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Positive solutions for a class of nonlinear singular boundary value problems at resonance. (English) Zbl 0805.34019
The paper deals with the singular differential equation ${1 \over p(t)} (p(t)y'(t))' = q(t) f(t,y(t), p(t),y'(t)),\quad t \in(0,1), \tag{1}$ with homogeneous two-point boundary conditions (of Dirichlet, Neumann, Robin or periodic type). Supposing the existence of upper and lower solutions and the growth conditions of the Bernstein-Nagumo type for $$f$$, the authors prove the existence of a solution of the above boundary value problems. As a corollary they get the existence of a nonnegative solution. Further they study the singular Dirichlet problem (2) $${1 \over p(t)} (p(t)y'(t)' + \mu_ 0y(t) = q(t) f(t,y(t), p(t),y'(t))$$, $$0<t<1$$, $$y(0) = y(1)=0$$, where $$\mu_ 0>0$$ is the smallest eigenvalue of the associated linear problem. Two theorems which guarantee the existence of a nonnegative solution of (2) are presented here. The proofs are based on the upper and lower solutions method, the coincidence degree arguments and the topological transversality method.

##### MSC:
 34B15 Nonlinear boundary value problems for ordinary differential equations 34B18 Positive solutions to nonlinear boundary value problems for ordinary differential equations
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