CHENG, S. S., and B.-G. ZHANG. “QUALITATIVE THEORY OF PARTIAL DIFFERENCE EQUATIONS (I): OSCILLATION OF NONLINEAR PARTIAL DIFFERENCE EQUATIONS”. Tamkang Journal of Mathematics, vol. 25, no. 3, Sept. 1994, pp. 279-88, doi:10.5556/j.tkjm.25.1994.4455.