Sectorial oscillation of linear differential equations and iterated order. Wu, Zhao-Jun; Sun, Dao-chun — 2007 Electronic Journal of Differential Equations (EJDE) [electronic only]
The singular directions of solutions of some equations. Wu, Zhao-Jun; Sun, Dao-Chun — 2008 Acta Mathematica Academiae Paedagogicae Nyí regyháziensis. New Series [electronic only]