Is it possible for matrices to be equal but their columns not equal?

  • Thread starter yaganon
  • Start date
In summary, the conversation discusses the possibility of matrices B and C being different even if AB = AC. This is because A does not necessarily have to be singular, and even if it is, the matrices may not be square. The example provided shows how this is possible through the use of a nullspace vector.
  • #1
yaganon
17
0
If AB = AC, it's possible that B /= C

conversely:
if BA = CA, is it possible that C /= B?

/= means doesn't equal
 
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  • #2
Certainly. In both cases let A = 0, B = Anything, C = Anything else.
 
  • #3
More generally, if A is a matrix that does not have an inverse, then we can have A(B- C)= 0 without B- C being equal to 0. And, similarly, (B- C)A= 0 without B- C= 0.
 
  • #4
hallsofivy

why does A have to be singular?
 
  • #5
yaganon said:
hallsofivy

why does A have to be singular?

say
AB=AC
suppose A has an inverse D such that AD=DA=
DAB=DAC
B=C
thus if B/=C A must be singular

Also the matrices might not be square
[[1,1,1]]*[[1,1,1]]=[[1,1,1]]*[[3,0,0]]=[[3]]
where * is the adjoint
[[1,1,1]]/=[[3,0,0]]
 
  • #6
A basic example for you...

[itex] \begin{pmatrix}0 & 0 & 0\\0 &1 & 0\\ 0 & 0 &0\end{pmatrix} = \begin{pmatrix}1 & 0 & 1\\0 &1 & 0\\ 1 & 0 &1\end{pmatrix} \begin{pmatrix}1 & 0 & -1\\0 &1 & 0\\ -1 & 0 &1\end{pmatrix} = \begin{pmatrix}1 & 0 & 1\\0 &1 & 0\\ 1 & 0 &1\end{pmatrix} \begin{pmatrix}2 & 0 & -5\\0 &1 & 0\\ -2 & 0 &5\end{pmatrix} =
\begin{pmatrix}0 & 0 & 0\\0 &1 & 0\\ 0 & 0 &0\end{pmatrix}
[/itex]

But

[itex] \begin{pmatrix}1 & 0 & -1\\0 &1 & 0\\ -1 & 0 &1\end{pmatrix} \neq \begin{pmatrix}2 & 0 & -5\\0 &1 & 0\\ -2 & 0 &5\end{pmatrix}[/itex]

If you look at the rank of A and the vector that is in the nullspace of A, you can see why this holds with the columns of the B and C are chosen as such.
 
Last edited:

FAQ: Is it possible for matrices to be equal but their columns not equal?

What is a matrix?

A matrix is a rectangular array of numbers, symbols, or expressions arranged in rows and columns. It is commonly represented using brackets or parentheses.

How do you add or subtract matrices?

To add or subtract matrices, the matrices must have the same dimensions. You simply add or subtract the corresponding elements in each matrix to get the resulting matrix.

How do you multiply matrices?

To multiply matrices, the number of columns in the first matrix must match the number of rows in the second matrix. You then multiply the corresponding elements in each row of the first matrix with the corresponding elements in each column of the second matrix and add the products.

What is the identity matrix?

The identity matrix is a square matrix with 1s along the main diagonal and 0s in all other positions. When multiplied with another matrix, the identity matrix acts as a neutral element and the other matrix remains unchanged.

What are some real-world applications of matrices?

Matrices are used in various fields such as computer graphics, economics, statistics, and physics. They are used to solve systems of linear equations, analyze data, and perform transformations in 3D graphics, among other applications.

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