Periodic solutions for a class of Duffing differential equations

We provide sufficient conditions for the existence of periodic solutions for the class of Duffing differential equations

x″+c(t)x′+a(t)x+b(t)x3=h(t,x),x'' + c(t)x' + a(t)x + b(t){x^3} = h(t,x),

where the functions a(t), b(t), c(t) and h(t, x) are C² and T–periodic in the variable t.