We investigate multiplicity of solutions u(x, t) for a piecewise linea
r perturbation -(bu(+) - au(-)) of the one-dimensional beam operator u
(tt) + u(xxxx) under Dirichlet boundary condition on the interval (-pi
/2, pi/2) and periodic codition on the varible t. Our concern is to in
vestigate multiplicity of solutions of the equation when the nonlinear
ity crosses finite eigenvalues and the source term is generated by two
eigenfunctions.