Why Is (A^{T}A)^{-1}A^{T}=A^{-1}(A^{T})^{-1}A^{T}=A^{-1} Incorrect?

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Homework Help Overview

The discussion revolves around the properties of matrix operations, specifically focusing on the expression \((A^{T}A)^{-1}A^{T}\) and its comparison to \(A^{-1}(A^{T})^{-1}A^{T}\). Participants are exploring the conditions under which these expressions are valid and the implications for solving linear systems.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants are questioning the validity of the expression \((A^{T}A)^{-1}A^{T} = A^{-1}(A^{T})^{-1}A^{T}\) and discussing the context in which these matrix operations are applied, particularly in relation to least squares approximation.

Discussion Status

The discussion is active, with participants clarifying concepts and questioning assumptions about matrix inverses. Some have provided context regarding the application of the expressions in solving inconsistent linear systems, indicating a productive exploration of the topic.

Contextual Notes

There is mention of the least squares approximation and the conditions under which the expressions are used, particularly in the context of inconsistent linear systems.

Yegor
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I know that for matrices A, B and C is correct to write: (AB)C=A(BC)
Also [tex](BA)^{-1}=A^{-1}B^{-1}[/tex]
Why [tex](A^{T}A)^{-1}A^{T}=A^{-1}(A^{T})^{-1}A^{T}=A^{-1}[/tex] is not correct?
 
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who says it is incorrect?
 
[tex](BA)^{-1}=A^{-1}B^{-1}[/tex]

Yes, provided A and B are both invertible matrices...
 
[tex](A^{T}A)^{-1}A^{T}[/tex] such expression comes in chapter about least squares aproximation.
e.g. if we have inconsistent linear system Ax=b, then [tex]x=(A^{T}A)^{-1}A^{T}b[/tex] is best approximation. It is not equal to [tex]x=A^{-1}b[/tex]
 
Oh, yes. Now i see. Thank you very much, Hurky!
 

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