On positive periodic solutions of second-order semipositone differential equations

Using the Krasnosel'skii's fixed point theorem, we establish the existence and multiplicity of positive $\mathrm{T}$-periodic solutions of second-order semipositone system $ \left \{\begin{array}{lcr} x''(t)+a(t)x(t)=\lambda f(t,x(t)),\\ x(0)=x(\mathrm{T}),x'(0)=x'(\mathrm{T}), \end{array}\right. $ where $x=(x_1,x_2,\cdots,x_n)$, $f(t,x)=(f_1(t,x),f_2(t,x),\cdots,f_n(t,x))$ is bounded below.