LMAM and School of Mathematical Sciences Peking University Beijing 100871 China;
LMAM and School of Mathematical Sciences Peking University Beijing 100871 China;
LMAM and School of Mathematical Sciences Peking University Beijing 100871 China;
LMAM and School of Mathematical Sciences Peking University Beijing 100871 China;
critical point theory; nonsmooth analysis; combinatorial optimization; polytope; spectral graph theory;