LI, W.-T., and S.-S. CHENG. “AN OSCILLATiON THEOREM FOR A NEUTRAL DIFFERENCE EQUATION WITH POSITIVE AND NEGATIVE COEFFICIENTS”. Tamkang Journal of Mathematics, vol. 30, no. 1, Mar. 1999, pp. 39-46, doi:10.5556/j.tkjm.30.1999.4204.