References:

S.J. Aarseth (1994) in Galactic Dynamics and N-Body Simulations,
eds. G. Contopoulos, N.K. Spyrou & L. Vlahos (Springer), p. 277.

S.J. Aarseth (1996), in Dynamical Evolution of Star Clusters,
eds. P. Hut & J. Makino (Kluwer), p. 161.

Seppo Mikkola and Sverre Aarseth (1993), Cel. Mech. 57, 439,
An implementation of N-body chain regularization.
