Talk on String functions and quantum affine algebras
abstract
The character of an irreducible representation with dominant integral highest weight of an affine Lie algebra can be written as a linear combination of theta functions, with coefficients given by string functions which are modular forms. There are still many aspects of string functions that are not well-understood. In this talk I will review the basic properties of them, and explain certain connections with finite-dimensional representations of quantum affine algebras.
key message
- string functions know about Kirillov-Reshetikhin modules
review of affine Lie algebras and their integrable representations
affine Lie algebras
- Affine Kac-Moody algebra
- Let $\overline{\mathfrak{g}}$ be a complex simple Lie algebra of rank $r$
- $(a_{ij})_{i,j\in \overline{I}}$ Cartan matrix, $\overline{I}=\{1,\cdots, r\}$ index set
- \((a_{ij})_{i,j\in I}\) : extended Cartan matrix $I=\{0\}\cup \overline{I}$
- untwisted affine Kac-Moody algebra $\tilde{\mathfrak{g}}$ : Lie algebra with generators \(e_i,h_i,f_i , (i=0,1,2,\cdots, r)\) and relations
- \(\left[h,h'\right]=0\)
- \(\left[e_i,f_j\right]=\delta _{i,j}h_i\)
- \(\left[h,e_j\right]=\alpha_{j}(h)e_j\)
- \(\left[h,f_j\right]=-\alpha_{j}(h)f_j\)
- \(\left(\text{ad} e_i\right){}^{1-a_{i,j}}\left(e_j\right)=0\) (\(i\neq j\))
- \(\left(\text{ad} f_i\right){}^{1-a_{i,j}}\left(f_j\right)=0\) (\(i\neq j\))
- basis of the Cartan subalgebra $\mathfrak{h}$; \(h_0,h_ 1,\cdots,h_r,d\)
- dual basis for $\mathfrak{h}^{*}$; \(\Lambda_0,\Lambda_1,\cdots,\Lambda_r,\delta\)
- we call \(\Lambda_0,\Lambda_1,\cdots,\Lambda_r\) the fundamental weights and \(\delta\) the imaginary root
- simple roots \(\alpha_0,\alpha_1,\cdots,\alpha_r\)
- distinguished elements
- central element \(c=\sum_{i=0}^{r}a_i^{\vee}h _i\)
- imaginary root \(\delta=\sum_{i=0}^{r}a_i\alpha_i\)
- Weyl vector \(\rho=\sum_{i=0}^{r}\Lambda_i\)
- normalize the bilinear form $(\cdot|\cdot)$ on $\mathfrak{h}^{*}$ so that $(\theta|\theta)=2$
- let $Q=\sum_{i=1}^{r}\Z\, \alpha_i\subseteq \mathfrak{h}^{*}$ (root lattice of $\overline{\mathfrak{g}}$)
- define $M\subseteq Q$ : $M=\{\sum_{i=1}^{r}\Z\, \alpha_i^{\vee}\}$ where $\alpha_i^{\vee}=t_i\alpha_i$ where $t_i=\frac{2}{(\alpha_i|\alpha_i)}$
affine Weyl group
- Affine Weyl group
- The affine Weyl group $W$ is generated by $s_0,s_1,\cdots, s_r\in \operatorname{Aut}\,\mathfrak{h}^{*}$ defined by
$$s_{i}\lambda = \lambda -\lambda(h_i)\alpha_i$$ for $i=0,1, \cdots, r$.
- for $\gamma\in \mathfrak{h}^{*}$, define $t_{\gamma} : \mathfrak{h}^{*}\to \mathfrak{h}^{*}$ by
$$t_{\gamma}(\lambda)=\lambda+\lambda(c)\gamma-\left(\frac{1}{2}\lambda(c)|\gamma|^2+(\gamma|\lambda)\right)\delta $$
- thm
Let $T=\{t_{\gamma}|\gamma\in M\}$. Then $W=\overline{W} \ltimes T$
integrable representations and characters
- Unitary representations of affine Kac-Moody algebras
- for each $\lambda\in \mathfrak{h}^{*}$, we get an irreducible $\tilde{\mathfrak{g}}$-module $L(\lambda)$ (which is a quotient of the Verma module)
- character of an irreducible highest weight representation \(L(\lambda)\)
$$\operatorname{ch} L(\lambda):=\sum_{\beta\in \mathfrak{h}^{*}}\operatorname{mult}_{\lambda}(\beta) e^{\beta}$$
- dominant integral weights $\lambda(\mathfrak{h}_i)\in \mathbb{Z}_{\geq 0},\, i=0,1,\cdots,r$
- weight lattice
\[ P_{+}=\{\lambda\in \mathfrak{h}^{*}|\lambda=\sum_{i=0}^{l}\lambda_{i}\Lambda_i+\xi \delta, \lambda_i \in\mathbb{Z}_{\geq 0},\xi \in \mathbb{C}\} \]
- A $\tilde{\mathfrak{g}}$-module $V$ is called integrable if $V=\oplus_{\lambda\in \mathfrak{h}^{*}}V_{\lambda}$ and if $e_i : V\to V$ and $f_i : V\to V$ are locally nilpotent for all $i=0,1,\cdots, r$
- thm
If $\lambda\in P_{+}$, then $L(\lambda)$ is an integrable representation.
