When is matrix multiplication commutative

In summary, matrix multiplication is usually not commutative, meaning that changing the order of multiplication will result in different products. However, there are some cases where it is commutative, such as when the matrices are scalar or equal. The reason for this is because matrix multiplication involves combining elements in a specific order, and the position of each element affects its value in the final product. To determine if matrix multiplication is commutative, one must check the dimensions of the matrices and also consider if they are equal or scalar. Commutative matrix multiplication is significant in certain mathematical applications, making equations and calculations simpler, but it is not a common property and should not be assumed without proper justification.
  • #1
jaejoon89
195
0
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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  • #2
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.
 

1. Is matrix multiplication commutative?

No, in most cases, matrix multiplication is not commutative. This means that the order in which matrices are multiplied matters and swapping the order of matrices will result in different products.

2. Are there any cases where matrix multiplication is commutative?

Yes, matrix multiplication is commutative when the matrices involved are scalar matrices (matrices with only one element) or when the matrices are equal.

3. Why is matrix multiplication not commutative?

Matrix multiplication is not commutative because the operation involves combining elements from two matrices in a specific order. This order matters because the position of each element in a matrix affects its value in the final product.

4. How do you know if matrix multiplication is commutative?

The general rule is that if the dimensions of the matrices do not match, then matrix multiplication is not commutative. However, it is always important to check each specific case to determine if the matrices are equal or scalar, which would result in commutativity.

5. What is the significance of commutative matrix multiplication?

Commutative matrix multiplication is significant in certain mathematical applications, such as linear algebra, where it allows for easier manipulation of equations and simplification of calculations. However, it is not a common property in most cases and should not be assumed without proper justification.

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