Based on the eigensystem {λj, φj} of -△, the multiple solutions for nonlinear problem △u + f(u)= 0 in Ω,u = 0 on Ω are approximated. A new search-extension method (SEM) is proposed, which consists of three algorithms in three level subspaces.Numerical experiments for f(u)= u3 in a square and L-shape domain are presented. The results show that there exist at least 3κ - 1 distinct nonzero solutions corresponding to each κ-ple eigenvalue of -△ (Conjecture 1).
By applying the continuous finite element methods of ordinary differential equations,the linear element methods are proved having second-order pseudo-symplectic scheme and the quadratic element methods are proved having third-order pseudo- symplectic scheme respectively for general Hamiltonian systems,and they both keep energy conservative.The finite element methods are proved to be symplectic as well as energy conservative for linear Hamiltonian systems.The numerical results are in agree- ment with theory.
For numerical computations of multiple solutions of the nonlinear elliptic problemΔu+ f（u）=0 inΩ, u=0 onΓ, a search-extension method （SEM） was proposed and systematically studied by the authors. This paper shall complete its theoretical analysis. It is assumed that the nonlinearity is non-convex and its solution is isolated, under some conditions the corresponding linearized problem has a unique solution. By use of the compactness of the solution family and the contradiction argument, in general conditions, the high order regularity of the solution u∈H1+α,α>0 is proved. Assume that some initial value searched by suitably many eigenbases is already fallen into the neighborhood of the isolated solution, then the optimal error estimates of its nonlinear finite element approximation are shown by the duality argument and continuation method.
