Problematic nested subscript

Question

Hello and sorry if I'm reiterating an already asked question. The two i-subscripts have different positions in relation to m, the one on the right hand side of the inequality being the correct one presumably. Is there any way to fix this?

\documentclass[]{article}
\usepackage{mathtools}
\usepackage{amssymb}
\begin{document}
\begin{equation*}
\dim_{\mathbb{F}_{2^{m_{i}}}} W \geq \frac{n}{m_i}
\end{equation*}
\end{document}

Answer 1

Generally, when a font is scaled to a lower point size, the glyphs can change to retain legibility. However, your unhappiness over the relation of the i and m indicates you don't like the choices made by the font.

Originally I used the scalerel package to take a normalsize m_i and scale it down to an appropriate size, but then Hendrik Vogt pointed out that the same could be more simply accomplished using a \scalebox{0.6}{$m_i$} for the sub-sub-superscript, since \usebox does not apply the \mathpalette wrapper.

The \scalebox creates the subscripted m_i to be an exact scale of the normal size m_i. Of course, the drawback is that, when scaled down, the strokes become thinner, and some may consider that more detrimental to the appearance that the relation of the i to the m. In any event, here it is:

\documentclass{article}
\usepackage{mathtools}
\usepackage{amssymb}
\usepackage{graphicx}
\begin{document}
\begin{equation*}
\dim_{\mathbb{F}_{2^{%
\scalebox{0.6}{$m_i$}%
}}} W \geq \frac{n}{m_i}
\end{equation*}
\end{document}

Answer 2

I strongly advise against using these four-level sub- and superscripts. Using two levels is OK, three levels is already dubious. In my opinion, the major problem in the output is not the distance between m and the index i but the fact that the \mathbb{F} and the m are almost on the same level. This way it almost looks like the product of \mathbb{F}_2 and m_i. If you really want to do it, here are some manual tweaks:

I pushed the index 2 further down by adding an empty superscript to the \mathbb{F}, and I reduced the horizontal space at three places, most notably before the W.

\documentclass{article}
\usepackage{amssymb}
\begin{document}
$\dim_{\mathbb{F}^{}_{\mkern-1mu 2^{m_{\mkern-1mu i}}}} \mkern-5mu W$
\end{document}

However, the better option is to use the recommendations from the comments to the question, e.g., to define \mathbb{F}(m_i) or \mathbb{F}(2,m_i) as a notation for the finite field with 2^{m_i} elements.