METAPLECTIC ICEBEN BRUBAKER, DANIEL BUMP, GAUTAM CHINTA, SOLOMON FRIEDBERG,
AND PAUL E. GUNNELLS
Abstract. We study spherical Whittaker functions on a metaplectic cover ofGL(r + 1) over a nonarchimedean local field using lattice models from statisticalmechanics. An explicit description of this Whittaker function was given in termsof Gelfand-Tsetlin patterns in [5, 17], and we translate this description into an
expression of the values of the Whittaker function as partition functions of a sixvertex model. Properties of the Whittaker function may then be expressed in termsof the commutativity of row transfer matrices potentially amenable to proof using
the Yang-Baxter equation. We give two examples of this: first, the equivalence of
two different Gelfand-Tsetlin definitions, and second, the effect of the Weyl group
action on the Langlands parameters. The second example is closely connectedwith another construction of the metaplectic Whittaker function by averaging overa Weyl group action [9, 10].

1. Introduction
The study of spherical Whittaker functions of reductive groups over local fields isof fundamental importance in number theory and representation theory. Recently, in
two separate series of papers, the authors and their collaborators have studied Whittaker functions on metaplectic covers of such groups. The goal of this paper is tointroduce a new method for describing such p-adic metaplectic Whittaker functions:
two-dimensional lattice models of statistical mechanics. In such a model, one defines
the partition function to be a weighted sum over states of the model. We showthat there exists a choice of weights for which the partition functions are metaplectic Whittaker functions. Baxter [2] developed important techniques for evaluating
the partition functions of lattice models including the so-called “commutativity oftransfer matrices” and the use of the Yang-Baxter equation. We discuss how thesemethods relate to our descriptions of Whittaker functions and to prior work.
Two different explicit formulas have been given in [9] and [17] for the spherical
Whittaker function on a metaplectic cover of GL(r + 1) over a non-archimedean local
field. The first of these is expressed in terms of a Weyl group action described in [10],1991 Mathematics Subject Classification. Primary: 11F68, Secondary: 11F70, 16T25, 22E50.
Key words and phrases. p-adic Whittaker function, metaplectic group, Gelfand-Tsetlin pattern,six-vertex model, statistical mechanics, Yang-Baxter equation, multiple Dirichlet series.
12

B. BRUBAKER, D. BUMP, G. CHINTA, S. FRIEDBERG, AND P. E. GUNNELLS

the second in terms of a function on Gelfand-Tsetlin patterns initially introducedin [7]. In fact, this latter representation belongs to a family of explicit formulas, one
for each reduced expression of the long element of the Weyl group as a product ofsimple reflections. Two such reduced expressions in type A are particularly nice,and lead to representations of the Whittaker function as sums over Gelfand-Tsetlin
patterns. In keeping with earlier works, we refer to the two different descriptions as“Gamma” and “Delta” rules. The main result of [6] is a combinatorial proof thatthese two definitions are in fact equal. This equality allows one to prove the analytic
properties of an associated global object (a multiple Dirichlet series) by applying
Bochner’s convexity principle.
In the following section we demonstrate that the Gelfand-Tsetlin patterns we areconcerned with are in bijection with admissible states of the six-vertex model having
certain fixed boundary conditions. After recalling the description of the metaplectic
Whittaker function as a function on Gelfand-Tsetlin patterns in Section 3, we use thebijection with the lattice model in Section 4 to express both the Gamma and Delta
descriptions of the Whittaker function as partition functions for certain respective
choices of Boltzmann weights.
In Section 5, we take the connection with statistical models further. We showthat the necessary result for demonstrating the equivalence of the Gamma and Delta
descriptions may be reformulated in terms of the commutativity of transfer matrices.
Baxter [2] advocated the use of the Yang-Baxter equation for demonstrating thiscommutativity. In Section 6, we explain how this is carried out in the context of thesix-vertex model and we speculate about the possibility of such an equation in themetaplectic case.
Finally we discuss the Weyl group action on metaplectic Whittaker functions,
initially established by Kazhdan and Patterson [13], which plays a critical role in
the explicit formulas of [9]. When the degree of the cover is 1, i.e. the linear case,the p-adic spherical Whittaker function is essentially a Schur polynomial by results
going back to Shintani [19]. The Weyl group action is thus closely related to thestandard permutation action on polynomials in r + 1 variables. In [4], this Whittaker
function (or equivalently, the Schur polynomial multiplied by a q-deformation of the
Weyl denominator) is realized as a partition function on a six-vertex model and its
properties are studied via instances of the Yang-Baxter equation. On the other hand,
as soon as the degree of the cover is greater than 1, the action looks rather different(cf. (31)–(33)). Nevertheless, we may ask whether these functional equations may
also be phrased in terms of transfer matrices and a Yang-Baxter equation, and in
this final section, we present evidence towards an affirmative answer.
This work was partially supported by the following grants: NSF grants DMS0844185 (Brubaker), DMS-0652817 and DMS-1001079 (Bump), DMS-0847586 (Chinta),

METAPLECTIC ICE

3

DMS-0652609 and DMS-1001326 (Friedberg), DMS-0801214 (Gunnells) and NSA
grant H98230-10-1-0183 (Friedberg).
2. Six Vertex Model and Gelfand-Tsetlin Patterns
In this section, we demonstrate a bijection between strict Gelfand-Tsetlin patterns
and admissible states of the six vertex model (or “square ice”) on a finite squarelattice with certain fixed boundary conditions. The boundary conditions on ice were
known to Hamel and King, who presented bijections between ice and patterns related
to the symplectic group in [12]. A treatment tailored to the aims of the present paper
was given in [4], whose terminology we now recall.
A Gelfand-Tsetlin pattern of rank r is a triangular array of integers
a0,0a0,1 · · · a0,r−1
a0,r 





a1,1
···
a1,r
(1)
T=
..
.


.
..




ar,r
in which the rows interleave: ai−1,j−1 > ai,j > ai−1,j . The set of all GelfandTsetlin patterns with fixed top row is in bijection with basis vectors of a corresponding highest weight representation of GL(r + 1, C). Indeed, any given top row(a0,0 , a0,1 , . . . , a0,r ) is a partition which may be regarded as a dominant weight of
the GL(r + 1, C) weight lattice. Each successive row of a pattern then records a
branching rule down to a highest weight representation on a subgroup of rank one
less. We will focus mainly on the set of strict Gelfand-Tsetlin patterns, whose entries
in horizontal rows are strictly decreasing. In terms of representation theory, thesepatterns result from branching through strictly dominant highest weights. Top rowsof strict Gelfand-Tsetlin patterns are then indexed by strictly dominant weights λ+ρ
where λ is a dominant weight and ρ is the Weyl vector (r, r − 1, . . . , 0).
Now we come to lattice models. The six-vertex model consists of labelings ofedges in a square grid where each vertex has adjacent edges in one of six admissible
configurations. This model is sometimes referred to as “square ice” where each vertexof the grid represents an oxygen atom and the 6 admissible ways of labeling adjacentedges correspond to the number of ways in which two of the four edges include anearby hydrogen atom. If we represent adjacent hydrogen atoms by incoming arrows,
and locations where there is no adjacent hydrogen atom by outgoing arrows, the six
admissible states are as follows.

