K+k k Au 0 K+k K+k → 0 m K+kk →0 m K+2k 2k       = + +  =  = =  = +   =  =       −      −  +  = −  + − −  − + m 2k l g m K 2k l g m K m k m K k 0 m k m K k 0 m K k m k m k m K k 2 1 2 2 2 2 2