Linear Algebra Proof: Invertible Idempotent Matrix Must be Identity Matrix

Click For Summary
SUMMARY

The discussion centers on the proof that if A is an invertible idempotent matrix, then A must be the identity matrix I_n. The participant demonstrates that assuming A is not equal to I_n leads to a contradiction, confirming that A must indeed equal I_n. The proof utilizes properties of idempotent matrices, specifically A^2 = A, and the implications of invertibility. The conclusion is that any invertible idempotent matrix cannot be anything other than the identity matrix.

PREREQUISITES
  • Understanding of idempotent matrices and their properties
  • Knowledge of matrix invertibility and the concept of the identity matrix
  • Familiarity with basic linear algebra proofs and notation
  • Ability to manipulate matrix equations and understand their implications
NEXT STEPS
  • Study the properties of idempotent matrices in depth
  • Learn about the implications of matrix invertibility in linear algebra
  • Explore proofs involving matrix multiplication and commutativity
  • Investigate additional theorems related to idempotent matrices and their applications
USEFUL FOR

Students of linear algebra, mathematicians focusing on matrix theory, and anyone interested in understanding the properties of idempotent and invertible matrices.

Dosmascerveza
Messages
23
Reaction score
0

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...
 
Physics news on Phys.org
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.
 

Similar threads

  • · Replies 69 ·
3
Replies
69
Views
10K
  • · Replies 7 ·
Replies
7
Views
3K
  • · Replies 32 ·
2
Replies
32
Views
3K
  • · Replies 4 ·
Replies
4
Views
2K
Replies
4
Views
23K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 13 ·
Replies
13
Views
3K
  • · Replies 9 ·
Replies
9
Views
4K
  • · Replies 7 ·
Replies
7
Views
3K
Replies
17
Views
3K