Why Do the Positions of C and C^-1 Switch in Matrix Diagonalization Definitions?

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Homework Help Overview

The discussion revolves around the definitions of similar matrices and their diagonalization in linear algebra, specifically addressing the placement of the matrices C and C^-1 in the equations. The original poster questions the consistency of these definitions and whether they are correctly represented in their lecture notes.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to understand why the positions of C and C^-1 switch between the definitions of similar matrices and diagonalized matrices. They express confusion about the implications of this switching on the correctness of the definitions and whether it indicates an error in their notes.

Discussion Status

Participants are exploring the definitions and questioning the assumptions behind them. Some participants seek clarification on whether additional context or definitions were omitted from the original poster's explanation. There is an acknowledgment of the potential for misunderstanding due to reliance on lecture notes rather than a standard textbook.

Contextual Notes

The discussion highlights that the definitions being referenced come from lecture notes rather than a published textbook, which may contribute to the confusion. The original poster is also considering the implications of these definitions for calculating powers of matrices.

cookiesyum
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Homework Statement


My linear algebra textbook defines...

similar matrices: A = C^-1BC
diagonalized similar matrices: A = CDC^-1
A^n = C^-1*D^n*C

Why do the C^-1 and C's get switched around between the definitions? Doesn't order of multiplication matter? Are these the correct definitions? Is A^n really the opposite of the definition for diagonalized similar matrices, or is this an error?


Homework Equations





The Attempt at a Solution

 
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cookiesyum said:
My linear algebra textbook defines...

similar matrices: A = C^-1BC
That couldn't possibly have been the definition your book gave. What did you omit?
 
Hurkyl said:
That couldn't possibly have been the definition your book gave. What did you omit?

Nope, that's it! He has us using his own lecture notes, instead of a published textbook. This is why I thought it could potentially be an error.

So if I'm diagonalizing a matrix, there exists C such that A = CDC^-1 but when I want to find A^n, I solve C^-1*D^n*C (the C inverse and C switch spots). Is that on purpose?
 
cookiesyum said:
Nope, that's it! He has us using his own lecture notes, instead of a published textbook. This is why I thought it could potentially be an error.
Nothing else, really? Nothing like
A and B are similar matrices if ...​
or
... if there exists an invertible matrix C such that ...​
?




P.S. if [itex]A = C D C^{-1}[/itex], then we do indeed have [itex]A^n = C D^n C^{-1}[/itex]. (For nonnegative integers n. Negative is allowed if D is invertible) And, of course, if we have the equation [itex]A = C^{-1} D C[/itex] then we can infer [itex]A^n = C^{-1} D^n C[/itex]
 

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