Multispecies Weighted Hurwitz Numbers
The construction of hypergeometric 2D Toda τ-functions as generating functions for weighted Hurwitz numbers is extended to multispecies families. Both the enumerative geometrical significance of multispecies weighted Hurwitz numbers, as weighted enumerations of branched coverings of the Riemann sphe...
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irk-123456789-1471642019-02-14T01:24:57Z Multispecies Weighted Hurwitz Numbers Harnad, J. The construction of hypergeometric 2D Toda τ-functions as generating functions for weighted Hurwitz numbers is extended to multispecies families. Both the enumerative geometrical significance of multispecies weighted Hurwitz numbers, as weighted enumerations of branched coverings of the Riemann sphere, and their combinatorial significance in terms of weighted paths in the Cayley graph of Sn are derived. The particular case of multispecies quantum weighted Hurwitz numbers is studied in detail. 2015 Article Multispecies Weighted Hurwitz Numbers / J. Harnad // Symmetry, Integrability and Geometry: Methods and Applications. — 2015. — Т. 11. — Бібліогр.: 30 назв. — англ. 1815-0659 2010 Mathematics Subject Classification: 05A15; 14H30; 33C70; 57M12 DOI:10.3842/SIGMA.2015.097 http://dspace.nbuv.gov.ua/handle/123456789/147164 en Symmetry, Integrability and Geometry: Methods and Applications Інститут математики НАН України |
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The construction of hypergeometric 2D Toda τ-functions as generating functions for weighted Hurwitz numbers is extended to multispecies families. Both the enumerative geometrical significance of multispecies weighted Hurwitz numbers, as weighted enumerations of branched coverings of the Riemann sphere, and their combinatorial significance in terms of weighted paths in the Cayley graph of Sn are derived. The particular case of multispecies quantum weighted Hurwitz numbers is studied in detail. |
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Harnad, J. Multispecies Weighted Hurwitz Numbers Symmetry, Integrability and Geometry: Methods and Applications |
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Harnad, J. |
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Harnad, J. |
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Multispecies Weighted Hurwitz Numbers |
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Multispecies Weighted Hurwitz Numbers |
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Multispecies Weighted Hurwitz Numbers |
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Multispecies Weighted Hurwitz Numbers |
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Multispecies Weighted Hurwitz Numbers |
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multispecies weighted hurwitz numbers |
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Інститут математики НАН України |
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Multispecies Weighted Hurwitz Numbers / J. Harnad // Symmetry, Integrability and Geometry: Methods and Applications. — 2015. — Т. 11. — Бібліогр.: 30 назв. — англ. |
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Symmetry, Integrability and Geometry: Methods and Applications |
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Symmetry, Integrability and Geometry: Methods and Applications SIGMA 11 (2015), 097, 19 pages
Multispecies Weighted Hurwitz Numbers?
J. HARNAD †‡
† Centre de recherches mathématiques, Université de Montréal,
C.P. 6128, succ. Centre-ville, Montréal (QC) H3C 3J7, Canada
E-mail: harnad@crm.umontreal.ca
‡ Department of Mathematics and Statistics, Concordia University,
7141 Sherbrooke W., Montréal (QC) H4B 1R6, Canada
Received March 31, 2015, in final form November 16, 2015; Published online December 02, 2015
http://dx.doi.org/10.3842/SIGMA.2015.097
Abstract. The construction of hypergeometric 2D Toda τ -functions as generating functions
for weighted Hurwitz numbers is extended to multispecies families. Both the enumerative
geometrical significance of multispecies weighted Hurwitz numbers, as weighted enumer-
ations of branched coverings of the Riemann sphere, and their combinatorial significance
in terms of weighted paths in the Cayley graph of Sn are derived. The particular case of
multispecies quantum weighted Hurwitz numbers is studied in detail.
Key words: weighted Hurwitz number; τ -function; multispecies
2010 Mathematics Subject Classification: 05A15; 14H30; 33C70; 57M12
1 Introduction
In [11, 12] a simple method was developed for constructing parametric families of KP and 2D
Toda τ -functions [28, 29, 30] of hypergeometric type [10, 25] that serve as generating functions
for the weighted enumeration of n-sheeted branched coverings of the Riemann sphere. These
are characterized by the fact that their expansions in the basis of products of Schur functions
have only diagonal coefficients, and these are of special content product form. When expanded
instead in the basis of products of power sum symmetric functions, the coefficients turn out to be
weighted sums of Hurwitz numbers, with weighting dependent generally on an infinite sequence
of parameters c = (c1, c2, . . . ) determined by an associated weight generating function G(z).
Such weighted sums may be interpreted equivalently as weighted enumeration of paths in the
Cayley graph of the symmetric group Sn generated by transpositions.
The special choice G(z) = exp(z) gives rise to the generating functions for simple and double
Hurwitz numbers introduced by Pandharipande [26] and Okounkov [24], in which all branch
points other than a single one or a specified pair have simple branching profiles. Further insight
into the significance of these hypergeometric τ -functions as generating functions in terms of
recurrence relations for triangulations was developed in [9]. More generally, the “topological
recursion” program was successfully applied to the study of Hurwitz numbers in [1, 3, 4, 5].