4

B. BRUBAKER, D. BUMP, G. CHINTA, S. FRIEDBERG, AND P. E. GUNNELLS

We will use a representation consisting of a lattice whose edges are labeled withsigns + or −, called spins. To relate this to the previous description, interpret aright-pointing or down-pointing arrow as a +, and a left-pointing or up-pointing
arrow as −. We then find the following six configurations. (The index i in the tableindicates the row to which the vertex belongs, and will be used in later sections.)
i

i

i

i

i

i

The rectangular lattices we consider will be finite, with boundary conditions chosen
so that the admissible configurations are in bijection with strict Gelfand-Tsetlin
patterns with fixed rank r and top row λ + ρ as above. Here λ = (λr , . . . , λ1 , λ0 )with λj ≥ λj−1 for all j, and we suppose that λ0 = 0.
Boundary Conditions. The rectangular grid is to have λr + r + 1 columns (labeled
0 through λr + r increasing from right to left) and r + 1 rows. Then with λ + ρ =(λr + r, λr−1 + r − 1, . . . , 0), we place a − spin at the top of each column whose label
is one of the distinct parts of λ + ρ, i.e. at columns labeled λj + j for 0 6 j 6 r. We
place a + spin at the top of each of the remaining columns. Furthermore, we placea + spin at the bottom of every column and on the left-hand side of each row and a− spin on the right-hand side of each row.
For example, put r = 2, and take λ = (3, 2, 0), so that λ + ρ = (5, 3, 0). Then we
have the following boundary conditions for the ice:
5

(2)

4

3

2

1

0

3

3

3

3

3

3

2

2

2

2

2

2

1

1

1

1

1

1

The column labels are written above each column, and row labels have been placednext to each vertex. These row labels will be used in Section 4, but need not concern
us now. The edge spins have been placed inside circles located along the boundary.
The remaining open circles indicate interior spins not determined by our boundary
conditions, though any filling of the grid must use only the six admissible configurations in the above table. Such an admissible filling of the finite lattice having above
boundary conditions will be referred to as a state of ice.

METAPLECTIC ICE

5Proposition 1. Given a fixed rank r and a dominant weight λ = (λr , . . . , λ1 , 0),
there is a bijection between strict Gelfand-Tsetlin patterns with top row λ + ρ andadmissible states of ice having boundary conditions determined by λ as above.
Proof. We begin with a strict Gelfand-Tsetlin pattern. Each row of the GelfandTsetlin pattern will correspond to the set of spins located between numbered rowsof ice, the so-called “vertical spins” since they lie on vertical edges of the grid, as
follows. To each entry ai,j in the Gelfand-Tsetlin pattern, we assign a − to thevertical spin between rows labeled r + 2 − i and r + 1 − i in the column labeled ai,j .
(Recall that we are using decreasing row labels from top to bottom as in the example(2).) The remaining vertical spins are assigned +.
It remains to assign horizontal spins, but these are already uniquely determinedsince the left and right edge horizontal spins have been assigned and each admissible
vertex configuration has an even number of adjacent + spins. We must only verify
that the resulting configuration uses only the 6 admissible configurations (from the 8
having an even number of + signs) for the corresponding ice. This is easily implied by
the interleaving condition on entries in the Gelfand-Tsetlin pattern, which is violated
if one of the two inadmissible configurations appears. See Lemma 2 of [4] for moredetails.

A simple example illustrates the bijection:
5

(3)


 5


3
3

1
3


0 

←→

4

3

2

1

0

3

3

3

3

3

3

2

2

2

2

2

2

1

1

1

1

1

1



3. Metaplectic Whittaker Functions and Patterns
We now discuss the relation between the spherical metaplectic Whittaker function
on the n-fold cover of of GL(r + 1) over a non-archimedean local field and GelfandTsetlin patterns. Such a relationship, described globally, was conjectured in [7] andwas established in [5]. Though it is possible to pass from the global result to its
local analogue (cf. [11]), a direct local proof was given by McNamara [17], expressing
a metaplectic spherical Whittaker function as a generating function supported onstrict Gelfand-Tsetlin patterns. In this section, we recall two formulations of thisexplicit description, following [6]. In Section 4, we will explain their translations tosquare ice via the bijection of Proposition 1.

6

B. BRUBAKER, D. BUMP, G. CHINTA, S. FRIEDBERG, AND P. E. GUNNELLS]) denote the n-fold metaplectic cover of G(F ) = GL(r + 1, F ), where F
Let G(Fdenotes a nonarchimedean local field having ring of integers oF and residue field oforder q. (There are several related such extensions, but we will use the one in [13]where their parameter c = 0.) We assume 2n divides q − 1, which guarantees that F]) is a central extension
contains the group µ2n of 2nth roots of unity. The group G(Fof G(F ) by µn :π]) −→1 −→ µn → G(FG(F ) −→ 1.
We will identify µn ⊂ F with the group µn ⊂ C of complex n-th roots of unity by]) as just G,esome fixed isomorphism. For convenience, we will sometimes denote G(Fe the preimage of H(F )and if H is an algebraic subgroup of G, we may denote by Hein G.
For details of the construction of the metaplectic group and results about its representations, see [18] in this volume. Let B(F ) be the standard Borel subgroup of
upper triangular matrices in G(F ), and let T (F ) be the diagonal maximal torus.
Then B(F ) = T (F )U (F ) where U (F ) is the unipotent radical of B(F ). The metaplectic cover splits over various subgroups of G(F ); for us it is relevant that it splits
over U (F ) and over K := G(oF ), the standard maximal compact subgroup. By
abuse of notation, we will sometimes denote by K the homomorphic image of K ine under this splitting.
Ge be any map such that π ◦ s is the identity map on G(F ). Then
Let s : G(F ) → G
the map σ : G(F ) × G(F ) → µn such thats(g1 )s(g2 ) = σ(g1 , g2 )s(g1 g2 )is a 2-cocycle defining a class in H 2 (G(F ), µn ). A particular such cocycle was considered by Matsumoto [16], Kazhdan and Patterson [13] and Banks, Levy and Sepanski [1]. By these references, such as [13] Section 0.1, the map s may be chosen sothat the restriction of σ to T (F ) is given by the formula

 

t1
u1
Y
..
..
 =
,
(ti , uj )n .
(4)
σ 
.
.
i<j
ur+1
tr+1
The cocycle σ also has the property that σ(u, g) = σ(g, u) = 1 if u ∈ U (F ), and soe
the restriction of s to U (F ) is a homomorphism to G.
e
We will call a representation π of G or any subgroup genuine if π(ζg) = ζπ(g) whenζ ∈ µn . Recall that Te denotes the inverse image under π of the maximal torus T (F ).
The center Z(Te) of Te has finite index. Since Te is a Heisenberg group, charactersof Z(Te) parametrize the irreducible genuine representations of Te as follows. Let χ
be a quasi-character of Z(Te) that is unramified. This means that it is trivial on

METAPLECTIC ICE

7the maximal compact subgroup of this abelian group. Let A be a maximal abeliansubgroup of Te. Extend χ to A, then induce it to Te. By Clifford theory, the resulting
representation i(χ) of Te is genuine, irreducible, and finite-dimensional. It does notdepend on either A or the extension of χ to A.
e of B(F ) in such a way that s(U (F )) acts
We extend i(χ) to the inverse image Be obtained by normalized induction.
trivially. We then consider the representation of G
We call the vector space of the resulting representation I(χ). It has a one-dimensional
space of K-fixed, i.e. spherical, vectors.
Let ψ0 : F → C be an additive character that is trivial on oF but on no larger
fractional ideal. Given a spherical vector f in I(χ), we may construct the sphericalmetaplectic Whittaker function via the integral
Z(5)
W (g) =
f (w0 s(u)g)ψ(u)du,U (F )

where ψ is the character of U (F ) given by
1 x1,2 · · ·
x1,n
X

1 x2,3 · · · x2,n  = ψ0 (
xi,i+1 )
ψ
.
.

