Linear Dependence of Matrix Vectors

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Discussion Overview

The discussion centers on proving the linear dependence of the set {I, A, A^2,..., A^n} where A is any nxn matrix. The context involves theoretical exploration within linear algebra, specifically regarding matrix properties and vector spaces.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant seeks a proof of linear dependence for the set of matrices {I, A, A^2,..., A^n} within the context of nxn matrices.
  • Another participant suggests starting with the identity matrix I and exploring linear combinations to express I in terms of the matrices A, A^2, ..., A^n.
  • A different participant discusses using the Jordan form of the matrix A and attempts to prove linear dependence through the properties of nilpotent and diagonal matrices.
  • One participant claims to have arrived at a solution based on the condition (A - lambda I)^k = 0 for some k < n, linking it to the Jordan canonical form.
  • Another participant proposes using the Cayley-Hamilton theorem, noting that the characteristic polynomial of the matrix is of degree at most n and is satisfied by the matrix.

Areas of Agreement / Disagreement

There is no clear consensus on the proof method, as participants propose different approaches and some express uncertainty about their claims. Multiple competing views remain regarding the best way to demonstrate linear dependence.

Contextual Notes

Participants reference various mathematical concepts such as Jordan matrices, nilpotent matrices, and the Cayley-Hamilton theorem, but the discussion does not resolve the dependencies or assumptions underlying these approaches.

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I need to prove that the set {I, A, A^2,..., A^n} is linear dependent where A is any nxn matrix. The vector space is the set of nxn matrix, considered as a nxn dimensional vector space.

Does anybody have an idea how to prove it?
Thank you very much.
 
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well I maybe wrong but to get you started:

I is in the nxn vector space and all the other matrices A, A^2, ..A^n

You might start by showing the linear combinations of matrices to get I and then relate that to the matrices A, of which matrices' can be written as nxn A^n=PA(^n)P^(-1)
 
Well, I tried to write A=P^(-1) J P where J is a Jordan matrix. Even more, I also tried to put J=N + D where N is nilpotent such that N^n=0 and D diagonal but I could not prove that the set is linear dependent. Thank you for your help. I f you have any further details or ideas I would be pleased.
 
At the end I arrived at the right solution. (A-lambda I)^k must be zero for some k<n. It comes from the Jordan canonical matrix, where lambda belongs to its spectrum. Logically then, the set {I, A, A^2,..., A^n} is linear dependent.

Thank you
 
An alternative method, which gives you the exact dependence right off the bat, is to use the Cayley-Hamilton theorem. The characteristic polynomial is of degree at most n, and is satisfied by the matrix.
 
Great! Thats certainly a much better, simpler and more elegant solution.
Thank you very much, Henry.
 

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