Find all matrices commuting with

> A:=Matrix([[1,0],[0,2]]);

> B:=Matrix([[a,b],[c,d]]);

> A.B;

> B.A;

> A.B-B.A;

We want to claim AB=BA, so we get B=

> C:=Matrix([[a,0],[0,d]]);

> C;

> A.C-C.A;

>