How can I reduce the size in array environment?

Question

My code is

\[
    D_{k+1}(x_{k+1}) = \min_{\begin{array}{c}
    \text{all } x_k \text{ such that}\\
    p_{{x_k}{x_{k+1}}} > 0
    \end{array}}
    \left[ D_k(x_k)-\ln\left(p_{{x_{k-1}}{x_k}}
    r\left(z_k; x_{k-1}, x_k\right)\right) \right]
\]

However, I want to make all x_k such that p_{{x_k}{x_{k+1}} > 0 smaller.

How can I reduce the size of that part?

Answer 1

You could use the \substack macro of the amsmath package. Compared with the array method used in your example, the material in the argument of \substack is automatically set in \scriptstyle. Also, do abbreviate "such that" to "s.t.".

\documentclass{article}
\usepackage{amsmath} % for \text and \substack macros
\begin{document} 
\[
    D_{k+1}(x_{k+1}) = \min_{%
    \substack{\text{all $x_k$ s.t.} \\[0.25ex] 
              p_{x_k x_{k+1}} > 0}}
    \bigl[ D_k(x_k)-\ln\bigl(p_{x_{k-1} x_k}
    r(z_k; x_{k-1}, x_k)\bigr) \bigr]
\]
\end{document}