# Show that a group of operators generates a Lie algebra

1. Jun 27, 2010

### russel

Hello there! Above is a problem that has to do with Lie Theory. Here it is:
The operators $P_{i},J,T (i,j=1,2)$ satisfy the following permutation relations:
$[J,P_{i}]= \epsilon_{ij}P_{ij},[P_{i},P_{j}]= \epsilon_{ij}T, [J,T]=[P_{i},T]=0$
Show that these operators generate a Lie algebra. Is that algebra a semisimple one? Also show that
$e^{uJ}P_{i}e^{-uJ}=P_{i} \cos{u}+ \epsilon_{ij}P_{j} \sin{u}$

Does anyone know how to deal with it?

Last edited: Jun 27, 2010
2. Jun 27, 2010

### Martin Rattigan

Changing "MATH" to "itex" will probably generate more response.

3. Jun 27, 2010

### russel

ok! I fixed it! I didn't know the exact code, sorry!

4. Jun 27, 2010

### Office_Shredder

Staff Emeritus
You just have to prove the three things:
1) The bracket is bilinear
2) The bracket is anti-commutative
3) The bracket satisfies the Jacobi identity.

Since you're given a proposed list of generators, the bracket is defined to be the bilinear operator satisfying the properties that you're given, so 1 is already done.

Let's say I'm given two arbitrary elements A and B. Do you see how you could prove that in general, [A,B]=-[B,A] using only what you know about the Lie bracket on the generators?

5. Jun 27, 2010

### russel

I think I got it! Thanks a lot! What about the last equation? How do I prove it?

6. Jul 2, 2010

### Simon_Tyler

The last one is the group adjoint action. Try taking the derivative of the LHS wrt u.
This should show you how to rewrite the group adjoint as an "exponentiated commutator".
I'll let you do the rest.

Also have you proved that it's semi-simple yet?

Last edited: Jul 2, 2010
7. Jul 7, 2010

### russel

I'll haven't had enough time, so I didn't really managed to solve it. As far as I know, a lie algebra is a semi-simple one if it is a direct sum of simple lie algebras. I will follow this path? Or should I think of a different way to prove it?

8. Jul 7, 2010

### Office_Shredder

Staff Emeritus
Also, a semi-simple Lie algebra L satisfies [L,L]=L.

9. Jul 7, 2010

### russel

Oh, I see. So, I'll choose every element and see if this condition is satisfied.

10. Jul 7, 2010

### tmccullough

The $L$ Office Shredder is referring to is the whole algebra, not a single element. The Lie bracket of an element with itself is 0, by definition.

Look at the presentation for the algebra. Then, consider $$L,L$.$ Can you get each generator from the Lie bracket of other elements?

11. Aug 1, 2010

### vigvig

To prove that you have a lie algebra, you wave to show that your bracket is a bilinear operator, and also that the Jacobi identity holds: i.e given any arbitrary 3 operators A,B,C,
[[A,B],C]+[[B,C],A]+[[C,A],B]= 0. That would involve simple computations.
As for showing that it is semi-simple, this might actually involve some more work since you have to show that the nilradical of the group is zero.
Vignon S. Oussa

Last edited: Aug 1, 2010
12. Aug 21, 2010

### maxverywell

How do we do that in the case with 4 operators (let's say A,B,C and D) ?

[[A,B],C]+[[B,C],D]+[[C,D],A]+[[D,A],B]= 0

Is it right?

13. Aug 21, 2010

### maxverywell

Oops, it's wrong and I think I figure it out. It's

[[A,B],C]+[[B,C],A]+[[C,A],B]= 0

[[A,B],D]+[[B,D],A]+[[D,A],B]= 0

[[A,C],D]+[[C,D],A]+[[D,A],C]= 0

[[B,C],D]+[[C,D],B]+[[D,B],C]= 0

So now we have 4 Jacobi identities which must be satisfied to form a Lie algebra.

In case of N operators (generators of Lie group) there are $\binom{N}{3}$ Jacobi identities.

Am I right? Please let me know.

Last edited: Aug 21, 2010
14. Aug 21, 2010

### Office_Shredder

Staff Emeritus
That is correct

15. Aug 21, 2010

### maxverywell

Thnx! So there are a lot of work if we have many generators...

16. Sep 5, 2010

### maxverywell

Is there any other way of doing this? If we have 10 generators than there are 120 Jacobi identities that we need to prove...

17. Sep 5, 2010

### Fredrik

Staff Emeritus
Just an idea...I haven't thought about the details. How about finding a Lie group that has a Lie algebra that satisfies the other requirements? Then the Jacobi identity will be automatic.

18. Sep 5, 2010

### maxverywell

Are you saying that if the following two properties are satisfied than the third is sutisfied automaticaly?

19. Sep 5, 2010

### maxverywell

Ok let me show you one example:

I want to prove that the algebra of octanions with generators $$X_a$$, $$a=1,...,7$$ that sutisfy the following relation: $$X_a X_b=\epsilon_{abc}X_c$$ is Lie algebra.

My proof is the following:

1) $$[X_a, X_b]=X_a X_b-X_b X_a=2\epsilon_{abc}X_c$$

2) $$\epsilon_{abc}=-\epsilon_{bac}$$

3) $$[[X_a, X_b],X_c]=[X_a X_b, X_c]-[X_b X_a, X_c]=[X_a, X_c]X_b+X_a [X_b,X_c]-[X_b, X_c]X_a-X_b[X_a, X_c]=X_a X_c X_b - X_c X_a X_b + ...=0$$

because $$X_i X_j X_k = 0$$ for $$i\neq j \neq k$$

Is it correct?

20. Sep 6, 2010

### Fredrik

Staff Emeritus
No, that would be a false claim. I'm thinking along these lines: Suppose that they're all satisfied. Then the bracket defines a Lie algebra, and we can try to find a group that has that Lie algebra as the tangent space at the identity element. If you find such a group, the claim you're trying to prove must be true.

I haven't tried to solve your problem with this method or any other. It's just the first idea that occurred to me.