@article {BOSCAGGIN20122900,
	title = {Pairs of positive periodic solutions of second order nonlinear equations with indefinite weight},
	journal = {Journal of Differential Equations},
	volume = {252},
	number = {3},
	year = {2012},
	pages = {2900 - 2921},
	abstract = {<p>We study the problem of the existence and multiplicity of positive periodic solutions to the scalar ODEu"+λa(t)g(u)=0,λ\&gt;0, where g(x) is a positive function on R+, superlinear at zero and sublinear at infinity, and a(t) is a T-periodic and sign indefinite weight with negative mean value. We first show the nonexistence of solutions for some classes of nonlinearities g(x) when λ is small. Then, using critical point theory, we prove the existence of at least two positive T-periodic solutions for λ large. Some examples are also provided.</p>},
	keywords = {Critical points, Necessary conditions, Pairs of positive solutions, Periodic solutions},
	issn = {0022-0396},
	doi = {https://doi.org/10.1016/j.jde.2011.09.011},
	url = {http://www.sciencedirect.com/science/article/pii/S0022039611003895},
	author = {Alberto Boscaggin and Fabio Zanolin}
}