..
.. 
1

i

and w0 is a representative in K of the long Weyl group element. Strictly speaking,W (g) as we have defined it is an i(χ)-valued function and should be composed with anatural choice of linear functional on i(χ) to obtain a complex-valued function. For
brevity we will sometimes refer to W (g) as the metaplectic Whittaker function.
Note that χ on Z(Te) may be described by a choice of r complex numbers s =(s1 , . . . , sr ). The transformation property W (s(u)gk) = ψ(u)W (g) for all u ∈U (F ), k ∈ K, implies that it suffices to determine W on the inverse image of thetorus T (F ).PMoreover, since W is genuine, it is sufficient to specify W on s(T (F )).
Given λ = i λi ωi , where ωi are fundamental weights, let tλ be the element of thetorus T (F ) given by λ1 +λ2 +···+λr
ppλ2 +···+λr.
 where p is a uniformizer for oF .
..
tλ = 

p λr 
1
Let tλ = s(tλ ). Due to our assumption that F contains the 2n-th roots of unity,(p, p)n = 1 and by (4) it follows that tλ+µ = tλ tµ .
It is not hard to show that W (tλ ) = 0 unless λ is a dominant weight.

8

B. BRUBAKER, D. BUMP, G. CHINTA, S. FRIEDBERG, AND P. E. GUNNELLS

Given any dominant weight λ, the metaplectic Whittaker function W (tλ ) may
thus be expressed as a Dirichlet series in r complex variables s = (s1 , . . . , sr ) of the
form
X
(6)
Z(s; λ) =H(pk1 , . . . , pkr ; λ) q k1 (1−2s1 )+···+kr (1−2sr ) ,k=(k1 ,...,kr )∈Nr

where q denotes the cardinality of the residue field oF /p. We now describe thefunction H(pk1 , . . . , pkr ; λ). See [17] for further details.
The positive integer n will continue to denote the degree of the metaplectic cover.
We define the Gauss sumZ(u, p)bn ψ0 pa−b u du,g(a, b) =o×Fwhere (·, ·)n denotes the n-th power Hilbert symbol, and we normalize the Haar
measure so that µ(oF ) = 1. As a further shorthand, for any positive integer b, we set(7)g(b) = g(b − 1, b),h(b) = g(b, b).

Note that for a fixed base field F , these values depend only on b mod n. If n divides
b, in particular if n = 1, we have11(8)g(b) = − , h(b) = 1 − .
qq
We caution the reader that the q-powers that appear in the g(a, b) are normalized
differently than in the previous works [5–7]; these are the functions denoted g [ and h[in [6]. The function h is a degenerate Gauss sum whose values may be made explicit,
while (if n - b) g(b) is a “true” Gauss sum.
Any strict Gelfand-Tsetlin pattern T with entries indexed as in (1), we associatea weighting function γ to each entry ai,j with i ≥ 1 as follows:r g(bi,j ) if ai,j = ai−1,j−1 ,Xh(bi,j ) if ai−1,j 6= ai,j 6= ai−1,j−1 ,(ai,l −ai−1,l ).
(9) γ(ai,j ) =where bi,j = 1
if ai,j = ai−1,j ,l=jThen we define(10)Γ

G (T) =r Y
rY

γ(ai,j ).

i=1 j=iIf T is a Gelfand-Tsetlin pattern that is not strict, we define GΓ (T) = 0. We also
define
r
X
Γ
Γ
Γ
Γ
(11)
k (T) = (k1 (T), . . . , kr (T)) where ki (T) =ai,l − a0,l .
l=i

METAPLECTIC ICE

9

In particular, note that both GΓ and kΓ are defined using differences of elements
above and to the right of ai,j . The superscript Γ may be regarded as indicator thatthese quantities are defined using such “right-hand” differences.
We present these definitions in this ad-hoc fashion in order to give a brief and selfcontained treatment, but in fact they have very natural descriptions when reinterpreted as functions on a Kashiwara crystal graph. See [6] for an extensive discussion.
As an example, consider the Gelfand-Tsetlin pattern T in (3). Then(12)
(b1,1 , b1,2 , b2,2 ) = (1, 1, 2) so that GΓ (T) = h(1)g(2), and (k1 , k2 ) = (1, 3).
Theorem 1. (Brubaker, Bump, and Friedberg [5]; McNamara [17]) Givena dominant weight λ and a fixed r-tuple of non-negative integers k = (k1 , . . . , kr ),the function H(pk1 , . . . , pkr ; λ) appearing in the p-adic Whittaker function W (tλ ) is
given byX
H(pk ; λ) := H(pk1 , . . . , pkr ; λ) =GΓ (T),kΓ (T)=k

where the sum is over all Gelfand-Tsetlin patterns with top row corresponding to λ+ρsatisfying the subscripted condition.
There is a second explicit description of H(pk ; λ) in terms of “left-hand” differencesusing functions G∆ and k∆ that are analogous to those defined in (10) and (11)respectively. Assuming that T is strict, set(13)j g(ci,j ) if ai,j = ai−1,j ,Xh(ci,j ) if ai−1,j 6= ai,j 6= ai−1,j−1 ,where ci,j =(ai−1,l−1 − ai,l )δ(ai,j ) =1
if ai,j = ai−1,j−1 ,l=1
and defineG∆ (T) =

(14)

r Y
r
Y

δ(ai,j ).

i=1 j=i∆If T is not strict, define G (T) = 0. We also set
(15)

k

∆

(T) = (k1∆ (T), . . . , kr∆ (T))

whereki∆ (T) =i
Xa0,l−1 − ar+1−i,r+1−l .

l=1The main theorem of [6] is as follows:
Theorem 2. (Statement A of Brubaker, Bump and Friedberg [6]) Given adominant weight λ and a fixed r-tuple of non-negative integers k = (k1 , . . . , kr ),XXG∆ (T),(16)
GΓ (T) =kΓ (T)=kk∆ (T)=k

