[Tex/LaTex] Using a math operator inside a ‘cases’ environment

Question

When I do this:

\begin{equation}
 b_i =
\begin{cases}
arg \min_{\forall p \in B} d(a_i, p) & \text{if } d(a_i, b_i) < D_{max}
\\
\text{undefined} & \text{otherwise}
\end{cases}
\end{equation}

the _ doesn't cause the following text to go below \min, but rather appear as a subscript. Is it possible to make a \min expression with the following text below it inside a cases environment?

Answer 1

does adding \displaystyle do it for you? id est:

\begin{equation}
    b_i =
    \begin{cases}
    arg \displaystyle\min_{\forall p \in B} d(a_i, p) & \text{if } d(a_i, b_i) < D_{max}
    \\
    \text{undefined} & \text{otherwise}
    \end{cases}
\end{equation}