Proving Inverses (Matrix Multiplications)

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Discussion Overview

The discussion revolves around the properties of matrix inverses, specifically focusing on the invertibility of the matrix I + BA given that I + AB is invertible and B is an invertible matrix. The scope includes mathematical reasoning and proof techniques related to matrix operations.

Discussion Character

  • Mathematical reasoning

Main Points Raised

  • One participant states that if B is invertible, then I + BA is also invertible, but expresses uncertainty about how to prove this.
  • Another participant inquires about the properties of the matrix (I + BA)B.
  • A different participant asserts that (I + BA)B is invertible because it involves only elementary matrices, but indicates difficulty with the mathematical steps required for the proof.
  • One participant suggests that there may be confusion between what is known and what needs to be proven, and provides a mathematical transformation: (I + BA)B = B + BAB = B(I + AB), prompting consideration of which matrices are known to be invertible.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the proof of invertibility for I + BA; instead, multiple viewpoints and approaches are presented, with some expressing confidence in the result while others seek clarification on the proof process.

Contextual Notes

Participants express uncertainty regarding the mathematical steps necessary to establish the invertibility of I + BA, indicating potential limitations in their current understanding or approach.

jwb
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Let A and B be n x n matrices such that I+AB is invertible.

If B is invertible what can you deduce about the matrix I+BA.

I know that I+BA is invertible but not sure how to go about proving this.
 
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What can you tell about the matrix (I+BA)B?
 
it is also invertible due to being a multiplication or addition of only elementary matrices. Its not the theory I am having trouble with its the mathematical steps needed to prove that it is invertible
 
jwb said:
it is also invertible due to being a multiplication or addition of only elementary matrices.

I think you are gettng confused between what you "know" (i.e. you know or you have guessed what the answer is) and what you are trying to prove.

(I+BA)B = B + BAB = B(I + AB)

Now think about which of those matrices you KNOW are invertible, and which you want to PROVE are invertible.
 

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