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\index{reference cells}
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The following five reference cells are covered by the UFC specification:
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the reference \emph{interval},
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the reference \emph{triangle},
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the reference \emph{quadrilateral},
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the reference \emph{tetrahedron} and
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the reference \emph{hexahedron} (see Table~\ref{tab:ufc_reference_cells}).
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\linespread{1.2}\selectfont
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\begin{tabular}{|l|c|c|c|}
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Reference cell & Dimension & \#Vertices & \#Facets \\
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The reference interval & 1 & 2 & 2 \\
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The reference triangle & 2 & 3 & 3 \\
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The reference quadrilateral & 2 & 4 & 4 \\
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The reference tetrahedron & 3 & 4 & 4 \\
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The reference hexahedron & 3 & 8 & 6 \\
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\caption{Reference cells covered by the UFC specification.}
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\label{tab:ufc_reference_cells}
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The UFC specification assumes that each cell in a finite element mesh
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is always isomorphic to one of the reference cells.
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\section{The reference interval}
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The reference interval is shown in Figure~\ref{fig:interval} and is
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defined by its two vertices with coordinates as specified in
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Table~\ref{tab:interval,vertices}.
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\includegraphics[width=10cm]{eps/interval.eps}
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\caption{The reference interval.}
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\linespread{1.2}\selectfont
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\begin{tabular}{|c|c|}
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Vertex & Coordinate \\
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\caption{Vertex coordinates of the reference interval.}
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\label{tab:interval,vertices}
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\section{The reference triangle}
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The reference triangle is shown in Figure~\ref{fig:triangle} and is
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defined by its three vertices with coordinates as specified in
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Table~\ref{tab:triangle,vertices}.
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\includegraphics[width=8cm]{eps/triangle.eps}
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\caption{The reference triangle.}
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\linespread{1.2}\selectfont
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\begin{tabular}{|c|c|}
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Vertex & Coordinate \\
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$v_0$ & $x = (0, 0)$ \\
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$v_1$ & $x = (1, 0)$ \\
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$v_2$ & $x = (0, 1)$ \\
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\caption{Vertex coordinates of the reference triangle.}
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\label{tab:triangle,vertices}
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\section{The reference quadrilateral}
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\index{quadrilateral}
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The reference quadrilateral is shown in Figure~\ref{fig:quadrilateral}
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and is defined by its four vertices with coordinates as specified in
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Table~\ref{tab:quadrilateral,vertices}.
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\psfrag{v0}{$(0, 0)$}
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\psfrag{v1}{$(1, 0)$}
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\psfrag{v2}{$(1, 1)$}
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\psfrag{v3}{$(0, 1)$}
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\includegraphics[width=8cm]{eps/quadrilateral.eps}
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\caption{The reference quadrilateral.}
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\label{fig:quadrilateral}
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\linespread{1.2}\selectfont
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\begin{tabular}{|c|c|}
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Vertex & Coordinate \\
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$v_0$ & $x = (0, 0)$ \\
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$v_1$ & $x = (1, 0)$ \\
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$v_2$ & $x = (1, 1)$ \\
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$v_3$ & $x = (0, 1)$ \\
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\caption{Vertex coordinates of the reference quadrilateral.}
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\label{tab:quadrilateral,vertices}
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\section{The reference tetrahedron}
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The reference tetrahedron is shown in Figure~\ref{fig:tetrahedron} and
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is defined by its four vertices with coordinates as specified in
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Table~\ref{tab:tetrahedron,vertices}.
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\psfrag{v0}{$(0, 0, 0)$}
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\psfrag{v1}{$(1, 0, 0)$}
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\psfrag{v2}{$(0, 1, 0)$}
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\psfrag{v3}{$(0, 0, 1)$}
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\includegraphics[width=6cm]{eps/tetrahedron.eps}
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\caption{The reference tetrahedron.}
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\label{fig:tetrahedron}
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\linespread{1.2}\selectfont
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\begin{tabular}{|c|c|}
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Vertex & Coordinate \\
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$v_0$ & $x = (0, 0, 0)$ \\
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$v_1$ & $x = (1, 0, 0)$ \\
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$v_2$ & $x = (0, 1, 0)$ \\
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$v_3$ & $x = (0, 0, 1)$ \\
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\caption{Vertex coordinates of the reference tetrahedron.}
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\label{tab:tetrahedron,vertices}
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\section{The reference hexahedron}
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The reference hexahedron is shown in Figure~\ref{fig:hexahedron} and
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is defined by its eight vertices with coordinates as specified in
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Table~\ref{tab:hexahedron,vertices}.
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\linespread{1.2}\selectfont
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\psfrag{v0}{$(0, 0, 0)$}
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\psfrag{v1}{$(1, 0, 0)$}
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\psfrag{v2}{$(1, 1, 0)$}
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\psfrag{v3}{$(0, 1, 0)$}
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\psfrag{v4}{$(0, 0, 1)$}
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\psfrag{v5}{$(1, 0, 1)$}
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\psfrag{v6}{$(1, 1, 1)$}
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\psfrag{v7}{$(0, 1, 1)$}
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\includegraphics[width=9cm]{eps/hexahedron.eps}
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\caption{The reference hexahedron.}
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\label{fig:hexahedron}
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\linespread{1.2}\selectfont
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\begin{tabular}{|c|c|}
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Vertex & Coordinate \\
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$v_0$ & $x = (0, 0, 0)$ \\
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$v_1$ & $x = (1, 0, 0)$ \\
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$v_2$ & $x = (1, 1, 0)$ \\
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$v_3$ & $x = (0, 1, 0)$ \\
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\begin{tabular}{|c|c|}
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Vertex & Coordinate \\
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$v_4$ & $x = (0, 0, 1)$ \\
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$v_5$ & $x = (1, 0, 1)$ \\
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$v_6$ & $x = (1, 1, 1)$ \\
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$v_7$ & $x = (0, 1, 1)$ \\
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\caption{Vertex coordinates of the reference hexahedron.}
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\label{tab:hexahedron,vertices}