A = 1 1
-1 -1
B = b11 b12
b21 b22
AB = BA
AB= b11+b21 b12+b22
-b11-b12 -b21-b12
BA = b11-b12 b11-b12
b21-b22 b21-b22
=> b11+ b21 = b11 - b12 => b21 = -b12
b12 + b22 = b11 - b12 => 2b12 = b11 - b22 => b11 = b22 = b
-b11 - b12 = b21 - b22 => 3b12 = -b21 => b12 = 0
-b21 - b22 = b21 - b22 => b21 = 0
=> B = b 0
0 b
для всех b из R