmath$\mathsf{#1}$}}

taking advantage of \mathsf working correctly in the sans math version.

\documentclass[11pt]{scrbook}

\usepackage{sansmath}

\newcommand{\mycmd}[1]{\mbox{\sansmath$\mathsf{#1}$}}

\begin{document}

$x = \mycmd{a5b_3},\quad y \ne \mathsf{a5b_3}$
\end{document}