Affine sl(2)

Gannon 190p, 193p, 196p,371p

 

 

construction

• this is borrowed from affine Kac-Moody algebra entry
• Let \(\mathfrak{g}\) be a semisimple Lie algebra with root system \(\Phi\) and the invariant form \(<\cdot,\cdot>\)
• say \(\mathfrak{g}=A_1\),  \(\Phi=\{\alpha\}\)
• Cartan matrix
  \(\mathbf{A} = \begin{pmatrix} 2 \end{pmatrix}\)
  
• Find the highest root 
  
  • \(\alpha\)
    
• Add another simple root \(\alpha_0\) to the root system \(\Phi\)
  
  • \(\alpha_0=-\alpha\)
    
• Construct a new Cartan matrix
  \(A' = \begin{pmatrix} 2 & -2 \\ -2 & 2 \end{pmatrix}\)
  
• Note that this matrix has rank 1 since \((1,1)\) belongs to the null space
  
• construct a Lie algebra from the new Cartan matrix \(A'\)
  
• Add a outer derivation\(d=-l_0\) to compensate the degeneracy of the Cartan matrix
  \(\begin{pmatrix} 2 & -2 & 1\\ -2 & 2 &0 \\ 1 &0 & 0 \end{pmatrix}\)
  

 

 

basic quantities

• a_i=1
  
• c_i=a_i^{\vee}=1
  
• a_{ij}
  
• dual Coxeter number
  
• Weyl vector
  

 

 

root systems

• \(\Phi=\{\alpha+n\delta|\alpha\in\Phi^{0},n\in\mathbb{Z}\}\cup \{n\delta|n\in\mathbb{Z},n\neq 0\}\)
• real roots
  
  • \(\{\alpha+n\delta|\alpha\in\Phi^{0},n\in\mathbb{Z}\}\)
    
• imaginary roots   
  
  • \(\{n\delta|n\in\mathbb{Z},n\neq 0\}\)
• simple roots
  
  •  
    \(\alpha_0,\alpha_1\)
    
• positive roots      
  
• \(\Phi^{+}=\{\alpha+n\delta|\alpha\in\Phi^{0},n>0\}\cup (\Phi^{0})^{+}\cup \{n\delta|n\in\mathbb{Z},n> 0\}\)
  
•  
  

 

 

fixing a Cartan subalgebra and its dual

• basis of the Cartan subalgebra H
  \(h_0=C-h_1\)
  \(h_1\)
  \(d=-l_0\)
  
• dual basis for H
  \(\omega_0,\omega_1,\delta\)
  

 

 

killing form

• invariant symmetric non-deg bilinear forms
  \(<h_i,h_j>=A_{ij}\)
  \(<h_0,d>=1\)
  \(<h_1,d>=0\)
  \(<d,d>=0\)
  

 

 

level k highest weight representation

• integrable highest weight
  \(\lambda=\lambda_{0}\omega_0+\lambda_{1}\omega_1\), \(\lambda_{i}\in\mathbb{N}\)
  
• level
  \(k=\lambda_{0}+\lambda_{1}\)
  
• therefore \(\lambda_{0}\in\{0,1,\cdots,k\}\)