This paper was inspired by the works of Chiappinelli ([3]) and Schmitt and Smith ([7]). We study the problem ℒu = λau + f(·,u,u',u'',u''') with separated boundary conditions on [0,π], where ℒ is a composition of two operators of Sturm-Liouville type. We assume that the nonlinear perturbation f satisfies the inequality |f(x,u,u',u'',u''')| ≤ M|u|. Because of the presence of f the considered equation does not in general have a linearization about 0. For this reason the global bifurcation theorem of Rabinowitz ([5], [6]) is not applicable here. We use the properties of Leray-Schauder degree to establish the existence of nontrivial solutions and describe their location. The results obtained are similar to those proved by Chiappinelli for Sturm-Liouville operators.
We are presenting a numerical method which detects the presence and position of a bifurcation simplex, the regular $(k+1)$-dimensional simplex, which may be considered as "fat bifurcation point", in the curve of zeroes of the $C^1$ map $f:{\mathbb R}^{k+1}\to{\mathbb R}^k$. On the other hand the bifurcation simplex appears in the neighbourhood of the bifurcation point, meaning that we have the method to locate the bifurcation point as well. The method does not require any estimation of the derivative of the function $f$ and refers to the values of the map $f$ only in the vertices of certain triangulation. The bifurcation simplex is detected by change of the Brouwer degree value of the restriction of the map $f$ to the appropriate $k$-simplex.This publication is co-financed by the European Union as part of the European Social Fund within the project Center for Applications of Mathematics.
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