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

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