Infinite periodic solutions to a class of second-order Sturm–Liouville neutral differential equations

By means of variational structure and Z2Z2-group index theory, we obtain infinite periodic solutions to a class second-order Sturm–Liouville neutral delay equations (p(t)x′(t−sτ))′−q(t)x(t−sτ)+f(t,x(t),x(t−τ),x(t−2τ),…,x(t−2sτ))=0.(p(t)x′(t−sτ))′−q(t)x(t−sτ)+f(t,x(t),x(t−τ),x(t−2τ),…,x(t−2sτ))=0.