Asked by Bob
                Find all $2 \times 2$ matrices $\bold{A}$ that have the property that for any $2 \times 2$ matrix $\bold{B}$,
\[\bold{A} \bold{B} = \bold{B} \bold{A}.\]
            
        \[\bold{A} \bold{B} = \bold{B} \bold{A}.\]
Answers
                    Answered by
            Steve
            
    Recall that Matrix multiplication is always commutative if ...
... one matrix is the Identity matrix.
... one matrix is the Zero matrix.
... both matrices are 2×2 rotation matrices. (basically case #2)
... both matrices are Diagonal matrices.
    
... one matrix is the Identity matrix.
... one matrix is the Zero matrix.
... both matrices are 2×2 rotation matrices. (basically case #2)
... both matrices are Diagonal matrices.
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