@article{DBLP:journals/ijcm/JiangZOA04,
  author    = {Daqing Jiang and
               Wenjie Zuo and
               Donal O'Regan and
               Ravi P. Agarwal},
  title     = {Optimal existence conditions for second order periodic solutions
               of delay differential equations with upper and lower solutions
               in the reverse order},
  journal   = {Int. J. Comput. Math.},
  volume    = {81},
  number    = {6},
  year      = {2004},
  pages     = {707-717},
  ee        = {http://dx.doi.org/10.1080/00207160310001597215},
  bibsource = {DBLP, http://dblp.uni-trier.de}
}