where g(t,x(t)) may be unbounded as x → 0+. Equation (1.1) is of repulsive type (resp. attractive type) if g(t,x(t))→ –∞ (resp. g(t,x(t))→ +∞) as x→ 0+. Using Mawhin’s continuation theorem, the author proved that Eq. (1.1) has at least one T-periodic solution. Zhang’s work has attracted much attention of many specialists in differential equations. In 2014,Wang [2] investigated the existence of ...