MULTIPLE PERIODIC SOLUTIONS TO A CLASS OF SECOND-ORDER FUNCTIONAL DIFFERENTIAL EQUATIONS
一类二阶泛函微分方程的多重周期解

By means of variational principle and $Z_{2}$ group index theory, we obtain the existence of multiple periodic solutions to second-order functional differential equations $x'(t-\tau)+c(x(t)-x(t-2\tau))'-x(t-\tau)+\lambda f(t,x(t),x(t-\tau),x(t-2\tau))=0$.