http://www.eecs.berkeley.edu/~demmel/cs267/lecture26/lecture26.html
http://www.cs.princeton.edu/courses/archive/fall04/cos126/assignments/barnes-hut.html
http://physics.princeton.edu/~fpretori/Nbody/intro.htm
http://nptel.ac.in/courses/111107063/module2/lecture3/lecture3.pdf
http://en.wikiversity.org/wiki/Adams-bashforth_and_Adams-moulton_methods