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\subsection*{Curved non-polynomial surfaces}
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\cindex{convergence!versus mesh}
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\includegraphics{cvge-helmholtz-s-sphere-l2.pdf} &
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\includegraphics{cvge-helmholtz-s-sphere-linf.pdf} \\
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\includegraphics{cvge-helmholtz-s-sphere-h1.pdf} &
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\caption{Curved non-polynomial surface:
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error analysis in $L^2$, $L^\infty$ and $H^1$ norms.}
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\label{fig-surface-err}