“…Worth mentioning, Zaslavski [6] generalized [1]'s result and proved the existence of a nonconstant periodic solution under the condition that µ < inf{f (t, 0, 0)|t ∈ R} for a class of functions f . In our situation, either [1] or [6] leads to the same conclusion. The proofs are based on the methods developed by V. J. Mizel, L. A. Peletier, and W. C. Troy [5,3].…”