| Abstract: |
| The study of periodic solution problem to delay differential equations, which has extensively received a lot of attention, has had a long history. Many excellent results on existence of their periodic solutions were obtained by using some fixed point theorems, Hopf bifurcation theorems, coincidence degree theory and Poincar\`e-Bendixson theorem, etc. Since 1997, variational methods were applied to study the existence and multiplicity of periodic solutions to delay differential equations. A series of interesting and significant results are obtained via variational approaches in the literature. In this talk, we present a detailed survey of basic results and recent development on this research area. Some interesting open problems are also addressed. |
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