CHENG, S. S., XIE, S.-L. and ZHANG, B. G. (1996) “QUALITATIVE THEORY OF PARTIAL DIFFERENCE EQUATIONS (V): STURMIAN THEOREMS FOR A CLASS OF PARTIAL DIFFERENCE EQUATIONS”, Tamkang Journal of Mathematics, 27(1), pp. 89–97. doi: 10.5556/j.tkjm.27.1996.4366.