Can a Matrix Satisfy \(A^2 = A\) and be Non-Singular?

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Discussion Overview

The discussion revolves around the question of whether a matrix \(A\) can satisfy the equation \(A^2 = A\) while also being non-singular. Participants explore the implications of this equation and the characteristics of matrices that meet this condition.

Discussion Character

  • Debate/contested

Main Points Raised

  • One participant proposes a proof that if \(A^2 = A\), then either \(A = I\) or \(A\) is singular, assuming \(A\) is non-singular.
  • Another participant reiterates the proof steps but questions the necessity of stating \(A^2 = A\) as it is given by hypothesis.
  • A third participant suggests that the conclusion drawn may be redundant and questions the placement of the word "therefore" in the argument.
  • A fourth participant agrees that the conclusion should precede the statement \(A = I\) and suggests stopping at that point.

Areas of Agreement / Disagreement

Participants express differing views on the clarity and necessity of certain steps in the proof, indicating a lack of consensus on the presentation of the argument.

Contextual Notes

There are unresolved questions regarding the redundancy of certain statements in the proof and the logical flow of the argument.

skoker
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i have a simple proof is this correct?

prove that if \(A^2=A\), then either A=I or A is singular.

let A be a non singular matrix. then \(A^2=A, \quad A^{-1}A^2=A^{-1}A, \quad IA=I, \quad A=I\) therefore \(A^2=A.\)
 
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skoker said:
let A be a non singular matrix. then \(A^2=A, \quad A^{-1}A^2=A^{-1}A, \quad IA=I, \quad A=I\)

Right

therefore \(A^2=A.\)

Why do you write this? $A^2=A$ just by hypothesis.
 
Fernando Revilla said:
Right
Why do you write this? $A^2=A$ just by hypothesis.

i suppose that is redundant or unnecessary. i was not sure if it needs a conclusion with the 'therefore'.
 
skoker said:
i suppose that is redundant or unnecessary. i was not sure if it needs a conclusion with the 'therefore'.

The therefore should go before the \(A=I\) and you should stop at that point.

CB
 

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