\documentclass{article}
\usepackage{iftex}
\usepackage{ifthen}
\usepackage{epsfig}
\usepackage[utf8]{inputenc}
\usepackage{xcolor}
\ifPDFTeX
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\pagestyle{empty}
\begin{document}
$\rho$
\pagebreak

\[\rho(\xi) \approx \rho_{h}(\xi) = \sum_{j}c_{j}\phi_{j}(\xi)
\]
\pagebreak

$c_{j}$
\pagebreak

$j^{th}$
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$\phi_{j}$
\pagebreak

$\xi$
\pagebreak

$q$
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$E$
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\[E_{qj} = \phi_{j}(\xi_{q})
\]
\pagebreak

\[\rho_{h}(\xi_{q}) = E\mathbf{c}
\]
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$E^{T}$
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\[\rho_h(\xi) = \sum_{j=1}^{n} c_j \phi_j(\xi).
\]
\pagebreak

\[E_{qj} = \phi_j(\xi_q).
\]
\pagebreak

\[\rho_h(\xi_q) = \sum_{j=1}^{n} E_{qj} c_j.
\]
\pagebreak

\[\boldsymbol{\rho}_h = E\mathbf{c}.
\]
\pagebreak

\end{document}