10

B. BRUBAKER, D. BUMP, G. CHINTA, S. FRIEDBERG, AND P. E. GUNNELLS

where the sums each run over all Gelfand-Tsetlin patterns with top row corresponding
to λ + ρ satisfying the subscripted condition.
As an immediate corollary, we have a second description of the p-adic Whittaker
function in terms of G∆ and k∆ . We refer to these two recipes on the left- andright-hand sides of (16) as the Γ- and ∆-rules, respectively.
In fact, there are many other descriptions for the Whittaker function, though theseare generally much more difficult to write down as explicitly. Indeed, as explainedin [3, 15], there exist bases for highest weight representations corresponding to any
reduced expression for the long element w0 of the Weyl group of GL(r + 1) — Sr , thesymmetric group on r letters — as a product of simple reflections σi . These makeuse of the Kashiwara crystal graph and are commonly called string bases. Using
these bases, one may make a correspondence between long words and recipes for the
Whittaker function (cf. [6, Chapter 2]). From this perspective, the Γ-rule corresponds
to the word
w0 = σ1 (σ2 σ1 ) · · · (σr σr−1 · · · σ1 ),whereas the ∆-rule corresponds to the word
w0 = σr (σr−1 σr ) · · · (σ1 σ2 · · · σr ).
These two words are as far apart as possible in the lexicographic ordering of all
reduced decompositions. The proof of Theorem 2 as given in [6] uses a blend of
combinatorial arguments to give various equivalent forms of the identity (16) as wemove through the space of long words. We highlight various aspects of the proof in
more detail now.
The proof is by induction on the rank r. The inductive hypothesis allows us toequate any two recipes for the Whittaker function whose associated long words differ
by a sequence of relations obtained from a lower rank case. For example, assuming
the rank 2 case allows us to perform a braid relation σ1 σ2 σ1 = σ2 σ1 σ2 , which could
be applied to the word corresponding to the Γ-rule above. After a series of such
identities, we arrive at two descriptions for the Whittaker function as a weighted sum
over Gelfand-Tsetlin patterns that agree on the bottom r − 2 rows of the pattern.
Thus, we may restrict our attention to the top three rows of a rank r pattern. We
refer to such three-row arrays of interleaving integers, where we fix both the top andbottom of the three rows, as “short Gelfand-Tsetlin patterns” and re-index the threerows as follows:


`1
···
`r−1
`r 
 `0
a1
a2
ar−1
ar
(17)
t=
.


m1
m2
mr−1
These two recipes for the Whittaker function will be called GΓ∆ (as this recipe uses
a right-hand rule for the entries ai and a left-hand rule for the entries mj ) and G∆ΓMETAPLECTIC ICE

11(where the use of rules is reversed). To be exact, using the definitions in (9) and(13), we have
Γ∆

G

(t) =

r
Y

γ(ai )i=1

r−1
Yδ(mj ),

and

∆Γ

G

j=1

(t) =

r
Y
i=1

δ(ai )r−1
Yγ(mj ).

j=1

Rather than define functions k Γ∆ and k ∆Γ on short patterns in analogy to the recipes
above, it is enough to specify the middle row sum as the other rows are fixed.
Before stating the reduction, we require one final ingredient. There is a natural
involution qr on short Gelfand-Tsetlin patterns of rank r, given by acting on middle
row entries according toqr :ai 7−→ max(`i−1 , mi−1 ) + min(`i , mi ) − ai =: a0i ,where if i = 0 we understand that max(`0 , m0 ) = `0 and if i = r, min(`r , mr ) =`r . This involution qr is used by Berenstein and Kirillov (cf. [14]) to define a
Schützenberger involution on Gelfand-Tsetlin patterns. Brubaker, Bump and Friedberg use the involution qr to give the following reduction of Statement A in [6].
Theorem 3. (Brubaker, Bump and Friedberg; Statement B of [6]) Fix an(r + 1)-tuple of positive integers ` = (`0 , . . . , `r ), an (r − 1)-tuple of positive integersm = (m1 , . . . , mr−1 ), and a positive integer k. Then
X
X
G∆Γ (qr (t)),
GΓ∆ (t) =
P

P

ai =k

a0i =k0

P
where a0i are the entries of qr (t), k 0 = i `i + j mj − k, and the sums range over all
short patterns with top row ` and bottom row m satisfying the indicated condition.
P

See [8] and Chapter 6 of [6] for a full proof of the reduction from Statement A to
Statement B. As noted above, the proof of Statement B proceeds through a series ofadditional reductions which occupy thirteen chapters of [6]. In brief, for “generic”
short patterns t, the Schützenberger involution qr gives a finer equality GΓ∆ (t) =G∆Γ (qr (t)), which implies the equality of Statement B summand by summand. By“generic” we mean that the entries of the short pattern are in general position – inparticular, for all i, `i 6= mi using the notation of (17). Note that the Schützenberger
involution does not necessarily preserve strictness for all patterns in the remaining
non-generic cases, and one needs much more subtle arguments to handle these shortpatterns. For such patterns, Statement B is not in fact true summand by summand,and one does need to sum over all short patterns with fixed row sum to obtainequality.
As an alternative to establishing Statement B, we mention that one could also
prove Theorem 2 by computing the Whittaker integral in two ways, mimicking thetechniques of [5], thus obtaining a proof via decomposition theorems in algebraic

12

B. BRUBAKER, D. BUMP, G. CHINTA, S. FRIEDBERG, AND P. E. GUNNELLS

groups which respect the metaplectic cover. In subsequent sections of this paper,
we propose a third way of viewing these theorems using ice-type models, which portends new connections between number theory/representation theory and statisticalphysics.
4. Ice and Metaplectic Whittaker Functions
In statistical mechanics, one attempts to infer global behavior from local interactions. In the context of lattice models, this means that we attach a Boltzmann weightto each vertex in the grid and for each admissible state of the model, we consider
the product of all Boltzmann weights ranging over all vertices of the grid. Then
one can attempt to determine the partition function of the lattice model, which issimply the sum over all admissible states of the associated weights. In this section,
we explain how to obtain the metaplectic spherical Whittaker function of Section 3
as the partition function of a lattice model with boundary conditions as defined in
Section 2.
We make use of the two sets of Boltzmann weights B Γ and B ∆ . When these weights
are applied to an admissible state of ice, we refer to the resulting configuration as
Gamma ice or Delta ice, respectively. In order to indicate which set of weights isbeing used at a particular vertex, we use • for Gamma ice and ◦ for Delta ice.
Gamma
Ice

(18)

Boltzmannweight B Γ

i

1

Delta
Ice
Boltzmannweight B ∆

zi
i

1

i

i

g(a)
i

g(a)ziizi
i

1h(a)zi
i

zi

i

i

1

i

h(a)zii

1

In giving these Boltzmann weights, we have made use of the notation in (7). For
Gamma ice, the constant a equals the number of + signs in the i-th row to the rightof the vertex •. For Delta ice, the constant a equals the number of − signs in thei-th row to the left of the vertex ◦. In either case, we refer to this constant as the“charge” at the vertex. Note by our definitions in (7), the Boltzmann weights only
depend on the charge mod n. The weights B Γ and B ∆ also depend on parameters
zi , where i indicates the row in which the vertex is found. For Gamma ice, the row
numbers decrease from r + 1 to 1 as we move from top to bottom as in the example

METAPLECTIC ICE

13(2), while for Delta ice, the row numbers increase from 1 to r + 1. These zi arereferred to as “spectral parameters.” We often suppress the dependence of B Γ andB ∆ on the spectral parameters zi , 1 ≤ i ≤ r + 1. Let z = (z1 , . . . , zr+1 ).
Given an admissible state of Gamma ice (or Delta ice, respectively) S, we defineYY(19)G Γ (S, z) =B Γ (v), G ∆ (S, z) =B ∆ (v),v∈Sv∈S

where the product (in either case) is taken over all vertices in the state of ice S.
Proposition 2. Under the bijection of Proposition 1, with strict pattern T corresponding to an admissible state of Gamma ice S, then GΓ (T) as defined in (10) is
related to G Γ (S, z) in (19) as follows:
d (T)−d1 (T) d1 (T)−d2 (T)d(T)−dr (T) dr (T)zr· · · z2 r−1
z1 ,0
G Γ (S, z) = GΓ (T)zr+1

where di (T) is the sum of the entries in the i-th row of the pattern T.
Similarly, for an admissible state of Delta ice S, G∆ (T) as defined in (14) is related
to G ∆ (S, z) in (19) byd (T)−d1 (T) d1 (T)−d2 (T)dr (T)
z2· · · zrdr−1 (T)−dr (T) zr+1.

