Analytic study of the three-urn model for separation of sand
  We present an analytic study of the three-urn model for separation of sand. We solve analytically the master equation and the first-passage problem. We find that the stationary probability distribution obeys the detailed balance and is governed by the {\it free energy}. We find that the characteristic lifetime of a cluster diverges algebraically with exponent 1/3 at the limit of stability. 
G. M. Shim, B. Y. Park, J. D. Noh, and Hoyun Lee