environment of cases mess up the math operator

Question

Thanks to another question, I have defined argmin operator. It works fine when I use it in equations except for when I apply it in cases environment. As you see, the x becomes an index instead of being underneath of argmin.

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{amsmath,amssymb}

\DeclareMathOperator*{\argmin}{argmin}

\begin{document}

\begin{align}
x^*=\argmin_x x^2-3x+1
\end{align}

\begin{align}
\begin{cases}
x^*=\argmin_x x^2-3x+1 \\
x>0
\end{cases}
\end{align}


\end{document}

Answer 1

This is the expected behaviour, since each entry within a cases environment is set using \textstyle, making sub-/superscripts to an operator be set beside rather below/above it. You either need to force \displaystyle, or use the dcases environment from mathtools:

\documentclass{article}

\usepackage{mathtools}

\DeclareMathOperator*{\argmin}{argmin}

\begin{document}

\begin{equation}
  x^* = \argmin_x x^2 - 3x + 1
\end{equation}

\begin{equation}
  \begin{dcases}
    x^* = \argmin_x x^2 - 3x + 1 \\
    x > 0
  \end{dcases}
\end{equation}

\end{document}