Linear Algebra Proof: Invertible Idempotent Matrix Must be Identity Matrix

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Homework Help Overview

The discussion revolves around the properties of idempotent matrices, specifically focusing on the proof that if an invertible idempotent matrix A exists, then it must be the identity matrix I_n. Participants are examining the implications of A being both idempotent and invertible.

Discussion Character

  • Conceptual clarification, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants explore the implications of the idempotent property A^2 = A and question the validity of certain steps taken in the proof. There is a discussion about the assumption that A is not equal to I_n and whether this leads to contradictions. Some participants also raise concerns about the reasoning related to the invertibility of A.

Discussion Status

The discussion is ongoing, with participants providing feedback on each other's reasoning. Some guidance has been offered regarding the necessity of stating that A is invertible, and there is an exploration of the implications of the assumptions made. Multiple interpretations of the problem are being considered, and there is no explicit consensus yet.

Contextual Notes

Participants are working under the constraints of homework rules, which may limit the information they can use or the methods they can apply. The discussion includes a separate proof regarding the idempotency of the product of two idempotent matrices under certain conditions, which raises additional questions about the validity of the assumptions made.

Dosmascerveza
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Homework Statement


If A is an invertible idempotent matrix, then A must be the Identity matrix I_n.

Homework Equations


A^2==A ; A^2==AA; A^(-1); I==A^(-1)

The Attempt at a Solution



Suppose A is an nxn matrix =/= I_n.

s.t. A^(2)==A

so A^(2)==A ==> AA==A

==> A^(-1)AA==A^(-1)A ==> A==I==> A^(-1)A==A^(-1)I==>I==A^(-1)I==A^(-1)==A

which yeilds a contradiction because we supposed our A =/= I_n.

Therefore A==I_nIs this correct please help me understand where I have failed...
 
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Dosmascerveza said:
Suppose A is an nxn matrix =/= I_n.

s.t. A^(2)==A

so A^(2)==A ==> AA==A
==> A^(-1)AA==A^(-1)A
==> A==I
You should have stopped right here. You should have also stated that A is invertible.
==> A^(-1)A==A^(-1)I
==> I==A^(-1)I==A^(-1)==A
This is wrong. You don't know that the inverse of A is equal to A.
 
Okay so if i stated A an invertible nxn matrix =/= I_n

s.t A^(2)==A(idempotent)... truncating the last bit of foolishness. I was correct?
 
another proof...
problem statement.
prove if A and B are idempotent and AB==BA then AB is idempotent.

AB==BA ==> A^(-1), B^(-1) exist

Since A and B are idempotent invertible matrices, from previously proven theorem, we know A=I and B=I. and since II==I ==> AB==I Therefore AB==BA and AB is Idempotent,
 
Dosmascerveza said:
AB==BA ==> A^(-1), B^(-1) exist
This isn't true.
 

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