G ∆ (S, z) = G∆ (T)z1 0

We first illustrate this for Gamma ice with our working example from (3) in rank 2.
The admissible Gamma ice S and its associated Boltzmann weights are pictured
below.
5

4

3

2

1

0

3

3

3

3

3

3

2

2

2

2

2

2

1

1

1

1

1

1

7−→

Boltzmann weights
5 4
3
2
1
0
1 z3 z3 z3 h(1)z3 1
1 1 g(2) 1
1
z2
1 1
1 z1
z1
z1

Taking the product over all these weights, we obtain G Γ (S, z) = h(1)g(2)z34 z2 z13 ,d (T)−d1 (T) d1 (T)−d2 (T) d2 (T)which indeed matches GΓ (T)z3 0
z2
z1
with T as in (3) and GΓ (T)as in (12).
Remark 1. The relevant terms in the metaplectic Whittaker function take the form
(20)

Γ

Γ

GΓ (T)q k1 (T)(1−2s1 )+···+kr (T)(1−2sr )as given in Theorem 1. However, kΓ (T) = (k1Γ , . . . , krΓ ) may be easily recovered fromour fixed choice of highest weight λ + ρ = (`1 , · · · , `r , 0) and the row sums di := di (T)14

B. BRUBAKER, D. BUMP, G. CHINTA, S. FRIEDBERG, AND P. E. GUNNELLS

used in the monomial above. Indeed,k1Γ = d1 − (`2 + · · · + `r ),k2Γ = d2 − (`3 + · · · + `r ), . . . ,krΓ = dr .

Hence, upon performing this simple transformation, we may recover the monomials
in q 1−2si in (20) from those in zj appearing in G Γ (S, z) of the above proposition. A
similar set of transformations holds for the Delta rules.
Proof. Proposition 2 is a consequence of the bijection given in Proposition 1. We
sketch the proof for Gamma ice, as the proof for Delta ice is similar. Recall that −vertical spins correspond to the entries of the pattern, so the values γ(ai,j ) given in(9) should appear in the Boltzmann weights for vertices sitting above a − vertical
spin. The particular cases of (9) to be used are determined by the vertical spin above
the vertex in question. We now show that each bi,j in (9) matches the charge, thenumber of + signs to the right of the vertex in row i. Equivalently, we must showthat every spin between column ai,j and column ai−1,j in row i is assigned a +. So
suppose that ai,j > ai−1,j and let v be the vertex in row i, column ai,j , and let w be
the vertex in row i, column ai−1,j . Then the north and south spins for v are (+, −)which, by the six admissible configurations in Gamma ice, forces the east spin to be+. All the vertices between v and w have north and south spins (+, +) according
to our bijection. The east spin + for v becomes the west spin for the neighboringvertex v 0 to the right of v, forcing the east spin of v 0 to be + as well. This effectpropogates down the row, forcing all row spins between v and w to be +. Finally wemust show that the spectral parameters for G are given by differences of consecutive
row sums. This is Lemma 3 of [4].
Given a fixed set of boundary conditions for the vertex model and an assignment
B of Boltzmann weights associated to each admissible vertex, we refer to the set of
all admissible states S as a “system.” Given a system S, its partition function Z(S)is defined as
XY(21)
Z(S) := Z(S, z) =B(S, z), with B(S, z) :=
B(v),
S∈S

v∈S

where this latter product is taken over all vertices v in the state S. In particular, letSΓ denote the system with boundary conditions as in Section 2, Boltzmann weights
B Γ , and rows labeled in descending order from top to bottom. Similarly, let S∆
denote the system with the same boundary conditions, but with Boltzmann weights
B ∆ and rows labeled in ascending order from top to bottom. Using this language,
we may now summarize the results of the past two sections in a single theorem.
Theorem 4. Given a dominant weight λ for GLr+1 , the metaplectic Whittaker function W (tλ ) is expressible as either of the two partition functions Z(SΓ ) or Z(S∆ ).
This is merely the combination of Theorems 1 and 2 together with Proposition 2.

METAPLECTIC ICE

15

5. Transfer Matrices
Baxter considered the problem of computing partition functions for solvable latticemodels (cf. [2]). His approach is based on the idea of using the Yang-Baxter equation (called the “star-triangle identity” by Baxter) to prove the commutativity ofrow transfer matrices. We will show that basic properties of metaplectic Whittaker
functions can be interpreted as commutativity of such transfer matrices, and at least
when the metaplectic degree n = 1, the Yang-Baxter equation can be used to give
proofs of these.
The row transfer matrices shall now be described. Let us consider a row of vertices
that all have the same Boltzmann weights. If B = (a1 , a2 , b1 , b2 , c1 , c2 ) then we use
the assignment of Boltzmann weights in the following table.

Boltzmannweight

B

B

B

B

B

B

a1

a2

b1

b2

c1

c2

The vertical edge spins in the top and bottom boundaries will be collected into
vectors α = (αN , · · · , α0 ) and β = (βN , · · · , β0 ). The subscripts correspond to thecolumns which, we recall, are numbered in ascending order from right to left. For
example, if α = (−, +, −, +, +, −) and β = (+, −, +, +, +, −), we would consider
the partition function of the following one-layer system of ice:
5

(22)

4
B

3
B

2
B

1
B

0
B

B

Let VB (α, β) denote the partition function. Recall that we compute this as follows.
We complete the state by assigning values to the interior edges (unlabeled in thisfigure) and sum over all such completions. Let VB be the 2N +1 × 2N +1 matrix whose
entries are all possible partition functions VB (α, β), where the choices of α and β
index the rows and columns of the matrix, respectively. This is referred to as thetransfer matrix for the one-layer system of size N with Boltzmann weights B at every
vertex.

16

B. BRUBAKER, D. BUMP, G. CHINTA, S. FRIEDBERG, AND P. E. GUNNELLS

Now let us consider a two-layer system:
5

4

3

2

1

0

B1

B1

B1

B1

B1

B1

B2

B2

B2

B2

B2

B2

(23)

