Linear algebra proof - inverses

In summary, the conversation discusses the question of whether the product of two non-square matrices, A and B, can be equal to the identity matrix. It is determined that this is not possible, as a matrix can only be invertible if it is square. Therefore, the statement AB /= In is proven.
  • #1
miky87
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Homework Statement



Let m < n. Let A be an n × m matrix, and let B be an m × n matrix. Prove that AB /= In .


Homework Equations





The Attempt at a Solution


since m<n, the reduced form of matrix B will have free variables.
I know that if A and B are invertible matrices, AB will be as well but does the converse also hold?
 
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  • #2
Hi miky87! :smile:

miky87 said:
I know that if A and B are invertible matrices, AB will be as well but does the converse also hold?

This makes no sense. A matrix can only be invertible when it's square. And these are not square matrices.

What can you tell us about the rank of A, B, AB and In??
 
  • #3
The question make perfect sense: he is being asked to show that A*B cannot be the identity matrix (i.e., that a non-square matrix cannot have a left- or right-inverse). This seems closely related to another question in this Forum, and the same advice given there applies here.

RGV
 
  • #4
Ray Vickson said:
The question make perfect sense: he is being asked to show that A*B cannot be the identity matrix (i.e., that a non-square matrix cannot have a left- or right-inverse). This seems closely related to another question in this Forum, and the same advice given there applies here.

RGV

A matrix A is invertible if and only if there is a matrix B such that AB=BA=I. This can only be satisfied for square matrices. So calling A and B invertible in his question does not make any sense.
Left and right inverses do make sense, but I doubt he meant that.
 

FAQ: Linear algebra proof - inverses

What is an inverse in linear algebra?

An inverse in linear algebra refers to the reverse operation of a given mathematical process. In the context of linear algebra, an inverse is a matrix that, when multiplied with the original matrix, yields the identity matrix.

Why is finding the inverse important in linear algebra?

Finding the inverse of a matrix is important in linear algebra because it allows us to solve systems of linear equations, perform matrix division, and find the determinant and eigenvalues of a matrix. It also plays a crucial role in many other areas of mathematics and science, such as optimization and data analysis.

How do you prove that a matrix has an inverse?

To prove that a matrix has an inverse, we need to show that when the matrix is multiplied by its inverse, the result is the identity matrix. This can be done through various methods, such as using the adjugate matrix, the Gauss-Jordan elimination method, or using the determinant and cofactor formulas.

Can every matrix have an inverse?

No, not every matrix has an inverse. For a matrix to have an inverse, it must be a square matrix (number of rows = number of columns) and its determinant must not be equal to zero. If the determinant is zero, then the matrix is said to be singular and does not have an inverse.

Is the inverse of a matrix unique?

Yes, the inverse of a matrix is unique. This means that for a given matrix, there is only one matrix that can be its inverse. This is because if there were multiple inverses, then the inverse operation would not be well-defined and would lead to conflicting results.

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