"Affine Kac-Moody algebras as central extensions of loop algebras"의 두 판 사이의 차이
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imported>Pythagoras0 (새 문서: ==introduction== * Construct the loop algebra from a finite dimensional Lie algebra * Make a central extension * Add a outer derivation to compensate the degeneracy of the Cartan matr...) |
Pythagoras0 (토론 | 기여) |
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(사용자 2명의 중간 판 9개는 보이지 않습니다) | |||
3번째 줄: | 3번째 줄: | ||
* Make a central extension | * Make a central extension | ||
* Add a outer derivation to compensate the degeneracy of the Cartan matrix | * Add a outer derivation to compensate the degeneracy of the Cartan matrix | ||
+ | |||
+ | |||
+ | ==2-cocycle of loop algebra== | ||
+ | * <math>L\mathfrak{g}</math> : loop algebra | ||
+ | * <math>c(f,g) = \operatorname{Res}_0 \langle f dg \rangle</math> Here, <math>\langle \cdot \rangle : \mathfrak{g}\otimes \mathfrak{g}\to \mathbb{C}</math> denotes some invariant bilinear form on <math>\mathfrak{g}</math>, and <math>f dg</math> is the <math>\mathfrak{g}\otimes \mathfrak{g}</math>-valued differential given by multiplying <math>f</math> and <math>dg</math> | ||
+ | * in other words, | ||
+ | :<math> | ||
+ | c(\gamma_1,\gamma_2) = \int \langle \gamma_1(\theta), \gamma'_2(\theta)\rangle d\theta | ||
+ | </math> | ||
+ | |||
+ | |||
+ | ==derivarion== | ||
+ | * adding <math>d</math> gives <math>\hat{\mathfrak{g}}</math> a <math>\mathbb{Z}</math>-grading | ||
+ | * it makes the each root space finite-dimensional | ||
==explicit construction== | ==explicit construction== | ||
− | * start with a semisimple Lie algebra | + | * start with a semisimple Lie algebra <math>\mathfrak{g}</math> with invariant form <math>\langle \cdot,\cdot\rangle </math> |
* make a vector space from it | * make a vector space from it | ||
* construct the loop algbera | * construct the loop algbera | ||
− | + | :<math>\hat{\mathfrak{g}}=\mathfrak{g}\otimes\mathbb{C}[t,t^{-1}]</math> | |
− | + | :<math>\alpha(m)=\alpha\otimes t^m</math> | |
* Add a central element to get a central extension and give a bracket | * Add a central element to get a central extension and give a bracket | ||
− | + | :<math>\mathfrak{g}\otimes\mathbb{C}[t,t^{-1}]\oplus\mathbb{C}c</math> | |
− | + | :<math>[\alpha(m),\beta(n)]=[\alpha,\beta]\otimes t^{m+n}+m\delta_{m,-n}\langle \alpha,\beta\rangle c</math> | |
− | + | :<math>[c,x] =0, x\in \hat{\mathfrak{g}}</math> | |
− | * add a derivation | + | * add a derivation <math>d=t\frac{d}{dt}</math> to get |
− | + | :<math>\tilde{\mathfrak{g}}=\mathfrak{g}\otimes\mathbb{C}[t,t^{-1}]\oplus\mathbb{C}c \oplus\mathbb{C}d</math> | |
* define a Lie bracket | * define a Lie bracket | ||
− | + | :<math>[d,x]:=d(x)</math> | |
− | where | + | where <math>d(\alpha(n))=n\alpha(n), d(c)=0</math> |
+ | |||
+ | |||
+ | ==Chevalley generators== | ||
+ | * simple Lie algebra <math>\mathfrak{g}</math> | ||
+ | * l : rank of <math>\mathfrak{g}</math> | ||
+ | * <math>(a_{ij})</math> : extended Cartan matrix | ||
+ | * <math>\theta</math> : highest root | ||
+ | * generators <math>e_i,h_i,f_i , (i=0,1,2,\cdots, l)</math> | ||
+ | * Serre relations | ||
+ | ** <math>\left[h,h'\right]=0</math> | ||
+ | ** <math>\left[e_i,f_j\right]=\delta _{i,j}h_i</math> | ||
+ | ** <math>\left[h,e_j\right]=\alpha_{j}(h)e_j</math> | ||