Note that we are using two sets of Boltzmann weights B1 and B2 for the top andbottom layers, respectively. We may try to express the partition function V (α, γ)
for the two layer system pictured above having top row α as in (22) and bottom
row γ = (+, +, +, −, +, +) in terms of one-layer partition functions. However each
one-layer system is only determined upon a choice of vertical spins lying between
the rows. One such choice of edge spins is β as in the one-layer system in (22), but
we must sum over all possible choices to get the partition function of the two-layer
system. ThereforeX
VB1 (α, β)VB2 (β, γ),V (α, γ) =βwhich is precisely the entry V (α, γ) in the product of the two transfer matrices VB1and VB2 .
Cases where the transfer matrices commute are of special interest. Indeed, thiscommutativity means that one can interchange the roles of Boltzmann weights B1
and B2 in (23) and the value of the product of the transfer matrices is unchanged.
Baxter considers the case where B1 = (a, a, b, b, c, c) and B2 = (a0 , a0 , b0 , b0 , c0 , c0 ) for
arbitrary choices of a, a0 , b, b0 , c, c0 . However, his boundary conditions are toroidal;
that is, the boundary edges at the left and right edges of the each row are identified
and treated as interior edges, hence summed over. With this modification, Baxter
proves that if 4 = 40 , where 4 = (a2 + b2 − c2 )/2ab and 40 is similarly defined witha0 , b0 and c0 , then the transfer matrices commute. Obtaining a sufficiently large familyof commuting transfer matrices is a step towards evaluating the partition function,
since by doing so one can make the eigenspaces one-dimensional. Thus the problem
of simultaneously diagonalizing them has a unique solution and therefore becomes
tractable.
Let us now show how Statement B may be formulated in terms of commuting
transfer matrices. We consider a two layer system having a layer of Gamma ice andMETAPLECTIC ICE

17

a layer of Delta ice, thus:
B1Γ

B1Γ

B1Γ

B1Γ

B1Γ

B1Γ

B1Γ

B2∆

B2∆

B2∆

B2∆

B2∆

B2∆

B2∆

(24)

We use the values in (18); in the top row, the spectral parameter is z1 , and in thebottom row it is z2 . Regarding the boundary conditions, as always the rows of ice
must have + at the left edge and − at the right edge. Furthermore, we fix a choiceof spins for the top edge and the bottom edge of this two-layered ice such that the
top edge has two more − than the bottom row. In this example, the locations of the− along the top edge are (reading from right to left) 0, 1, 4, 6 and along the bottomedge, they are at 3, 4. We have labeled each vertex with • Γ1 and ◦∆2 to remind thereader of the Boltzmann weights that we are using. We will call this system SΓ∆and its partition function Z(SΓ∆ ).
On the other hand we may consider the same configuration with the roles of the
Boltzmann weights for Gamma and Delta ice switched, as in the figure below. Notethat the boundary conditions remain the same as in (24). We will refer to this system
as S∆Γ .
B2∆

B2∆

B2∆

B2∆

B2∆

B2∆

B2∆

B1Γ

B1Γ

B1Γ

B1Γ

B1Γ

B1Γ

B1Γ

Theorem 5. Given top and bottom boundary values as vectors of spins α and γ and
Boltzmann weights B1Γ and B2∆ as in (18), let SΓ∆ and S∆Γ be the systems describedabove. Then Z(SΓ∆ ) = Z(S∆Γ ).
We prove this by showing that the claim is equivalent to Statement B, stated as
Theorem 3 here and proved by combinatorial means in [6]. Note in particular that
we have reformulated Statement B as the commutativity of two transfer matrices.
Proof. We associate two strictly decreasing vectors of integers with α and γ, which
we call l and m. Namely, let l = (l0 , l1 , l2 , · · · ), where the li ’s are the integers suchthat αli = −, arranged in descending order; m is defined similarly with regard to γ.

18

B. BRUBAKER, D. BUMP, G. CHINTA, S. FRIEDBERG, AND P. E. GUNNELLS

Thus, in the example (24) above there are − spins in the 6, 4, 1, 0 columns of the top
row and so l = (6, 4, 1, 0), while m = (4, 3). Similarly, given any admissible state of
the system, let β be the middle row of edge spins, and associate in similar fashion asequence a = (a1 , a2 , · · · ) according to the location of − signs in β.
We observe that the sequences l, a, m interleave. This holds for the same reason that the rows of the pattern interleave in Proposition 1; it is a consequence ofLemma 2 of [4]. Therefore the legal states of either system S are in bijection withthe (strict) short Gelfand-Tsetlin patterns


`1
···
`r−1
`r 
 `0
a1
a2
ar−1
ar
t=
.


m1
m2
mr−1
These are not in bijection with the terms of the sum GΓ∆ (t) appearing in Theorem 3
because there is no condition on the middle row sum. Rather, the states of ice give all
possible middle row sums. However, letting G Γ∆ (S, z) denote the Boltzmann weight
for a state of Γ∆ ice, this may be regarded as a homogeneous polynomial in the two
spectral parameters z1 and z2 of our two-row system. In the notation of Proposition 2,
d (t)−d1 (t) d1 (t)−d2 (t), where di (t) denotes the i-th row sum in thez2this monomial is z1 0
short Gelfand-Tsetlin pattern above. Clearly, the middle row sum can be recoveredfrom knowledge of this monomial for fixed choice of boundary conditions α and γ,
which dictate the top and bottom row of the short pattern. A similar correspondence
may be obtained for the ∆Γ system whose short patterns t0 are associated to thed (t0 )−d1 (t0 ) d1 (t0 )−d2 (t0 )monomial z2 0
z1
. Of course, the boundary conditions remain constantwhether we are using the Γ∆ or ∆Γ system, so d0 (t) = d0 (t0 ) and d2 (t) = d2 (t0 ). Thus,
the monomials
d (t)−d1 (t) d1 (t)−d2 (t)
z2

z1 0

d (t0 )−d1 (t0 ) d1 (t0 )−d2 (t0 )z1and z2 0agree precisely whend1 (t) = d0 (t) + d2 (t) − d1 (t0 ),which is exactly the condition on the sum in Theorem 3. Hence we see that thecommutativity of transfer matrices – the statement that Z(SΓ∆ ) = Z(S∆Γ ) – isan equality of two homogeneous polynomials and the matching of each monomial
corresponds to the identity of Statement B for each possible middle row sum.

6. The Yang-Baxter Equation
The proof of Theorem 5, the commutativity of transfer matrices, uses the equivalence with Theorem 3 and hence implicitly relies on all of the combinatorial methodsof [6] in order to obtain this result. In this section, we want to explore the extent

METAPLECTIC ICE

19

to which Baxter’s methods for solving statistical lattice models, most notably the
Yang-Baxter equation, may be used to prove the commutativity of transfer matrices.
In our context of two-dimensional square lattice models, the Yang-Baxter equation
may be viewed as a fundamental identity between partition functions on two verysmall pieces of ice – each having 6 boundary edges to be fixed, 3 internal edges, and3 vertices each with an assigned set of Boltzmann weights.
Definition (Yang-Baxter Equation). Let R, S, and T be three collections of Boltzmann weights associated to each admissible vertex. Then for every fixed combinationof boundary conditions σ, τ, α, β, ρ, θ, we have the following equality of partition functions:


(25)


τ



Z


σ




β
ν
R
µ

S





β
T




τ





γ
=Z



T ρ

σ



δ

α

α

θ 

S

ψ

φ





 .
R


ρ

θ 

Recall that these partition functions are sums of Boltzmann weights over all admissible states. Hence, the left-hand side is a sum over all choices of internal edge labelsµ, ν, γ, while the right-hand side is a sum over internal edge labels φ, ψ, δ. Note thatthe roles of S and T are interchanged on the two sides of the equality.
In the diagram above one vertex, labeled R, has been rotated by 45◦ for ease ofdrawing the systems. It should be understood in the same way as S and T - it hasa Boltzmann weight associated to a set of admissible adjacent edge labels. However,vertices of this type have a distinguished role to play in the arguments that follow,
so we use the term R-vertex to refer to any vertex rotated by 45◦ like R in (25).
Once equipped with the Yang-Baxter equation, the commutativity of transfer matrices, i.e. invariance of the partition function under interchange of rows, may be
proved under certain assumptions. We illustrate the method with a three-layer system of ice S having boundary conditions and admissible vertices like those of thesystem SΓ , to give the basic idea. Suppose we wanted to analyze the effect of swapping the second and third rows in the following configuration:

