Let D = [d11 d12] [d21 d22] be a 2x2 matrix. Prove that D commutes with all other 2x2 matrices if and only if d12 = d21 = 0 and d11 = d22. I know if we can prove for every A, AD=DA should be true, but I really dont know how to proceed from there. I tried equating elements of AD with DA but that really didnt help. Can anyone help me with this problem. Thanks..