When is matrix multiplication commutative

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SUMMARY

Matrix multiplication is commutative when both matrices are diagonal and have the same dimensions, as well as when they are scalar multiples of each other. The example provided demonstrates that the product of two diagonal matrices, such as \begin{bmatrix}a & 0 & 0 \\ 0 & b & 0 \\ 0 & 0 & c\end{bmatrix} and \begin{bmatrix}x & 0 & 0 \\ 0 & y & 0 \\ 0 & 0 & z\end{bmatrix}, yields the same result regardless of the order of multiplication. This property can be generalized to any dimension, confirming that diagonal matrices commute under multiplication.

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  • Basic concepts of scalar multiplication
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jaejoon89
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Can somebody please explain to me when matrices commute? I've read that it's when they are diagonal with the same dimensions (and also scalar multiples?), but I don't understand why this is.
 
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Well, for example, what is
[tex]\begin{bmatrix}a & 0 & 0 \\ 0 & b & 0 \\ 0 & 0 & c\end{bmatrix}\begin{bmatrix}x & 0 & 0 \\ 0 & y & 0 \\ 0 & 0 & z\end{bmatrix}[/tex]
and what is
[tex]\begin{bmatrix}x & 0 & 0 \\ 0 & y & 0 \\ 0 & 0 & z\end{bmatrix}\begin{bmatrix}a & 0 & 0 \\ 0 & b & 0 \\ 0 & 0 & c\end{bmatrix}[/tex]

If you can see why those are the same, you should be able to generalize to any dimension.
 

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