Linear algebra proof - inverses

Click For Summary

Homework Help Overview

The discussion revolves around a proof involving an n × m matrix A and an m × n matrix B, specifically addressing the claim that the product AB cannot equal the identity matrix In when m < n.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants explore the implications of matrix dimensions on invertibility, questioning whether the converse of a property regarding invertible matrices holds true. They also discuss the ranks of the matrices involved and the conditions under which a matrix can be invertible.

Discussion Status

There is an ongoing exploration of the definitions and properties of invertible matrices, particularly in relation to non-square matrices. Some participants are clarifying the original poster's understanding of the problem and its implications, while others are drawing connections to similar discussions in the forum.

Contextual Notes

Participants note that the matrices in question are not square, which raises questions about their invertibility and the nature of their product. There is a suggestion to consider the ranks of the matrices involved as part of the discussion.

miky87
Messages
1
Reaction score
0

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?
 
Physics news on Phys.org
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??
 
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
 
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.
 

Similar threads

  • · Replies 7 ·
Replies
7
Views
1K
Replies
15
Views
3K
  • · Replies 10 ·
Replies
10
Views
2K
  • · Replies 25 ·
Replies
25
Views
4K
Replies
5
Views
1K
Replies
9
Views
2K
Replies
4
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 15 ·
Replies
15
Views
3K
Replies
34
Views
4K