Letters and numbers have a different dimension in subscripts

Question

I am writing in math mode symbols with long lists of subscripts, that can either be letters or numbers.

For example, I write something like

\begin{equation}
p_{51y_4y_3}
\end{equation}

to obtain

This looks rather inelegant though. Have you got any suggestion on how to improve it?

Answer 1

The dimensions of numbers and letters are different. As the y is not as high as the 1 you have maybe got confused as it looks a bit like a subscript y. But it isn't. Please have a look on my examples. As we don't know, what you actually want to write, you may combine my solutions by your needs:

% arara: pdflatex

\documentclass{article}
\usepackage{mathtools}

\begin{document}
$p_{51 y_{4}y_{3}}$ means $p$ with subscript $51y_4$ and $y_3$. 

If you prefer, you may write $p_{51\,y_4\,y_3}$, or $p_{51,y_4,y_3}$, 

or $p_{51\cdot y_4 \cdot y_3}$, or $p_{(51 y_{4}y_{3})}$.

Or did you mean $p_{51y_{4y_3}}$?
\end{document}

Answer 2

Regarding the question in the title: Characters have different dimensions (and line thicknesses) at very small sizes. They're not simply scaled-down versions of the normal-sized characters. This is a deliberate decision by the fontmakers to improve legibility.

Regarding the inelegance of your equation: I can only suggest rewriting it to avoid the subscripts. More than one level of subscript is inadvisable, in my opinion. You might consider presenting the subscripts as function arguments, though I don't know whether this is workable in your particular situation.

 p(5,1,y_4,y_3)