Proving Theorem: Column Space of Matrix A is a Subspace of R^m

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Discussion Overview

The discussion centers on proving the theorem that "the column space of an m x n matrix A is a subspace of R^m." Participants are exploring the definition of a subspace and how to demonstrate that the column space meets the necessary criteria for being a subspace.

Discussion Character

  • Homework-related
  • Mathematical reasoning

Main Points Raised

  • One participant requests guidance on proving the theorem using the definition of a subspace, which includes showing that the zero vector is in the column space, that it is closed under vector addition, and that it is closed under scalar multiplication.
  • Another participant suggests a step-by-step approach to the proof, emphasizing the need to define "column space" and demonstrate that it is a subset of R^m.
  • Some participants express confidence in showing the three properties of a subspace but seek clarification on how to establish that the column space is a subset of R^m.
  • One participant asserts that it is obvious that the column space is a subset of R^m since all columns are vectors from R^m.
  • Another participant prompts for the definition of "column space," indicating a need for clarity in the discussion.

Areas of Agreement / Disagreement

Participants generally agree on the importance of demonstrating the properties of a subspace but have not reached a consensus on how to show that the column space is a subset of R^m. There are varying levels of confidence and clarity regarding the definitions and steps involved in the proof.

Contextual Notes

Some participants have not provided explicit definitions or have not fully articulated their assumptions, which may affect the clarity of the discussion.

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How would I prove this theorem:

"The column space of an m x n matrix A is a subspace of R^m"

by using this definition: A subspace of a vector space V is a subset H of V that has three properties: a) the zero vector of V is in H. b) H is closed under vector addition. c) H is closed under multiplication by scalars.

Please help
 
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Okay, here's what you should do:
1. Write out the definition of "column space" since you already have the definition of "subspace".

2. Show that the column space is a subset of R^m.

3. Show that (a) is true: is the zero vector in the column space- does it satisfy the definition of vectors in the column space?

4. Show that (b) is true: if you add two vectors in the column space is the result in the column space?

5. Show that (c) is true: if you multiply a vector in the column space by a scalar is the result in the column space?
 
I think i can show the three properties, but how would i show the column space is a subset of R^m?
 
413 said:
I think i can show the three properties, but how would i show the column space is a subset of R^m?

Well, it's obvious since all the columns are from R^m !
 
Again, what is the definition of "column space"?
 

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