Positive periodic solutions of first-order functional differential equations with parameter

We prove the existence and multiplicity of positive T -periodic solution(s) for T -periodic equation x ′ ( t ) = h ( t , x ) − λ b ( t ) f ( x ( t − τ ( t ) ) ) by Krasnoselskii fixed point theorem, where f ( x ) may be singular at x = 0 . Our results improve some recent results in previous literature.