\documentclass{standalone}

\usepackage{bm}
\usepackage{tikz}

\begin{document}

\begin{tikzpicture}
  \node at (0,0) {$\bm{x}_i^{\prime} = \max_{j \in \mathcal{N}(i)} \textsc{MLP}_{\hspace{-1pt}\theta} \big([ \bm{x}_i, \bm{x}_j - \bm{x}_i ]\big)$};
\end{tikzpicture}

\end{document}
