Journal:
:bulletin of the iranian mathematical society2015
s. balamuralitharan
in this paper the periodic solutions of fourth order delay differential equation of the form $ddddot{x}(t)+adddot{x}(t)+f(ddot{x}(t-tau(t)))+g(dot{x}(t-tau(t)))+h({x}(t-tau(t)))=p(t)$ is investigated. some new positive periodic criteria are given.
