12 In N N g A1B2…B g A21B2 g 2k B 2 kk = = 1 2 11 12 1 21 22 2 1 2 g n n g g gn = 11 12 1 21 22 2 1 2 g g g g gg = 11 12 1 21 22 2 1 2 k k k k kk