3d-function-8.tex: convert argument of sin() to degrees.

[simon-budig]

tikz apparently expectes the argument to sin() in degrees. This is in strong contrast to the usual conventions in math, where the argument is expected in radians.

With this example it actually provides a bogus graph - it actually should approach f(x,y) = 1 for large x and y (since 1/(x^2+y^2) is small for large x,y; hence sin(1/(x^2+y^2)) can be approximated by 1/(x^2+y^2), resulting in f(x,y) -> 1 for large x,y).

Also I've adjusted the plot range a bit, since this then actually shows the "interesting" center of the function (x=y=0).

Note that this image is used in Wikipedia ( https://de.wikipedia.org/wiki/Datei:3d-function-8.svg ) and should be updated there as well.

[simon-budig]

also the roughness of the graph is probably due to numerical issues within tex. It really should be a quite smooth surface, making this a very misleading image in the article on Differenzierbarkeit (german wikipedia)