http://www.univie.ac.at/future.media/moe/galerie/diff1/diff1.html#ableitung
http://www.ecs.fullerton.edu/~mathews/a2001/Animations/Derivative/CentralD2.2f/CentralDfaa.html
http://math.fullerton.edu/mathews/a2001/Animations/Derivative/ForwardD2f/ForwardD2faa.html
http://www.ecs.fullerton.edu/~mathews/a2001/Animations/Derivative/