How to write a text under subscript

Question

I know two different ways to write the following:

We can either type $(a_{ij})_{\mathop{1\le i\le n}\limits_{1\le j\le n }}$ or, as shown here, $(a_{ij})_{\underset{1 \leq j \leq n}{1\le i\le n}}$. The output (the picture above) is the same in both cases. Notice that 1< j < n has a smaller size than 1 < i < n. I found this answer. So I typed $(a_{ij})_{\mathop{1\le i\le n}\limits_{\mbox{\tiny\(1\le j\le n\)}}}$. The result is pretty ugly:

Could you please tell me how to write it correctly?

Answer 1

This would do the job :

\documentclass{article}
\usepackage{amsmath}
\begin{document}
$$(a_{ij})_{\substack{1\leq i\leq n\\1\leq j\leq n}}$$
\end{document}

Here is the result :

All lines have the same font-size.

Please note that with substack your number of line is not limited.