Smaller font for subscript within subscript in math mode

Question

How to get smaller font for subscript within subscript in math mode? For example, I want the subscript $c$ to be much smaller than it is in the following example:

\documentclass{article}
\begin{document}
$a_{b_{c}}$
\end{document}

But "\scriptstyle" or "\scriptscriptstyle" is not working for some reason for subscripts within subscripts, i.e.

\documentclass{article}
\begin{document}
$a_{b_{\scriptscriptstyle c}}$
\end{document}

has the same result for the size of $c$. So have to make $c$ even smaller?

Answer 1

For even smaller, you could scale the sub-subscript:

\documentclass{article}
\usepackage{scalerel}
\begin{document}
$a_{b_{\scaleto{c}{1pt}}}$
\end{document}

If you need consistency across a range of subscripts, you may wish to add a normalizer like a \mathstrut:

\documentclass{article}
\usepackage{scalerel}
\begin{document}
$a_{b_{\scaleto{c\mathstrut}{2pt}}}$
$a_{b_{\scaleto{g\mathstrut}{2pt}}}$
\end{document}

Here, \scaleto from the scalerel package operates on its arguments in math mode by default, unlike \scalebox of the graphicx package, which operates in text mode.

Answer 2

You can use \DeclareMathSizes; declare also other sizes if needed.

\RequirePackage{fix-cm} % arbitrary font scaling
\documentclass{article}

\DeclareMathSizes{10}{10}{7}{4}

\begin{document}
$a_{b_{c}}$
\end{document}

Compare to the standard and decide for yourself if it's worth the trouble. I wouldn't go below four points.

Answer 3

Using someone's idea in How to get an even smaller font? to scale \scriptscriptstyle content down by 30%:

\documentclass{article}

\usepackage{graphicx}

\begin{document}

$a_{b_c}$

$a_{b_{\scalebox{.7}{$\scriptscriptstyle c$}}}$

\end{document}