[Tex/LaTex] 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.