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\begin{document}
\begin{equation*}
\begin{aligned}
  x(k+1) &= A x(k) + B u(k) + v(k) &\quad x &\in R^n, u \in R \\
  y(k) &= C x(k) + D u(k) + w(k) &  y &\in R
\end{aligned}
\end{equation*}
\end{document}