What Is the Adjoint Representation of SU(2)?

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Homework Help Overview

The discussion revolves around the adjoint representation of the special unitary group SU(2), specifically focusing on the relationship between the adjoint and coadjoint actions, as well as the computation of matrix exponentials related to these representations.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • The original poster attempts to understand the connection between the adjoint representation and matrix exponentiation, questioning the choice of matrices g and g-1 in the context of Ad(X). Other participants suggest starting with simpler matrices to avoid complications and share insights from previous work on SU(2) representations.

Discussion Status

Participants are actively engaging with the problem, sharing resources and insights. Some guidance has been offered regarding the complexity of matrix exponentiation and the potential for mistakes with larger matrices. There is an acknowledgment of the need for clarity in the calculations being performed.

Contextual Notes

There is mention of specific resources and prior articles that may contain relevant information, as well as the challenges posed by the calculations involving matrix exponentials of different sizes.

nigelscott
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Homework Statement


[/B]
I am looking at this document. http://www.math.columbia.edu/~woit/notes3.pdf

Homework Equations


[/B]
ad(x)y = [x,y]

Ad(X) = gXg-1

The Attempt at a Solution


[/B]
I understand how ad(S1) and X is found but I don't understand what g and g-1 to use to find Ad(X). Also I need to show that ad and Ad are connected via matrix exponentiation.
 
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To be honest, I don't really want to read four pages to answer a possibly simple question, the more as I've already written two insight articles, which deal with ##SU(2)## representations. I worked out quite a few explicit formulas for expressions in certain bases. So maybe you'll find there what you are looking for (you can skip the first two sections).

https://www.physicsforums.com/insights/journey-manifold-su2mathbbc-part/
https://www.physicsforums.com/insights/journey-manifold-su2-part-ii/
 
Thanks. I worked through your paper and understand how you got to part 2 (12) and (13). The part 1 am stuck on now is solving Ad(expX) = exp(ad(X)) for the matrices you have calculated. When I try to calculate the matrix exponential I get strange results. Any pointers you could give would be of great help. Thanks again.
 
nigelscott said:
When I try to calculate the matrix exponential I get strange results. Any pointers you could give would be of great help.
It's not really funny to calculate the exponentials of ##(3\times 3)-##matrices and easy to make mistakes. I guess, that's why I haven't really seen it anywhere. I would start with ##(2\times 2)-##matrices and the Lie group of matrices of the form ##\begin{bmatrix}a&b\\0&0\end{bmatrix}## with ##a \neq 0## and the Lie algebra ##[X,Y]=Y## which is easier than with simple Lie groups which have entries on both sides of the diagonal. It's also the reason why I used the differentiation of curves in the Lie group to show the connection to its Lie algebra. Exponentiation is basically the way back, an integration.
 

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