Multiple periodic solutions for a class of second-order nonlinear neutral delay equations (Q2491394)

The authors consider the second order neutral delay equation \[ x''(t-\tau)+\lambda(t)f(t,x(t),x(t-\tau),x(t-2\tau))=x(t-\tau),\;\lambda(t)>0,\;\tau>0 \] to establish multiple periodic solutions by means of a variational structure and \(Z_2\)-group index theory. One example is provided on the main theorem.