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\begin{figure}[tb]
\begin{center}
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% beliebige dicke und dŸnne Linie
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%single horizontal arrow
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\put(17.5,0.2){\makebox(0,0){\footnotesize Legendre}}
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%grid point space 1
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%grid point space 2
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%Fourier space 2
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%Fourier space 1
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\put(-1.5,21.5){\makebox(0,0){\footnotesize z}}
\put(-1.5,20.5){\makebox(0,0){\footnotesize levels}}
\put(10,17){\makebox(0,0){\footnotesize x longitudes}}
\put(21.5,19.2){\makebox(0,0){\footnotesize y}}
\put(23,18.2){\makebox(0,0){\footnotesize latitudes}}
\put(27,30.5){\makebox(0,0){\em grid point space}}
\put(7,-0.2){\makebox(0,0){\em spectral space}}
\put(28,-0.2){\makebox(0,0){\em Fourier space}}
\put(37,1.6){\makebox(0,0){\footnotesize $M$ wave numbers}}
\put(41,2.8){\makebox(0,0){\footnotesize 
$M=\left\lceil\frac{x-1}{3}\right\rceil$}}
\put(39.5,14.2){\makebox(0,0){\huge $\updownarrow$ \footnotesize FFT}}

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\put(18.5,1){\dashbox{0.5}(27,11){}}
\put(-3.2,16){\dashbox{0.5}(51.3,13.6){}}

\thicklines
\end{picture}
\end{center}
\begin{fig}
\label{ifs}
{\rm
Rough model for the 2D partitioning of the IFS data structure in grid point,
Fourier, and spectral space into $3\times 4$ parts. 
} % rm for fig
\end{fig}
\end{figure}
}

.