A note on the oscillation theory of certain second order differential equations. Wang, Shupei — 1995 Annales Academiae Scientiarum Fennicae. Series A I. Mathematica
Solutions of nonhomogeneous linear differential equations with exceptionally few zeros. Gundersen, Gary G.; Steinbart, Enid M.; Wang, Shupei — 1998 Annales Academiae Scientiarum Fennicae. Mathematica