Idempotent Matrices: Proving PQ+QP=0 implies PQ=QP=0

  • Thread starter Thread starter td21
  • Start date Start date
  • Tags Tags
    Matrix
Click For Summary

Homework Help Overview

The discussion revolves around the properties of idempotent matrices, specifically focusing on proving that if the sum of the products of two idempotent matrices \(P\) and \(Q\) satisfies the equation \(PQ + QP = 0\), then it follows that both \(PQ\) and \(QP\) must equal zero.

Discussion Character

  • Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants explore the implications of the equation \(PQ + QP = 0\) and attempt to manipulate the expressions to show that \(PQ = 0\) and \(QP = 0\). There is a focus on substituting and rearranging terms, with some participants questioning how to derive the final results from their current expressions.

Discussion Status

The discussion is ongoing, with participants providing hints and guidance to each other. Some have pointed out specific lines in the attempts that may lead to the desired conclusions, while others express uncertainty about how to proceed from their current findings.

Contextual Notes

Participants are working under the constraints of homework rules, which may limit the extent to which they can receive direct solutions. There is a shared understanding that the goal is to prove the relationship between \(PQ\) and \(QP\) without directly stating the outcomes.

td21
Gold Member
Messages
177
Reaction score
8

Homework Statement


P,Q is idempotent, i.e. P^2=P and Q^2=Q.
Show that: If PQ+QP=0, PQ=QP=0.


Homework Equations





The Attempt at a Solution


PQ+QP=0
PQ=-QP
PPQ=-PQP
PPQP=-PQPP
PQP=-PQP
PQP=0

I can only show PQP=0, but how to show PQ=QP=0?
Thanks.
 
Physics news on Phys.org
td21 said:

Homework Statement


P,Q is idempotent, i.e. P^2=P and Q^2=Q.
Show that: If PQ+QP=0, PQ=QP=0.


Homework Equations





The Attempt at a Solution


PQ+QP=0
PQ=-QP
PPQ=-PQP
PPQP=-PQPP
PQP=-PQP
PQP=0

I can only show PQP=0, but how to show PQ=QP=0?
Thanks.

I really don't think you want me to tell you. You are SO close. Look at the third line where you showed PPQ=-PQP.
 
Dick said:
I really don't think you want me to tell you. You are SO close. Look at the third line where you showed PPQ=-PQP.
Thanks! But i still have trouble...
From 3rd line:
PPQ=-PQP
then
PQ=-PQP
PQQ=-PQPQ
PQ=-PQPQ
PQ(PQ+I)=0
PQ=0 or PQ=-I

Similarly,QP=0 or PQ=-I

Conclusion:
PQ=QP=0 or PQ=QP=-I

but still cannot show PQ=QP=0 ??
 
td21 said:
Thanks! But i still have trouble...
From 3rd line:
PPQ=-PQP
then
PQ=-PQP
PQQ=-PQPQ
PQ=-PQPQ
PQ(PQ+I)=0
PQ=0 or PQ=-I

Similarly,QP=0 or PQ=-I

Conclusion:
PQ=QP=0 or PQ=QP=-I

but still cannot show PQ=QP=0 ??

You completely overshot the goal. You showed PQ=(-PQP) and PQP=0. Just substitute the second into the first. So PQ=(-0)=0.
 

Similar threads

  • · Replies 40 ·
2
Replies
40
Views
6K
  • · Replies 2 ·
Replies
2
Views
1K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 31 ·
2
Replies
31
Views
3K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
Replies
5
Views
3K
  • · Replies 2 ·
Replies
2
Views
1K
  • · Replies 4 ·
Replies
4
Views
2K