Matrix Theory problem

  • Thread starter arpitm08
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  • #1
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Homework Statement


If B≈A with (P^-1)*A*P = B and also (Q^-1)*A*Q = B, show that Q=RP where R is a nonsingular matrix that commutes with A.


Homework Equations


AR = RA


The Attempt at a Solution


Our professor told us that it is easier to just play around with these equations and get the answer. That breaking them down to their elements would be a ton of work. I tried multiplying by a bunch of various ways. First I changed the equivalence formulas to, AP=PB and AQ=QB, and I tried multiplying by R, because of the commuting factor, but I couldn't get anywhere doing that. If someone could give me a hint as to if I'm missing a property or something that could help me out, that'd be great. Thanks.
 

Answers and Replies

  • #2
tiny-tim
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hi arpitm08! :smile:

hint: what happens if you multiply either equation by Q ? :wink:
 
  • #3
Deveno
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i have an alternate approach:

note that B = P-1AP is the same as: A = PBP-1.

also, note that if R-1AR = A, then AR = RA.

what happens if you evaluate (QP-1)-1A(QP-1)?
 
  • #4
tiny-tim
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isn't that the same as mine, except using P instead of Q ? :wink:
 
  • #5
Deveno
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isn't that the same as mine, except using P instead of Q ? :wink:

your previous hint was:

"multiply either equation by Q".

i count 5 "=" in the OP's post, so it is unclear to me which two of them you mean. i suppose you mean:

1) P-1AP = B
2) Q-1AQ = B

note that "multiply by Q" is not unambiguously defined, since Mat(n,F) is a non-commutative monoid. presumably you meant "left-multiply by Q".

i think your IDEA is the same idea as mine, i think our hints are not.
 

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