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I saw this question in practice problems and have seen similar questions asked about this on here. I proof that I'm not sure if it's acceptable or not:

It is known that $(AB)^{-1} = B^{-1}A^{-1}$
So we have $(XY)^{-1} = Y^{-1}X^{-1}$ but as $X$ and $Y$ are non-square matrices, the right-hand side doesn't exist and therefore the left-hand side doesn't either.

Alex
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    What do $X$ and $Y$ have to with $A$ and $B$? You should put the actual problem in the body of the question too. – kipf Mar 06 '24 at 12:47
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    Try to be more detailed with the first statement you make. What is $A$ and $B$ in that statement? – Alex Mar 06 '24 at 12:47
  • You recieved 3 answers to your question. Is any of them what you needed? If so, consider accepting the best answer and upvoting all useful answers you got. That's how the site works. – 5xum Mar 12 '24 at 09:40

3 Answers3

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Even simpler: You can also think of it in terms of linear maps, without using the rank inequality.

If $B\in \mathbb R^{2 \times 3}$ then $B$ is a linear map $\mathbb R^3 \to \mathbb R^2$ which can not be injective, by dimensionality reasons. Hence $AB=A\circ B$ is not injective and therefore not invertible.

Alex
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By your logic, the matrix $[1]$ is not invertible, since $[1]$ is the product of the matrix $[1, 0]$ with the matrix $\begin{bmatrix}1\\0\end{bmatrix}$.

The statement you are trying to use is:

If $A$ and $B$ are inverible square matrices, then $C=A\cdot B$ is invertible, and $C^{-1} = B^{-1}\cdot A^{-1}$.

However, from the fact above, it does not follow that if $A$ and $B$ are nonsquare, then their product is not invertible. If $A$ and $B$ are non-square, then the above fact says absolutely nothing about their product. In fact, as my example shows, a product of two non-square matrices absolutely can be invertible.


However, in your case, the simplest way to think about your problem is using the rank of matrices. In particular, the rank of a product of matrices is at most the rank of either matrix (see this question for details).

Given that, you can answer the following questions:

  • What is the largest possible rank of $X$?
  • What is the largest possible rank of $Y$?
  • Therefore, what is the largest possible rank for $XY$?
  • What is the size of the matrix $XY$?
5xum
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By thinking in terms of rank, we have first that ${\rm rank} A B \le {\rm rank} B$, true for any matrices $A, B$. Since the maximum possible value of ${\rm rank} B$ is $2$, then we have ${\rm rank} A B \le 2$. So, $A B$ as a matrix representing a linear map from $\mathbb{R}^3$ to $\mathbb{R}^3$ cannot be surjective, and hence cannot be bijective.

K. Jiang
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