Upper and Lower Solution Method for Regular Discrete Second-Order Single-Variable BVPs

This paper investigates the existence of positive solutions for regular discrete second-order single-variable boundary value problems with mixed boundary conditions, including a nonhomogeneous Dirichlet boundary condition, of the form: \begin{equation*} u^{ΔΔ}(t-1)+h(t,\ u(t),\ u^Δ(t-1))=0 \mbox{ for }t\in[1,\ T+1];~u^Δ(0)=0;~u(T+2)=g(T+2) \end{equation*} where h is continuous on $[1, T + 1] \times \mathbb{R}^2$ and $g: [0, T + 2] \to \mathbb{R}^+$ is continuous. Using the concept of upper and lower solutions, we establish conditions under which the boundary value problem admits at least one positive solution. Our approach involves constructing an auxiliary problem with a modified nonlinearity and applying Brouwer Fixed Point Theorem to a carefully defined solution operator. We prove that any solution to this auxiliary problem that remains within the bounds of the upper and lower solutions is equivalent to a solution of the original problem.