The following is a selection of references for further information on chip firing.
Anderson, Richard, Laszlo Lovász, Peter Shor, Joel Spencer, Eva Tardos, and Shmuel Winograd, ``Disks, Balls, and Walls: Analysis of a Combinatorial Game, `` American Mathematical Monthly, volume 96, June-July 1989, pages 481-493.
Bitar, Javier, Eric Goles ``Parallel chip firing games on graphs'', Theoretical Computer Science, 92, (1992), pp.291-300.
Bjorner, Anders, Laszlo Lovász, and Peter Shor, ``Chip-firing games on graphs'', Europ. J. Combinatorics, 12 (1991), pp. 283-291.
Ericksson, Kimo, ``Chip-firing games on mutating graphs'', SIAM J. Discrete Math., vol. 9, no. 1, February 1996, pp 118-128.
Goles, Eric, and Marcos A. Kiwi, ``Games on line graphs and sand piles'', Theoretical Computer Science, 115 (1993), pp. 321-349.
Tardos, Gabor, ``Polynomial bound for a chip firing game on graphs,'' SIAM J. Discrete Math., vol. 1, no 3, August 1988, pp. 397-398.