Existence and uniqueness results for discrete second-order periodic boundary value problems (Q1827198)

The paper contains results on existence and uniqueness of a second order nonlinear difference equation \[ - \Delta^2 y(n-1) + q(n) y(n) = f(n,y(n)), \] \(\Delta\) the forward difference operator, with periodic boundary conditions. The methods used in the proofs are based on the notions of lower and upper solutions, Green's functions and Brouwer's fixed point theorem.