Nonlocal Kirchhoff superlinear equations with indefinite nonlinearity   and lack of compactness
  We study the following Kirchhoff equation $$- \left(1 + b \int_{\mathbb{R}^3} |\nabla u|^2 dx \right) \Delta u + V(x) u = f(x,u), \ x \in \mathbb{R}^3.$$ A special feature of this paper is that the nonlinearity $f$ and the potential $V$ are indefinite, hence sign-changing. Under some appropriate assumptions on $V$ and $f$, we prove the existence of two different solutions of the equation via the Ekeland variational principle and Mountain Pass Theorem. 
Lin Li, Vicen\c{t}iu D. R\u{a}dulescu, and Du\v{s}an D. Repov\v{s}
math.AP