+ | ** <math>\left[h,f_j\right]=-\alpha_{j}(h)f_j</math> | ||
+ | ** <math>\left(\text{ad} e_i\right){}^{1-a_{i,j}}\left(e_j\right)=0</math> (<math>i\neq j</math>) | ||
+ | ** <math>\left(\text{ad} f_i\right){}^{1-a_{i,j}}\left(f_j\right)=0</math> (<math>i\neq j</math>) | ||
+ | ===isomorphism=== | ||
+ | * <math>e_0=f_{\theta}\otimes x, f_0=e_{\theta}\otimes x^{-1}, h_0=-h_{\theta}\otimes 1+c</math> | ||
+ | * we choose <math>e_{\theta}</math> and <math>f_{\theta}</math> so that | ||
+ | :<math> | ||
+ | (e_{\theta},f_{\theta})=1 | ||
+ | </math> | ||
+ | ==related items== | ||
+ | * [[Affine Kac-Moody algebra]] | ||
+ | * [[Central extension of groups and Lie algebras]] | ||
+ | * [[Heisenberg group and Heisenberg algebra]] | ||
+ | ==expositions== | ||
+ | * http://mathoverflow.net/questions/24845/explicit-cocycle-for-the-central-extension-of-the-algebraic-loop-group-gct | ||
[[분류:Lie theory]] | [[분류:Lie theory]] |
2020년 11월 16일 (월) 11:01 기준 최신판
introduction
- Construct the loop algebra from a finite dimensional Lie algebra
- Make a central extension
- Add a outer derivation to compensate the degeneracy of the Cartan matrix
2-cocycle of loop algebra
- \(L\mathfrak{g}\) : loop algebra
- \(c(f,g) = \operatorname{Res}_0 \langle f dg \rangle\) Here, \(\langle \cdot \rangle : \mathfrak{g}\otimes \mathfrak{g}\to \mathbb{C}\) denotes some invariant bilinear form on \(\mathfrak{g}\), and \(f dg\) is the \(\mathfrak{g}\otimes \mathfrak{g}\)-valued differential given by multiplying \(f\) and \(dg\)
- in other words,
\[ c(\gamma_1,\gamma_2) = \int \langle \gamma_1(\theta), \gamma'_2(\theta)\rangle d\theta \]
derivarion
- adding \(d\) gives \(\hat{\mathfrak{g}}\) a \(\mathbb{Z}\)-grading
- it makes the each root space finite-dimensional
explicit construction
- start with a semisimple Lie algebra \(\mathfrak{g}\) with invariant form \(\langle \cdot,\cdot\rangle \)
- make a vector space from it
- construct the loop algbera
\[\hat{\mathfrak{g}}=\mathfrak{g}\otimes\mathbb{C}[t,t^{-1}]\] \[\alpha(m)=\alpha\otimes t^m\]
- Add a central element to get a central extension and give a bracket
\[\mathfrak{g}\otimes\mathbb{C}[t,t^{-1}]\oplus\mathbb{C}c\] \[[\alpha(m),\beta(n)]=[\alpha,\beta]\otimes t^{m+n}+m\delta_{m,-n}\langle \alpha,\beta\rangle c\] \[[c,x] =0, x\in \hat{\mathfrak{g}}\]
- add a derivation \(d=t\frac{d}{dt}\) to get
\[\tilde{\mathfrak{g}}=\mathfrak{g}\otimes\mathbb{C}[t,t^{-1}]\oplus\mathbb{C}c \oplus\mathbb{C}d\]
- define a Lie bracket
\[[d,x]:=d(x)\] where \(d(\alpha(n))=n\alpha(n), d(c)=0\)
Chevalley generators
- simple Lie algebra \(\mathfrak{g}\)
- l : rank of \(\mathfrak{g}\)
- \((a_{ij})\) : extended Cartan matrix
- \(\theta\) : highest root
- generators \(e_i,h_i,f_i , (i=0,1,2,\cdots, l)\)
- Serre relations
- \(\left[h,h'\right]=0\)
- \(\left[e_i,f_j\right]=\delta _{i,j}h_i\)
- \(\left[h,e_j\right]=\alpha_{j}(h)e_j\)
- \(\left[h,f_j\right]=-\alpha_{j}(h)f_j\)
- \(\left(\text{ad} e_i\right){}^{1-a_{i,j}}\left(e_j\right)=0\) (\(i\neq j\))
- \(\left(\text{ad} f_i\right){}^{1-a_{i,j}}\left(f_j\right)=0\) (\(i\neq j\))
isomorphism
- \(e_0=f_{\theta}\otimes x, f_0=e_{\theta}\otimes x^{-1}, h_0=-h_{\theta}\otimes 1+c\)
- we choose \(e_{\theta}\) and \(f_{\theta}\) so that
\[ (e_{\theta},f_{\theta})=1 \]
- Affine Kac-Moody algebra
- Central extension of groups and Lie algebras
- Heisenberg group and Heisenberg algebra