added 3 examples

tikz/3d-function-2/3d-function-2.tex

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+\documentclass{article}
+\usepackage[pdftex,active,tightpage]{preview}
+\setlength\PreviewBorder{2mm}
+\usepackage{pgfplots}
+
+\begin{document}
+\begin{preview}
+\pgfplotsset{
+    colormap={whitered}{
+        color(0cm)=(white);
+        color(1cm)=(orange!75!red)
+    }
+}
+\begin{tikzpicture}
+    \begin{axis}[
+    colormap name=whitered,
+    width=15cm,
+    view={340}{20},
+    enlargelimits=false,
+    grid=major,
+    domain=-5:5,
+    y domain=-5:5,
+    samples=56, %57 : TeX capacity exceeded, sorry [main memory size=3000000].
+                % see also http://tex.stackexchange.com/a/7954/5645
+    xlabel=$x$,
+    ylabel=$y$,
+    zlabel={$z$},
+    colorbar,
+    colorbar style={
+        at={(-0.1,0)},
+        anchor=south west,
+        height=0.25*\pgfkeysvalueof{/pgfplots/parent axis height},
+        title={$f(x,y)$}
+    }
+    ]
+      \addplot3[surf] {x^2 + y^2};
+    \end{axis} 
+\end{tikzpicture}
+\end{preview}
+\end{document}