>>920
 (A+B)^2 = E,
 (A-B)^2 = -E,
より
 A+B = [ 0, s ]
     [ 1/s, 0 ]

 A-B = [ 0, t ]
     [ -1/t, 0]
ここに st≠0,
よって
 A = (1/2)[ 0, s+t ]
       [1/s -1/t, 0 ]

 B = (1/2)[ 0, s-t ]
       [ 1/s +1/t, 0 ]