20

B. BRUBAKER, D. BUMP, G. CHINTA, S. FRIEDBERG, AND P. E. GUNNELLS

5

(26)

4

3

2

1

0

3

3

3

3

3

3

2

2

2

2

2

2

1

1

1

1

1

1

Suppose there exists only one admissible R-vertex having positive spins on theright; without loss of generality we take it to have all positive spins. Then thepartition function Z(S) for (26) multiplied by the Boltzmann weight for the R-vertex
with all + spins is equal to the partition function for the following configuration of
ice.
5

(27)

a

b

4

3

2

1

0

3

3

3

3

3

3

2

2

2

2

2

2

1

1

1

1

1

1

(By assumption, the only legal values for a and b are +, so every state of this problem
determines a unique state of the original problem.) Now we apply the Yang-Baxter
equation to move this R-vertex rightward, to obtain equality with the the following
configuration.
5

4

3

2

1

0

3

3

3

3

3

3

1

2

2

2

2

2

2

1

1

1

1

1

METAPLECTIC ICE

21

Repeatedly applying the Yang-Baxter equation, we eventually obtain the configuration in which the R-vertex is moved entirely to the right.
5

(28)

4

3

2

1

0

3

3

3

3

3

3

1

1

1

1

1

1 c

2

2

2

2

2

2

d

In drawing the above picture, we have again assumed that there is just one legal
configuration for the R-vertex having two − spins on the left, and assumed the spinsof this R-vertex were all −. If we let S0 denote the system with the same boundary
conditions as S shown in (26) but with the second and third row Boltzmann weights
interchanged, we have shown
!
!
(29)

BR

Z(S) = BR

Z(S0 )

where BR denotes the assignment of Boltzmann weight to each configuration. In
particular, if the two admissible R-vertices coming from the left- and right-handsides of (29) have equal Boltzmann weights, we obtain the exact equality of the two
configurations, i.e. the commutativity of transfer matrices.
We now explore the possibility of obtaining a Yang-Baxter equation with S and Tin (25) corresponding to the Boltzmann weights B Γ and B ∆ , respectively, from (18).
In light of our previous argument, this would give an alternate proof of Theorem 3.
However, the Boltzmann weights in (18) depend not only on spins + or − on adjacentedges, but also on a “charge” a mod n. Recall from Section 4 that using B Γ weights,
charge records the number of + signs in a row between the given vertex and the −boundary spin at the right-hand edge the row. Using B ∆ weights, charge counts thenumber of − signs between the vertex and the + boundary at the left.
In order to demonstrate a Yang-Baxter equation, we need Boltzmann weights thatare purely local — i.e., depend only on properties of adjacent edges — so we needa different way of interpreting charge. We do this by labeling horizontal edges withboth a spin and a number mod n. We declare the Boltzmann weight of these vertices
to be 0 unless the edge labels a, b mod n to the immediate left and right of the vertex
reflect the way charge is counted for the given spins. For example, with B Γ weights,

22

B. BRUBAKER, D. BUMP, G. CHINTA, S. FRIEDBERG, AND P. E. GUNNELLS

a = b + 1 if the spin below a is + and a = b if the spin below a is −. Using thisinterpretation, we record the non-zero vertices for both sets of Boltzmann weights:
(30)
Gamma
Ice
BΓ
weightDelta
Ice
B∆
weight

a+1

a

a

i

1

zi

a

i

1

a

i

a+1

a+1

a

i

1

a

a

a+1

h(a)zi

a

a

a+1

a

a

i

1

a

i

1

a

i

zi

a

i

zi

a

i

a

i

g(a)

a

g(a)zi

a

i

i

a+1

h(a)zi

The above vertices are admissible for any choice of a mod n (and the integers a + 1
are, of course, understood to be mod n as well). This means that we are generalizing
the six-vertex model, since due to the dependence on a, each vertex has more thansix admissible states.
For n = 1, the charge labels on horizontal edges are trivial as the Gauss sumsg(a) and h(a) are independent of a as evaluated in (8). For this special case, it was
shown in [4] that a Yang-Baxter equation exists with weights S and T as in (25)taken to be B Γ and B ∆ from the table above. We refer the reader to [4] for thecorresponding Boltzmann weights R for which the Yang-Baxter equation is satisfied.
Thus we obtain an alternate proof of Theorem 4, or equivalently Theorem 3, using
methods from lattice models.
In general, we know from [6] that Theorem 4 is true for any positive integer n.
It would be extremely interesting to find a local relation like (25) similarly provingthat the transfer matrices commute, and this is currently under investigation by theauthors.

7. Weyl group invariance and the Yang-Baxter equation
Kazhdan and Patterson [13, Lemma 1.3.3] describe how the metaplectic Whittaker
functions transform under the action of the Weyl group. This invariance—which does
not follow directly from the description of the coefficients H given in Theorem 1—
plays a key role in the proof of the metaplectic Casselman-Shalika formula for GLr+1

METAPLECTIC ICE

23

by Chinta and Offen [9], and was the main inspiration for the Weyl group actionin [10].
In this section we restate this Weyl group invariance in terms of the partition
functions defined in the previous sections. We content ourselves to describe how asimple reflection acts on the partition function. Let σi denote the simple reflection in
the Weyl group corresponding to the i-th simple root. We let σi act on the spectral
parameter z = (z1 , z2 , . . . , zr+1 ) by σi (z) = (z1 , . . . , zi−1 , zi+1 , zi , zi+2 , . . . , zr+1 ), i.e.
the ith and (i + 1)st coordinates are transposed. Here the notation Z(S, z) refers
to the partition function associated to the system S, where S is either of the twosystems SΓ or S∆ introduced in Section 4.
Further define, for j = 0, . . . , n − 1,
(31)

P (j) (x, y) = xj y n−j

1 − q −1
xn − y n
(j)
and
Q
(x,
y)
=
g(j)
,
xn − q −1 y n
xn − q −1 y n

where we again use the shorthand notation of (7) and interpret g(0) := g(n) = −q −1 .
The functions P ,Q are closely related to the functions τs1 ,τs2 of [13, Lemma 1.3.3].
For each 1 ≤ i ≤ r, we may decompose the partition function
X (j)
(32)
Z(S, z) =Zi (S, z),
0≤j<n
(j)

where Zi (S, z) is the sum over all states S ∈ S such that B(S, z) is equal to
ar+1
a constant times z1a1 · · · zr+1
where ai − ai+1 ≡ j (mod n). Then the Whittaker
function satisfies
(33)

(j)

(j)

(n−j)

Zi (S, σi (z)) = P (j) (zi+1 , zi ) · Zi (S, z) + Q(j) (zi+1 , zi ) · Zi

(S, z).

