Speaker：Ziqing Xie (Hunan Normal University)

Time：2019-04-18,10:30

Location：Conference Room 105 at Experiment Building at Haiyun Campus

Abstract: Saddle points, which are unstable critical points, have a widely range of applications in many fields of nonlinear science, such as nonlinear optics, condensed matter physics, chemical reactions, materials science etc.. Owning to the nonlinearity of model problems, the multiplicity instability of saddle points, it is extremely challenging to design a stable, efficient globally convergent numerical algorithm for finding saddle points. In this talk, a globally convergent Barzilai-Borwein-type local minimax method (GBB-LMM) is proposed for finding multiple saddle points of nonconvex functionals in Hilbert space, where the idea of the Barzilai-Borwein gradient method combining with the nonmonotone line search strategy in optimization in Euclidean space is applied to solve a two-level local optimization problem. Actually, the Barzilai-Borwein-type step-size is explicitly constructed as a trial step-size at each iteration step of the local minimax method, the nonmonotone step-size search rule is introduced to guarantee the global convergence. The feasibility global convergence of the GBB-LMM are rigorously verified. The GBB-LMM is then implemented to solve several typical nonlinear boundary value problems with variational structures for multiple unstable solutions. The numerical results indicate that our approach may greatly speed up the convergence of traditional local minimax methods.