- thm (Kac)
Let $\lambda\in P_{+}$. Then $$ \operatorname{ch} L(\lambda)=\frac{\sum_{w\in W} (-1)^{\ell(w)}w(e^{\lambda+\rho})}{\sum_{w\in W} (-1)^{\ell(w)}w(e^{\rho})} $$
- see Weyl-Kac character formula
- we call $k=\lambda(c)$ the level of $L(\lambda)$ and have \(k=\sum_{i=0}^{l}a_{i}^{\vee}\lambda_{i}\in \mathbb{Z}\)
Freudenthal multiplicity formula
- thm
$$ (|\Lambda+\rho|^2-|\lambda+\rho|^2)m_{\lambda}=2\sum_{\alpha\in \Delta_{+}}\sum_{j\geq 1}(\operatorname{mult} \alpha)(\lambda+j\alpha|\alpha)m_{\lambda+j\alpha} $$
theta functions
- Theta functions in Kac-Moody algebras
- for each $\lambda\in P^k$, define the theta function as
$$ \Theta_{\lambda}= e^{-\frac{|\lambda|^2\delta}{2k}}\sum_{\gamma \in M}e^{t_{\gamma}(\lambda)}=e^{k\Lambda_0}\sum_{\gamma\in M+\overline{\lambda}/k}e^{-\frac{1}{2}k|\gamma|^2 \delta + k \gamma} $$
string functions
- String functions and branching functions
- $\Lambda\in P_{+}^{k}$
- A weight $\mu$ of $L(\Lambda)$ is maximal if $\mu+\delta$ is not a weight
- for each $\mu$, there exists a unique integer $n\geq 0$ such that $\mu+n\delta$ is maximal
- define $m_{\Lambda}=\frac{(\Lambda+\rho)^2}{2(k+h^{\vee})}-\frac{\rho^2}{2h^{\vee}}$ and $m_{\Lambda,\lambda}=m_{\Lambda}-\frac{\lambda^2}{2k}$
- note that $m_{\Lambda}=h_{\Lambda}-\frac{c_{\Lambda}}{24}+\xi$ where $h_{\Lambda}=\frac{(\bar{\Lambda}+2\bar{\rho}|\bar{\Lambda})}{2(k+h^{\vee})}$ and $c_{\Lambda}=\frac{k}{k+h^{\vee}}\dim \mathfrak{\overline{g}}$
- the set $\max(\Lambda)$ of maximal weights is stable under $W$
- def
For each $\lambda\in \mathfrak{h}^{*}$, the string function $c_{\lambda }^{\Lambda}$ is defined by $$ c_{\lambda }^{\Lambda}=e^{-m_{\Lambda,\lambda}\delta}\sum_{n=-\infty}^{\infty}\operatorname{mult}_{\Lambda}(\lambda-n\delta)e^{-n \delta} $$
- an explicit expression for the string functions is not known in general
- the few that are known were guessed using the modular transformations
- modular form of weight $-r/2$
properties
- $c_{\lambda }^{\Lambda}=c_{w\lambda }^{\Lambda}$ for $w\in W$
- thm
We have $$ e^{-m_{\Lambda}\delta}\operatorname{ch} L(\Lambda)=\sum_{\lambda}c^{\Lambda}_{\lambda }\Theta_{\lambda} $$
- proof
$$ \begin{aligned} \operatorname{ch} L(\Lambda)&=\sum_{\lambda\in \max{L(\Lambda)}}\sum_{n=0}^{\infty}\operatorname{mult}_{\Lambda}(\lambda-n\delta)e^{\lambda-n \delta}\\ &=\sum_{\lambda\in \max{L(\Lambda)} \mod kQ^{\vee}} \left(\sum_{\gamma\in M}e^{t_{\gamma}(\lambda)}\right)\sum_{n=0}^{\infty}\left(\operatorname{mult}_{\Lambda}(\lambda-n\delta)e^{-n\delta}\right)\\ &=\sum_{\lambda\in \max{L(\Lambda)} \mod kQ^{\vee}} e^{m_{\Lambda,\lambda}\delta}e^{-m_{\Lambda,\lambda}\delta}\left(\sum_{\gamma\in