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Homework Statement
Let [itex]A\in Mat_{3,4}(K)[/itex]. Find all matrices X such that [itex]\forall X| A\cdot X = A'[/itex], where A' is the same as A with 2nd and 4th column swapped.
Homework Equations
The Attempt at a Solution
First we determine the size of matrix X. By definition the first factor must have as many columns as the second has rows and the end product is [itex]A_{m,n}\cdot B_{n,p} = C_{m,p}[/itex]. X must be 4 x 4. Let [itex]a'_{i,j}\in A', a_{i,j}\in A[/itex]
Haven't been able to (in my opinion) come up with anything sensible. Essentially, I have noticed that if we denote rows of A and columns of X as vectors, such that [itex]a_1 = (a_{1,1},a_{1,2},a_{1,3},a_{1,4})[/itex](by row up to a3) and [itex]x_1 = (x_{1,1},x_{2,1},x_{3,1},x_{4,1})[/itex](by column up to x4)
Then the dot products equal to the corresponding element of A except when we multiply by either x2 or x4. Problem is, where do I find the conditions for matrix X?
I can also write out the dot products individually, but then I have 3 equations and 4 variables..
I have concluded that:
[tex]
\begin{cases}
a'_{i,j} = a_{i,j}= \sum_{k=1}^4 a_{i,k}\cdot x_{k,j},& j\in\{1,3\}\\
a'_{i,2} = \sum_{k=1}^4 a_{i,k}\cdot x_{k, 2} = a_{i,4}\\
a'_{i,4} = \sum_{k=1}^4 a_{i,k}\cdot x_{k, 4} = a_{i,2}
\end{cases}
[/tex]
THIS LOOKS UGLY! I can't think of any other way to show conditions for all x in X :s
This definitely is not acceptable, suggestions?
(I want to avoid writing out individual x - what if the dimensions of the matrix are countible? Would take a long time to write them out xD)