We now consider the extent to which the functional equations (33) can be interpreted in the language of transfer matrices. First we consider the case n = 1. The
decomposition on the right-hand side of (32) has only one term, namely Z itself,
since the congruence condition is automatically satisfied by all monomials for any i.
The i-th functional equation (33) becomes
Z(S, σi (z)) = (P (0) (zi+1 , zi ) + Q(0) (zi+1 , zi ))Z(S, z),or(34)(zi − zi+1 /q)Z(S, z) = (−zi /q + zi+1 )Z(S, σi (z)).

Recalling the effect of σi on z defined above, the partition function on the righthand side is the result of swapping the spectral parameters associated to rows i and
i + 1 in the system S. Note that (34) is not exactly the same as “commutation of
two transfer matrices” because we do not have the identity Z(S, z) = Z(S, σi (z)).
Indeed, the partition function Z is not a symmetric function, but it is very close toone: it is a Schur polynomial times a q-deformation of the Weyl denominator (cf. [4]).

24

B. BRUBAKER, D. BUMP, G. CHINTA, S. FRIEDBERG, AND P. E. GUNNELLS

Nevertheless, with assumptions as in Section 6, we may ask for a Yang-Baxter
equation leading to a proof of (34). That is, we seek sets of Boltzmann weights R, S,
and T satisfying (25) where S = T = B Γ or S = T = B ∆ . Comparing (34) with(29), we further require Boltzmann weights BR for the R-vertices such that!
= zi − zi+1 /q,BR!

= −zi /q + zi+1 .

BR

It follows from results in [4] that we may use the following coefficients in the R-vertex
for Gamma ice:(35)

zi − q −1 zi+1 zi+1 − q −1 zi q −1 (zi+1 − zi ) zi+1 − zi (1 − q −1 )zi+1 (1 − q −1 )zi
We are taking all ti = −q −1 in Table 1 in [4], and observe that the order of the rows
in this paper are opposite those in that paper. Our convention here is the same asin [6].
For n > 1 the situation is more complicated, but rather suggestive. In general
(n−j)
(j)
(S), so we cannot rewrite (33) to look like (29) and (34). HoweverZi (S) 6= Zi
according to (31), the denominators of P and Q appearing in the ith functionaln/q. Thus,
equation (33) are equal and independent of j. For any j, they are zin − zi+1
clearing denominators, we may rewrite (33) as follows
(j)

n
(zi+1
− q −1 zin )Zi (S, σi (z)) =

(36)

(n−j)

(j)

p(j) (zi+1 , zi ) · Zi (S, z) + q (j) (zi+1 , zi ) · Zi

(S, z)

wherejp(j) (zi+1 , zi ) = (1 − q −1 )zi+1
zin−j ,nq (j) (zi+1 , zi ) = g(j)(zi+1− zin ).

Let S be a state of the system, and as before let a1 , · · · , ar+1 be the exponentsof z1 , · · · , zr+1 in B(S, z). We make the following observation. In the weights (30),there is a contribution of zi if and only if the charge is not augmented as we moveacross the vertex. Since (in Gamma ice) the charges at the right edge will have value
0, it follows that the charges at the left edge will have value ci where ai + ci is thenumber of vertices in the row. Therefore(37)

ai − ai+1 = ci+1 − ciMETAPLECTIC ICE

25

and we may therefore writeX(j)Zi (S, z) =B(S, z).

ci+1 − ci ≡ j mod n

We will now explain how, with a suitable R-vertex, (36) could also be interpreted
as an identity similar to (29), but now with sets of Boltzmann weights involving
charges. We will describe the characteristics that such an R-vertex might have. For
simplicity, we will assume that n is odd.
The value will depend on the spins and charges of the adjacent edges. Let us
assume first that the spins on these four edges are all +, with charges di+1 , di , d0i+1 ,d0i as follows:

(38)

di+1

d0i+1

di

d0i

If j = di+1 − di and j 0 = d0i+1 − d0i then we require that the Boltzmann weight of thisvertex v is zero unless j 0 ≡ j or n − j mod n. Moreover in these cases we requirethat the Boltzmann weight of (38) is
 (j)p (zi+1 , zi ) if j ≡ j 0 mod nq (j) (zi+1 , zi ) if j ≡ n − j 0 mod n
except when j ≡ 0. In this case the weight will benp(0) (zi+1 , zi ) + q (0) (zi+1 , zi ) = zin − q −1 zi+1,

since g(0) = −q −1 .
Regarding the case where the vertex has spin − on all four adjoining edges, we
require that the Boltzmann weight of
di+1

0

di

0

n
is zero unless di = di+1 = 0, in which case it is zi+1
− q −1 zin .
Assuming that the R-vertex has the above properties, we may now express thefunctional equation in a form similar to (29). Let us fix the vertical edge spins above
the zi+1 row and below the zi row, and work with just the two relevant rows; let S0

26

B. BRUBAKER, D. BUMP, G. CHINTA, S. FRIEDBERG, AND P. E. GUNNELLS

denote the two-layer system consisting of just rows i + 1 and i with these boundary
spins fixed. In order to establish (33), or equivalently (36), it suffices to show(j)n
− q −1 zin )Zi (S0 , σi (z)) =
(zi+1
(j)

(n−j)

p(j) (zi+1 , zi ) · Zi (S0 , z) + q (j) (zi+1 , zi ) · Zi

(S0 , z).

Since Z(S0 , z) is a homogeneous polynomial in the zi , and since only ai and ai+1 areallowed to vary, we have ai + ai+1 equal to a constant. Since we are assuming thatn is odd, there will be a unique pair of charges ci and ci+1 mod n such that (37) issatisfied, and such that ci+1 − ci ≡ j modulo n.
Now let us consider the partition function of the system
ci+1

±

±

±

±

±

±

ci

±

±

±

±

±

±

0

0

obtained by attaching the R-vertex to the left of S0 . From the above discussion, thisequals(n−j)(j)(S0 , z).
p(j) (zi+1 , zi ) · Zi (S0 , z) + q (j) (zi+1 , zi ) · Zi
Similarly the partition function of the system
ci+1 ±

±

±

±

±

±

ci

±

±

±

±

±

±

0

0

is
(j)

n
(zi+1
− q −1 zin )Zi (S0 , σi (z)).
The equality of these partition functions implies (36).
At this writing, we do not know if the values of the R-vertex that we have describedcan be completed to a full R-matrix such that the appropriate Yang-Baxter equation
is satisfied. We know that this can be done when n = 1, and since (36) is true, it
seems very plausible that this can be done in general. Thus we may conjecture thatwithin this scheme, or some similar one, it is possible to formulate a Yang-Baxter
equation adapted to these weights that gives a proof of (36). Such a “metaplectic”
Yang-Baxter equation might well have importance beyond the problems that we havediscussed in this paper.

METAPLECTIC ICE

27

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28

B. BRUBAKER, D. BUMP, G. CHINTA, S. FRIEDBERG, AND P. E. GUNNELLS

Department of Mathematics, MIT, Cambridge MA 02139-4307
E-mail address: brubaker@math.mit.edu
Department of Mathematics, Stanford University, Stanford CA 94305-2125
E-mail address: bump@math.stanford.edu
Department of Mathematics, The City College of CUNY, New York, NY 10031
E-mail address: chinta@sci.ccny.cuny.edu
Department of Mathematics, Boston College, Chestnut Hill MA 02467-3806
E-mail address: friedber@bc.edu
Department of Mathematics and Statistics, University of Massachusetts Amherst,
Amherst, MA 01003
E-mail address: gunnells@math.umass.edu