Let A=1â â â â 23â â â â 4 and B=aâ â â â 00â â â â b,a,bâN
Then there exists infinitely many Bâs such that AB = BA
there cannot exist B such that AB = BA
there exist more than one but finite number of Bâs such that AB = BA
there exists exactly one B such that AB = BA
If b=a,B=aI and B commutes with every matrix A.