Transpose of orthogonal matrix

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Homework Help Overview

The discussion revolves around the properties of orthogonal matrices, specifically the relationship between the transpose and the inverse of such matrices. The original poster questions why the inverse of an orthogonal matrix Q is given by its transpose Q^T, despite the condition Q^TQ=I not necessarily implying QQ^T=I.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants explore the definitions and properties of orthogonal matrices, questioning the validity of QQ^T=I and its implications. Some express confusion regarding the conditions under which these properties hold.

Discussion Status

There is an ongoing exploration of the properties of orthogonal matrices, with some participants providing insights into specific cases, such as when the matrix has more rows than columns. The discussion remains open, with no explicit consensus reached.

Contextual Notes

Participants are examining the implications of the definitions of orthogonal matrices and the conditions under which certain equalities hold, indicating a need for clarity on these mathematical concepts.

td21
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Homework Statement



Orthogonal matrix means [itex]Q^{T}Q=I[/itex], but not necessary [itex]QQ^{T}=I[/itex], so why can we say the inverse of Q is [itex]Q^{T}[/itex]?

Homework Equations





The Attempt at a Solution


the attempt is actually in my question. It's something i don't understand when doing revision.
 
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addition:
is [itex] QQ^{T}=I[/itex]
100% true?If it is, my problem is solved.
 
td21 said:
addition:
is [itex] QQ^{T}=I[/itex]
100% true?If it is, my problem is solved.

Sure. Q^TQ=I implies QQ^T=I.
 
An interesting variation is when Q has more rows than columns. In this case, QTQ still equals I but QQT doesn't.
 

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