A rectangular plate held at fixed temperatures on its four edges settles into a steady-state field satisfying Laplace's equation ∇²T = ∂²T/∂x² + ∂²T/∂y² = 0. No internal heat source, no time evolution — the field shown is the equilibrium solution, found here by Gauss-Seidel relaxation of the discretised 5-point stencil.