\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
    x[k+1] = x[k] + u_i[k] - u_o[k],\quad x\in {\mathbb Z} 
\end{equation*}
\end{document}