How to stack two lines of subscripts?

Question

How do I type two lines of subscript as shown below?

Answer 1

With use of the substack.

With using the amsmath package, you can simply define math operator for dist (it should be in upright letters):

\documentclass[varwidth, margin=3mm]{standalone}
\usepackage{amsmath}
\DeclareMathOperator{\dist}{dist}
\begin{document}
[
C=\sum_{\substack{1\leq i\leq n\ 1 <j <k}}\dist(d_i,c_j)
]
or
[
C=\sum\nolimits_{\substack{1\leq i\leq n\ 1 <j <k}}\dist(d_i,c_j)
]
\end{document}