Electronic Journal of Differential Equations,
Vol. 2005(2005), No. 26, pp. 1-5.

Title: Existence of periodic solutions  for  second-order neutral
       differential equations

Author: Yongjin Li (Sun Yat-sen Univ., Guangzhou, China)
 
Abstract: 
 By means of variational structure and critical point theory,
 we study the existence of periodic solutions for a second-order
 neutral differential equation
 $$\displaylines{
  (p(t) x' (t - \tau ))' + f(t, x(t), x(t-\tau ),
  x(t-2\tau) ) = g(t),\cr
  x(0) = x(2k\tau), x'(0) = x'(2k\tau).
 }$$
 where $k$ is a given positive integer and $\tau$ is a positive
 number.
 
Submitted January 6, 2005. Published March 6, 2005.
Math Subject Classifications: 34K13, 34K40, 65K10.
Key Words: Neutral differential equations; periodic solution;
           variational method; critical point.