Funes el memorioso

LaTeX Source

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\usepackage{concrete}
\usepackage{color}
\definecolor{MyGray}{rgb}{0.96,0.97,0.98}

\makeatletter\newenvironment{mybox}{%
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\usepackage{amsmath}
% Matrix norm
\newcommand{\norm}[1]{\ensuremath{\lVert{#1}\rVert }}
% bold face letters for matrix and vector
\newcommand{\Ebf}{\ensuremath{\mathbf{E}}}

\begin{document}
\pagestyle{empty}
\begin{mybox}
$\displaystyle \quad |\sigma_k - \beta_k| \leq \norm{\Ebf}_2\quad$
for $1\leq k\leq n$.
\label{eq:1}
\end{mybox}

This can be proved by the following fact.
\begin{equation}
  \label{eq:2}
  x^2 + y^2 = z^2
\end{equation}
\end{document}