In this paper we are concerned with questions of multiplicity and concentration behavior of positive solutions of the elliptic problem$$\left(P_{}\right)\left\{\begin{array}{c}\hfill {ℒ}_{}u=f\left(u\right)\text{in}{ℝ}^3,\\ \hfill u\>0\text{in}{ℝ}^3,\\ \hfill u\in H^1\left({ℝ}^3\right),\end{array}\right.$$( P ε ) ℒ ε u = f ( u ) in IR 3 , u > 0 in IR 3 , u ∈ H 1 ( IR 3 ) , whereε is a small positive parameter, f : ℝ → ℝ is a continuous function,$${ℒ}_{}$$ℒ ε is a nonlocal operator defined by$${ℒ}_{}u=M\left(\frac{1}{}{\int }_{{ℝ}^3}{\left|\nabla u\right|}^2+\frac{1}{{}^3}{\int }_{{ℝ}^3}V\left(x\right)u^2\right)\left[{-}^2\Delta u+V\left(x\right)u\right],$$ℒ ε u = M 1 ε ∫ IR 3 | ∇ u | 2 + 1 ε 3 ∫ IR 3 V ( x ) u 2 [ − ε 2 Δ u + V ( x ) u ] ,M : IR+ → IR+ and V : IR3 → IR are continuous functions which verify some hypotheses.

We study the existence and multiplicity of positive solutions of the nonlinear equation u''(x) + λh(x)f(u(x)) = 0 subject to nonlinear boundary conditions. The method of upper and lower solutions and degree theory arguments are used.

We study the existence and multiplicity of positive solutions of the nonlinear fourth order problem ⎧$u^{\left(4\right)}=\lambda f\left(u\right)$ in (0,1), ⎨ ⎩u(0) = a ≥ 0, u’(0) = a’ ≥ 0, u(1) = b ≥ 0, u(1) = -b’ ≤ 0 The methods employed are upper and lower solutions and degree theory arguments.

We study the existence of positive solutions to second order nonlinear differential equations with Neumann boundary conditions. The proof relies on a fixed point theorem in cones, and the positivity of Green's function plays a crucial role in our study.

We prove the existence of at least three solutions to the following fractional boundary value problem: ⎧ $-d/dt(1/2_0{D}_t^{-\sigma }\left(u^{\text{'}}\left(t\right)\right)+1/2_t{D}_T^{-\sigma }\left(u^{\text{'}}\left(t\right)\right))-\lambda \beta \left(t\right)f\left(u\left(t\right)\right)-\mu \gamma \left(t\right)g\left(u\left(t\right)\right)=0$, a.e. t ∈ [0, T], ⎨ ⎩ u (0) = u (T) = 0, where ${}_0{D}_t^{-\sigma }$ and ${}_t{D}_T^{-\sigma }$ are the left and right Riemann-Liouville fractional integrals of order 0 ≤ σ < 1 respectively. The approach is based on a recent three critical points theorem of Ricceri [B. Ricceri, A further refinement of a three critical points theorem, Nonlinear Anal. 74 (2011), 7446-7454].