Set of non-invertible matrices is unbounded

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Discussion Overview

The discussion revolves around the question of proving that the set of non-invertible matrices is unbounded. Participants explore definitions and properties related to non-invertible matrices, particularly focusing on the determinant and norms of matrices.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant attempts to relate the determinant of a matrix to the concept of unboundedness by suggesting that the line y=0 (representing det(A)=0) is unbounded.
  • Another participant questions the initial claim by stating that for the analogy to hold, the set of non-invertible matrices must be a subset of real numbers.
  • A challenge is posed to find a non-invertible matrix with a norm greater than 10, prompting a discussion about the choice of norms.
  • Several participants note that the determinant is not a norm and argue that if it were, the set of non-invertible matrices would actually be bounded.
  • There is an acknowledgment that there are infinitely many norms for matrices, and participants suggest checking specific course materials for the appropriate norm to use.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the unboundedness of the set of non-invertible matrices, with some arguing for boundedness based on the properties of the determinant, while others challenge this perspective and emphasize the need for clarity on norms.

Contextual Notes

There is uncertainty regarding the definition of norms for matrices and the implications of using the determinant in this context. The discussion highlights the need for clarity on mathematical definitions and assumptions.

Tigers64
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Question:
How do I prove the set of non-invertible matrices is unbounded?

Attempt:
Let A be an element of set of non-invertible matrices.
det(A)=0
det(A)=0 is just the line y=0 if you have det(A) as the y-axis and the set of non-invertible matrices on the x-axis. y=0 is unbounded, so the set of non invertible matrices is unbounded?
 
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Tigers64 said:
the set of non-invertible matrices on the x-axis
For this to make any sense at all, "the set of non-invertible matrices on the x-axis" would have to be a subset of "the set of real numbers"...


How do I prove the set of non-invertible matrices is unbounded?
Let's start with an easier question: can you find a non-invertible matrix whose norm is bigger than 10?

(p.s. what norm are you using?)
 
I guess the problem is that I don't know which norm to use, so I used det as the norm. How do you define a norm for matrices other than the det function?
 
How do you define a norm for matrices other than the det function?
There are infinitely many different norms you can define for matrices, several of which are in common use. This is a question I cannot answer for you -- you will have to check your homework problem / textbook / class notes to find out what norm you're supposed to be using.

(Incidentally, det isn't a norm. And even if it was, then the set of all non-invertible matrices would be bounded with respect to it)
 
Hurkyl said:
There are infinitely many different norms you can define for matrices, several of which are in common use. This is a question I cannot answer for you -- you will have to check your homework problem / textbook / class notes to find out what norm you're supposed to be using.

(Incidentally, det isn't a norm. And even if it was, then the set of all non-invertible matrices would be bounded with respect to it)
Since every non-invertible matrix has determinant 0, it would be very bounded!
 

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