Whit the following code the max subscript is displayed on the right of max, while I want it to be below. Removing the cases fixes the issue, but still I need it. How can I fix that ?
\begin{equation}
f(i)=\begin{cases}
0, & \text{if $S_{i}$ is empty}.\\
\max_{t_k \in S_{i}}\{f(k) + 42\}, & \text{otherwise}.
\end{cases}
\end{equation}
As mentioned in the comments, \max\limits_{} works well! However, if you want a general answer, read this answers (which is also mine):
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
f(i)=\begin{cases}
0, & \text{if $S_{i}$ is empty}.\\
\underset{t_k \in S_{i}}{\max}\{f(k) + 42\}, & \text{otherwise}.
\end{cases}
\end{equation}
\end{document}
\max\limits_{...}or thedcasesenvironment frommathtools. – campa Dec 03 '18 at 14:40\max\limits_{...}– David Carlisle Dec 03 '18 at 14:41