sigma08-049. 261KB Jun 04 2011 12:10:16 AM

Symmetry, Integrability and Geometry: Methods and Applications

SIGMA 4 (2008), 049, 13 pages

Free Field Approach to Solutions
of the Quantum Knizhnik–Zamolodchikov Equations
Kazunori KUROKI



and Atsushi NAKAYASHIKI





Department of Mathematics, Kyushu University, Hakozaki 6-10-1, Fukuoka 812-8581, Japan
E-mail: ma306012@math.kyushu-u.ac.jp




Department of Mathematics, Kyushu University, Ropponmatsu 4-2-1, Fukuoka 810-8560, Japan
E-mail: 6vertex@math.kyushu-u.ac.jp

Received February 18, 2008, in final form May 27, 2008; Published online June 03, 2008
Original article is available at http://www.emis.de/journals/SIGMA/2008/049/
Abstract. Solutions of the qKZ equation associated with the quantum affine algebra
b 2 ) and its two dimensional evaluation representation are studied. The integral formulae
Uq (sl
derived from the free field realization of intertwining operators of q-Wakimoto modules are
shown to coincide with those of Tarasov and Varchenko.
Key words: free field; vertex operator; qKZ equation; q-Wakimoto module
2000 Mathematics Subject Classification: 81R50; 20G42; 17B69

1

Introduction

In 1992 I. Frenkel and N. Reshetikhin [7] had developed the theory of intertwining operators
for quantum affine algebras and had shown that the matrix elements of intertwiners satisfy the
quantized Knizhnik–Zamolodchikov (qKZ) equations.

The theory of intertwiners and qKZ equations was successfully applied to the study of solvable
lattice models [9] (and references therein). As to the study of solutions of the qKZ equations,
bases are constructed by Tarasov and Varchenko [22] in the form of multi-dimensional hyperb 2 ). However solutions of the qKZ equations for other
geometric integrals in the case of Uq (sl
quantum affine algebras are not well studied [21].
The method of free fields is effective to compute correlation functions in conformal field theory (CFT) [3], in particular, solutions to the Knizhnik–Zamolodchikov (KZ) equations [18, 19].
A similar role is expected for those of quantum affine algebras. Unfortunately it is difficult to
say that this expectation is well realized, as we shall explain below.
Free field realizations of quantum affine algebras are constructed by Frenkel and Jing [6]
for level one integrable representations of ADE type algebras and by Matsuo [15], Shiraishi [16]
b 2 ). The latter results are
and Abada et al. [1] for representations with arbitrary level of Uq (sl
b
extended to Uq (slN ) in [2]. Free field realizations of intertwiners are constructed based on these
b 2 ) [9, 10, 12, 15, 4].
representations in the case of Uq (sl
The simplicity of the Frenkel–Jing realizations makes it possible not only to compute matrix
elements but also traces of intertwining operators [9], which are special solutions to the qKZ
equations. The case of q-Wakimoto modules with an arbitrary level becomes more complex and
the detailed study of the solutions of the qKZ equations making use of it is not well developed.

In [15] Matsuo derived his integral formulae [14] from the formulae obtained by the free field
calculation in the simplest case of one integration variable. However it is not known in general
whether the integral formulae derived from the free field realizations recover those of [14, 23, 22]1 .
1

See Note 1 in the end of the paper.

2

K. Kuroki and A. Nakayashiki

The aim of this paper is to study this problem in the case of the qKZ equation with the value
b 2 ). More general
in the tensor product of two dimensional irreducible representations of Uq (sl
cases will be studied in a subsequent paper.
There are mainly two reasons why the comparison of two formulae is difficult. One is that
the formulae derived from the free field calculations contain more integration variables than
in Tarasov–Varchenko’s (TV) formulae. This means that one has to carry out some integrals
explicitly to compare two formulae. The second reason is that the formulae from free fields
contains a certain sum. This stems from the fact that the current and screening operators are

written as a sum which is absent in the non-quantum case. Since TV formulae have a similar
structure to those for the solutions of the KZ equation [18, 19], one needs to sum up certain
terms explicitly for the comparison of two formulae. We carry out such calculations in the case
we mentioned.
The plan of this paper is as follows. In Section 2 the construction of the hypergeometric solutions of the qKZ equation due to Tarasov and Varchenko is reviewed. The free field construction
of intertwining operators is reviewed in Section 3. In Section 4 the formulae for the highest to
highest matrix elements of some operators are calculated. The main theorem is also stated in
this section. The transformation of the formulae from free fields to Tarasov–Varchenko’s formulae is described in Section 5. In Section 6 the proof of the main theorem is given. Remaining
problems are discussed in Section 7. The appendix contains the list of the operator product
expansions which is necessary to derive the integral formula.

2

Tarasov–Varchenko’s formula

Let V (1) = Cv0 ⊕ Cv1 be a two-dimensional irreducible representation of the algebra Uq (sl2 ),
and R(z) ∈ End(V (1)⊗2 ) be a trigonometric quantum R-matrix given by
R(z) (vǫ ⊗ vǫ ) = vǫ ⊗ vǫ ,
1−z
qv0 ⊗ v1 +

1 − q2z
1 − q2
R(z) (v1 ⊗ v0 ) =
zv0 ⊗ v1 +
1 − q2z

R(z) (v0 ⊗ v1 ) =

1 − q2
v1 ⊗ v0 ,
1 − q2z
1−z
qv1 ⊗ v0 ,
1 − q2z

Let p be a complex number such that |p| < 1 and Tj denote the multiplicative p-shift operator
of zj ,
Tj f (z1 , . . . , zn ) = f (z1 , . . . , pzj , . . . , zn ).
The qKZ equation for the V (1)⊗n -valued function Ψ(z1 , . . . , zn ) is
Tj Ψ = Rj,j−1 (pzj /zj−1 ) · · · Rj,1 (pzj /z1 )κ


1−hj
2

Rj,n (zj /zn ) · · · Rj,j+1 (zj /zj+1 )Ψ,

(1)

where Rij (z) signifies that R(z) acts on the i-th and j-th components, κ is a complex parameter,
κ

1−hj
2

acts on the j-th component as
κ

1−hj
2


vǫ = κǫ vǫ .

Let us briefly recall the construction of the hypergeometric solutions [22, 20] of the equation (1).
In the remaining part of the paper we assume |q| < 1. We set
(z)∞ = (z; p)∞ ,

(z; p)∞ =


Y

j=0

(1 − pj z),

θ(z) = (z)∞ (pz −1 )∞ (p)∞ .

Free Field Approach to Solutions of the Quantum Knizhnik–Zamolodchikov Equations

3


Let n and l be non-negative integers satisfying l ≤ n. For a sequence (ǫ) = (ǫ1 , . . . , ǫn ) ∈
{0, 1}n satisfying ♯{i|ǫi = 1} = l let


l
−1 t − z Y q −2 t − t
Y
X
Y
Y ta − tb
q
t
a
a
ai
j
ai
i
j

,

w(ǫ) (t, z) =
q −2 ta − tb 1≤a ,...,a ≤l
tai − q −1 zki
tai − q −1 zj
t a i − ta j
a

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