\documentclass[reqno]{amsart} \usepackage{hyperref} \AtBeginDocument{{\noindent\small {\em Electronic Journal of Differential Equations}, Vol. 2007(2007), No. 22, pp. 1--5.\newline ISSN: 1072-6691. URL: http://ejde.math.txstate.edu or http://ejde.math.unt.edu \newline ftp ejde.math.txstate.edu (login: ftp)} \thanks{\copyright 2007 Texas State University - San Marcos.} \vspace{9mm}} \begin{document} \title[\hfilneg EJDE-2007/22\hfil On asymptotic behaviour] {On asymptotic behaviour of oscillatory solutions for fourth order differential equations} \author[S. Padhi, C. Qian\hfil EJDE-2007/22\hfilneg] {Seshadev Padhi, Chuanxi Qian} % in alphabetical order \address{Seshadev Padhi\newline Department of Applied Mathematics, Birla Institute of Technology, Mesra, Ranchi-835 215, India} \email{ses\_2312@yahoo.co.in} \address{Chuanxi Qian \newline Department of Mathematics and Statistics \\ Mississippi State University \\ Mississippi state, MS 39762, USA} \email{qian@math.msstate.edu} \thanks{Submitted December 2, 2006. Published February 4, 2007.} \thanks{Supported by the Department of Science and Technology, New Delhi, Govt. of India, under \hfill\break\indent BOYSCAST Programme vide Sanc. No. 100/IFD/5071/2004-2005 Dated 04.01.2005} \subjclass[2000]{34C10} \keywords{Oscillatory solution; asymptotic behaviour} \begin{abstract} We establish sufficient conditions for the linear differential equations of fourth order $$(r(t)y'''(t))' =a(t)y(t)+b(t)y'(t)+c(t)y''(t)+f(t)$$ so that all oscillatory solutions of the equation satisfy $$\lim_{t\to\infty}y(t)=\lim_{t\to\infty}y'(t)=\lim_{t\to\infty}y''(t)= \lim_{t\to\infty}r(t)y'''(t)=0,$$ where $r:[0,\infty)\to(0,\infty),a,b,c$ and $f:[0,\infty)\to R$ are continuous functions. A suitable Green's function and its estimates are used in this paper. \end{abstract} \maketitle \numberwithin{equation}{section} \newtheorem{theorem}{Theorem}[section] \newtheorem{lemma}[theorem]{Lemma} \newtheorem{example}[theorem]{Example} \newtheorem{remark}[theorem]{Remak} \section{Introduction} Bainov and Dimitrova \cite{dimi} proved the following result (See also Theorem 3.1.1 with $n=4$ in \cite{bain}). \begin{theorem}\label{thm1.1} Assume \begin{gather}\label{e1.1} \lim_{t\to\infty}\int^{t}_{t_{0}}\frac{1}{r_{1}(s_{1})} \int^{s_{1}}_{t_{0}}\frac{1}{r_{2}(s_{2})} \int^{s_{2}}_{t_{0}}\frac{1}{r_{3}(s_{3})}\,ds_{3}\,ds_{2}\,ds_{1}\,<\infty, \\ \label{e1.2} \int^{\infty}_{t_{0}}|a(t)|\,dt<\infty, \\ \label{e1.3} \int^{\infty}_{t_{0}}|f(t)|\,dt<\infty, \end{gather} Then all solutions of $$\label{e1.4} (r_{3}(t)(r_{2}(t)(r_{1}(t)y'(t))')')'+a(t)F((Ay)(t))=f(t)$$ are bounded and all oscillatory solutions of \eqref{e1.4} tend to zero as $t\to\infty$, where $F\in C(R,R)$ and $F(u)$ is a bounded function on $R, r_{i}\in C^{4-i}([t_{0},\infty);(0,\infty)),1 \leq i \leq 3$, $a,f \in C([t_{0},\infty);R)$ and $A$ is an operator with certain properties. \end{theorem} The motivation of the present work has come from Theorem \ref{thm1.1}. Since $F(u)=u$ is not bounded, then Theorem \ref{thm1.1} cannot be applied to its corresponding linear equation. Our purpose is to show that under the conditions of Theorem \ref{thm1.1}, every oscillatory solution of the considered equation along with their first and second order derivatives tend to zero as $t\to\infty$. In fact, we consider the more general fourth order linear differential equations of the form $$\label{e1.5} (r(t)y'''(t))'=a(t)y(t)+b(t)y'(t)+c(t)y''(t)+f(t)$$ where $r:[0,\infty)\to(0,\infty),a,b,c$ and $f:[0,\infty)\to R$ are continuous functions. We shall show that all oscillatory solutions of \eqref{e1.5} along with their first and second order derivatives tend to zero as $t\to\infty$. Asymptotic behaviour of oscillatory solutions of second order differential equations have been studied by many authors, see (\cite{sing,bsin, bsig, bsih}). For higher order differential equations, one may see the paper due to Chen and Yeh \cite{chen} and the recent work due to Padhi \cite{spad} and the references cited therein. The monograph due to \cite{bain} gives a survey on the asymptotic decay of oscillatory solutions of differential equations. In \cite{spad}, Padhi considered a more general forced differential equation where he obtained a new sufficient condition under which all oscillatory solutions of the equation tend to zero as $t\to\infty$. The result improve all earlier existing results. In a recent note \cite{sesh}, Padhi studied the asymptotic behaviour of oscillatory solutions of third order linear differential equations. It seems that asymptotic behaviour of oscillatory solutions of fourth order differential equations of the form \eqref{e1.5} has not been studied in the literature. Motivated by the result in \cite{sesh}, this work pays an attention for the asymptotic behaviour of oscillatory solutions of the equations of the form \eqref{e1.5}. The technique used in the work is the help of a Green's function and its estimates. This technique was used by Padhi \cite{sesh}. The sufficient conditions given in this paper may be treated as a different set of condition given in \cite{spad}. Sufficient conditions for oscillations of equations of the form \eqref{e1.5} with $b(t)=0$ and $c(t)=0$ are given in \cite{bain}. The work is organized as follows: Section 1 is introductory where as the main result of the paper is given in Section 2 and an open problem is left to the reader. We note that a solution of the above mentioned equations is said to be oscillatory if it has arbitrarily large zeros. \section{Main Results} The main result of the paper is the following. \begin{theorem} \label{thm2.1} Let \eqref{e1.2} and \eqref{e1.3} hold. Further suppose that $$\label{e1.6} \int^{\infty}_{0}\frac{t^{2}}{r(t)}\,dt<\infty,\quad \int^{\infty}_{0}|b(t)|\,dt<\infty, \quad \int^{\infty}_{0}|c(t)|\,dt<\infty.$$ Then every oscillatory solution of the equation \eqref{e1.5} satisfies $$\label{e1.7} \lim_{t\to\infty}y(t)=\lim_{t\to\infty}y'(t)=\lim_{t\to\infty}y''(t)= \lim_{t\to\infty}r(t)y'''(t)=0.$$ \end{theorem} \begin{proof} Let $\{t_{k}\}^{\infty}_{k=1}$, \$1