IMA Journal of Applied Mathematics Advance Access published online on February 16, 2008
IMA Journal of Applied Mathematics, doi:10.1093/imamat/hxn001
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Existence, upper and lower solutions and quasilinearization for singular differential equations
Department of Mathematical Sciences, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia

Department of Mathematics, National University of Ireland, Galway, Ireland
Email: mgebeily{at}kfupm.edu.sa
Received on July 12, 2005; Revision received December 7, 2007. In this paper, we discuss existence theorems in the presence of upper and lower solutions as well as the method of quasilinearization (QSL) for general non-linear second-order singular ordinary differential equations. We show the existence of solutions under the assumption of weak continuity of the non-linear part. If the non-linear part is monotone decreasing, a solution may be obtained by the QSL method as the strong limit of a quadratically convergent sequence of approximate solutions. Under stronger assumptions on the linear and the non-linear parts, a solution is quadratically bracketed between two monotone sequences of approximate solutions of certain related linear equations.
Keywords: singular differential equations; self-adjoint operators; non-linear operators; upper and lower solutions; existence; quasilinearization methods.