## Existence of positive solutions in the superlinear case via coincidence degree: the Neumann and the periodic boundary value problems.(English)Zbl 1345.34031

The authors consider the positive solutions of the second order nonlinear boundary value problem $u'' + a(x)g(u) = 0, \;\; {\mathcal B}(u,u') = 0,$ where $$g : \mathbb R^+ \to \mathbb R^+$$ is continuous, $$g(0) = 0$$, $$g(s) > 0$$ for $$s > 0$$, $$a \in L^1(0,T)$$ and $${\mathcal B}(u) = (u'(0),u'(T))$$ or $$(u(T)-u(0),u'(T)-u'(0))$$. A necessary conditions for the existence of a positive solution is that $$a$$ changes sign and has negative mean value. Using coincidence degree together with delicate estimates, the authors prove the existence of a positive solution to the above problems when, in addition to the conditions above, the set on which $$a$$ is positive is made of a finite number of pairwise disjoint intervals $$J_k$$, $$g$$ is superlinear at $$0$$ and $$\liminf_{s \to + \infty}g(s)/s$$ is larger that all the first eigenvalues of the Dirichlet problem with weight $$a$$ on the intervals $$J_k$$. This is in particular the case when $$g$$ is superlinear at infinity. The sharpness of those conditions is discussed.

### MSC:

 34B18 Positive solutions to nonlinear boundary value problems for ordinary differential equations 34B15 Nonlinear boundary value problems for ordinary differential equations 34C25 Periodic solutions to ordinary differential equations 47H11 Degree theory for nonlinear operators
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