M}e^{t_{\gamma}(\lambda)}\right)\sum_{n=0}^{\infty}\left(\operatorname{mult}_{\Lambda}(\lambda-n\delta)e^{-n\delta}\right)\\ &=\sum_{\lambda\in \max{L(\Lambda)} \mod kQ^{\vee}} e^{(m_{\Lambda}-\frac{\lambda^2}{2k})\delta}e^{-m_{\Lambda,\lambda}\delta}\left(\sum_{\gamma\in M}e^{t_{\gamma}(\lambda)}\right)\sum_{n=0}^{\infty}\left(\operatorname{mult}_{\Lambda}(\lambda-n\delta)e^{-n\delta}\right)\\ &=\sum_{\lambda\in \max{L(\Lambda)} \mod kQ^{\vee}} e^{m_{\Lambda}\delta}c^{\Lambda}_{\lambda}\Theta_{\lambda} \end{aligned} $$ ■
modular transformations
- thm
We have $$ c_{\lambda }^{\Lambda}(-\frac{1}{\tau})=(\frac{\tau}{i})^{-r/2}\sum_{(\Lambda',\lambda')}b(\Lambda,\lambda,\Lambda',\lambda')c_{\lambda'}^{\Lambda'}(\tau) $$ where $$ b(\Lambda,\lambda,\Lambda',\lambda')=(*)\exp(\frac{2\pi i(\lambda|\lambda')}{k}) \sum_{w\in \overline{W}} (-1)^{\ell(w)}\exp \left(-{\frac{2\pi i ( w(\Lambda+\rho)|\Lambda'+\rho)}{k+h^{\vee}}}\right) $$ and the sum is over all $\Lambda'\in P_{+}^k$ and $\lambda' \in P^k \mod kM+\mathbb{C}\delta$
asymptotic growth of coefficients
- use the circle method
- thm (Kac-Peterson)
Let $\Lambda\in P_{k}^{+},\, \lambda\in \max(\Lambda)$. As $n\to \infty$, $$ \operatorname{mult}_{\Lambda}(\lambda-n\delta)\sim (\text{const})\times n^{-(1/4)(r+3)}e^{4\pi (a n)^{1/2}} $$ where $a=$
Rogers-Ramanujan identities for string functions
- Fermionic formula for string functions and parafermion characters
- now we denote the level by $\ell\in \mathbb{Z}$ and assume $\ell\geq 2$
- $H_\ell=\{(a,m)|a=1,\cdots, r, 1\leq m \leq t_a \ell-1\}$, $t_a=2/(\alpha_a|\alpha_a)$
- let
$$ K^{m n}_{a b} = \Bigl(\hbox{min}(t_bm, t_an) - {m n\over \ell}\Bigr) (\alpha_a \vert \alpha_b) $$
- conjecture [KNS93]
We have \begin{equation}\label{qkns} c^{\ell\Lambda_0}_\lambda(q)\cdot \eta(\tau)^r= \sum_{\{N^{(a)}_m\}}\frac{q^{\frac{1}{2}\sum_{(a,m), (b,n) \in H_\ell} K^{mn}_{ab}N^{(a)}_mN^{(b)}_n}} {\prod_{(a,m) \in H_\ell}(1-q)(1-q^2)\cdots (1-q^{N^{(a)}_m})} \end{equation} up to a rational power of $q$. The outer sum is over $N^{(a)}_m \in \Z_{\ge 0}$ such that $$\sum_{(a,m) \in H_\ell}mN^{(a)}_m\alpha_a \equiv \overline{\lambda} \mod \ell M.$$
example
- let $\mathfrak{g}=A_1$
- consider the vacuum representation of level $\ell$
- thm [Lepowski-Primc 1985]
$$ c^{\ell\Lambda_0}_{\ell\Lambda_0}(\tau)\cdot \eta(\tau)=\sum_{(N_1,\dots,N_{\ell-1})\in \mathbb{Z}_{\geq 0}^{\ell-1}}\frac{q^{\sum_{n,m=1}^{\ell-1} N_n N_m (\min (n,m) -\frac{nm}{\ell})}} {\prod_{m=1}^{\ell-1}(1-q)\cdots(1-q^{N_m})} $$ where the sum is under the constraint $ \sum_{m=1}^{\ell-1} m N_m \equiv 0 \ \mathrm{mod}\ \ell$.
- the associated matrix is $2\otimes \mathcal{C}(A_{\ell-1})^{-1}$