1. Bak, P., and Chen, K., Phys. D.
38, 4 (1989).
2. Chen, K. —, Sci. Am.
264, 46 (1990).
3. Bak, P., and Tang, C., J. Geophys. Res. B
94, 15, 635 (1989).
4. Bak, P., Tang, C., and Wiesenfeld, K., Phys. Rev. Lett.
59, 381 (1987).
5. Tang, C., and Wiesenfeld, K. —, Phys. Rev. A
38, 364 (1988).