“…Here, w(x) is a vertical downward load density on the beam, and -f (u(x), x) is the nonlinear and non-uniform elastic force density by the elastic foundation, which can depend on both the location x on the beam and the deflection u(x) at x. Beam deflection is one of the basic and important problems in structural mechanics and mechanical engineering, and it has a lot of applications [1,3,4,[7][8][9][10][11][12][13][15][16][17][18].…”