Can a Matrix with Identical Columns be Invertible?

In summary: The definition of "independent" I was thinking of is that the columns are not dependent on each other through any vector in the nullspace of the matrix.
  • #1
Chadlee88
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i need to be able to prove that an nxn matrix with two identical columns cannot be invertible. I know that if the columns of the matrix are linearly independent then the matrix is invertible. Could some please give me a hint on how to do this proof because i really don't know where to start. :frown:
 
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  • #2
Depending on what has been covered in your class, it may be easiest to work with the nullspace of the matrix A. A is invertible iff the nullspace of A contains only the zero vector.
Call, the matrix A. If you can find a nonzero vector x such that Ax=0, then you've shown A is not invertible.
 
  • #3
As Galileo said, it depends on what has been covered. It is relatively easy to show that the determinant of a matrix in which two columns are the same is 0.
 
  • #4
Chadlee88 said:
i need to be able to prove that an nxn matrix with two identical columns cannot be invertible. I know that if the columns of the matrix are linearly independent then the matrix is invertible. Could some please give me a hint on how to do this proof because i really don't know where to start. :frown:

what's it mean when two columns of a matrix are identical?

Compare this with what is meant by 'linearly independent'.

Can two identical columns in one matrix be independent?
 
  • #5
Actually, I've never seen a text that definedf "independent" for the columns of a matrix!

You can, of course, think of the columns of a matrix as vectors and then determine whether or not those vectors are independent.
 
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  • #6
hallsofivy, I think the columns being independent refers to them as vectors being linearly independent essentially (it's not proper terminology perhaps, but it does get the point across)
 
  • #7
Office_Shredder said:
hallsofivy, I think the columns being independent refers to them as vectors being linearly independent essentially (it's not proper terminology perhaps, but it does get the point across)
Yeh, I went back and edited my post just before I saw this.
 

FAQ: Can a Matrix with Identical Columns be Invertible?

What is a matrix inverse?

A matrix inverse is a mathematical concept that refers to the process of finding a matrix that, when multiplied by another matrix, results in the identity matrix. In simpler terms, it is the matrix version of division, where instead of dividing by a number, we multiply by its inverse.

What is the importance of invertible matrices in proofs?

Invertible matrices are important in proofs because they provide a way to solve systems of equations, and also have several useful properties that make them useful in various mathematical operations. In addition, the invertibility of a matrix is often a key factor in determining the existence of solutions to certain problems.

How can you determine if a matrix is invertible?

A matrix is invertible if its determinant is non-zero. This means that the matrix has an inverse that can be found using various methods, such as Gaussian elimination or the adjugate formula. In addition, square matrices with full rank are always invertible.

Can a matrix be invertible if it has a row or column of zeros?

No, a matrix cannot be invertible if it has a row or column of zeros. This is because a row or column of zeros will result in a determinant of zero, making the matrix non-invertible. However, if a matrix has a zero entry in a non-diagonal position, it can still be invertible.

Are all invertible matrices square matrices?

Yes, all invertible matrices are square matrices. This is because a non-square matrix cannot have an inverse, as the inverse must have the same number of rows and columns as the original matrix in order for the multiplication to result in the identity matrix.

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