It has been known since the pioneering works of Hurwitz [7, 8], Frobenius [16, 17] and
Schur [27] that these numbers may be reinterpreted combinatorially using the monodromy rep-
resentation of the fundamental group of the punctured sphere with values in Sn determined by
lifting closed paths to the covering surface. From this viewpoint, they enumerate factorizations
of the identity element into products of elements whose conjugacy classes correspond to the
ramifications profiles, and hence give uniformly weighted enumeration of paths in the Cayley
graph from one conjugacy class to another with a given number of steps.
?This paper is a contribution to the Special Issue on Exact Solvability and Symmetry Avatars in honour of
Luc Vinet. The full collection is available at http://www.emis.de/journals/SIGMA/ESSA2014.html
mailto:harnad@crm.umontreal.ca
http://dx.doi.org/10.3842/SIGMA.2015.097
http://www.emis.de/journals/SIGMA/ESSA2014.html
2 J. Harnad
Another choice of weight generating function that was studied in [12] is the quantum di-
logarithm function [6]. This amounts to equating the parameters ci to powers of a single quantum
deformation parameter q, and gives rise to four special versions of quantum weighted Hurwitz
numbers, whose distribution functions were linked to those of a Bosonic gas with linear energy
spectrum in [12]. Using a suitably extended class of weight generating functions, this was further
generalized in [13, 14] to include both the infinite family of classical weighting parameters c and
the pair (q, t) of quantum deformation parameters characterizing Macdonald polynomials.
In the present work, the notion of weighted Hurwitz numbers is extended to weighted enume-
rations of multispecies coverings involving arbitrary choices for the corresponding weighting para-
meters. The generating function depends on l+m expansion parameters (w1, . . . , wl; z1, . . . , zm),
corresponding to two classes and l +m subspecies (or “colours”) of branch points with ramifi-
cation profiles types
{
µ(β)
}
β=1,...,l
and
{
ν(β)
}
β=1,...,m
. The special case of signed multispecies
enumeration in the uniformly weighted case was studied in [15]. Its combinatorial significance
was explained in terms of enumeration of paths in the Cayley graph that are subdivided into
strictly or weakly monotonically increasing subsequences of transpositions having given lengths.
In the single species case [12], this was shown to be equivalent to (signed) enumeration of
branched covers of the Riemann sphere with the “coloured” branch points constrained to have
fixed total ramification index within each class. Another special case detailed here consists of
“multispecies quantum weighted Hurwitz numbers”, in which the weighting parameters consist
of powers of a sequence of auxiliary quantum deformation parameters q = (q1, . . . , ql), p =
(p1, . . . , pm).
In Sections 1.1–1.4 the basic notions regarding Hurwitz numbers will be recalled, together
with the construction of weighted Hurwitz numbers using infinite parameter families of weight
generating functions G(z), as developed in [11, 12, 15]. In Section 2, the single species case will
be extended to multispecies by introducing the idea of “coloured” branch points, of two classes.
Weight generating functions depending on a multiparametric set of expansion parameters mul-
tiplicatively provide multiparametric families of 2D Toda τ -functions of hypergeometric type
that are generating functions for such multispecies weighted Hurwitz numbers. As in the single
species case, these may be viewed both geometrically and combinatorially, in terms of weighted
coverings and paths in the Cayley graph. In Section 3 this is restricted to the special cases of
quantum weightings introduced in [12] to produce generating functions for multispecies quantum
Hurwitz numbers. These are interpreted, as in the single species case, both geometrically and
combinatorially1.
1.1 Hurwitz numbers
For a set of k ∈ N+ partitons
(
µ(1), . . . , µ(k)
)
of n ∈ N+, let H
(
µ(1), . . . , µ(k)
)
denote the
number of n-sheeted branched coverings of the Riemann sphere (not necessarily connected),
with k branch points, whose ramification profiles are given by the partitions, weighted by the
inverse of the order of the automorphism group. These are the geometrically defined Hurwitz
numbers as originally studied by Hurwitz [16, 17, 19]. The genus g of the covering surface is
determined by the Riemann Hurwitz formula for the Euler characteristic
2− 2g = 2n−
k∑
i=1
`∗
(
µ(i)
)
, (1.1)
where
`∗(µ) := |µ| − `(µ) (1.2)
1A different multiparametric family of generating functions for weighted Hurwitz numbers overRP2, consisting
of BKP generating functions of hypergeometric type, was considered in [22, 23].
Multispecies Weighted Hurwitz Numbers 3
is the colength of the partition µ, i.e., the complement of the length `(µ) with respect to its
weight |µ|, or the degree of degeneracy of the branched cover over a point with ramification
profile type µ.
The Frobenius–Schur formula [7, 8, 19, 27] expresses these as sums over irreducible Sn cha-
racters
H
(
µ(1), . . . , µ(k)
)
=
∑
λ,|λ|=n
hk−2λ
k∏
i=1
χλ(µ(i))
zµ(i)
, (1.3)
where χλ(µ) is the character of the irreducible representation of symmetry type λ, evaluated on
the conjugacy class cyc(µ) consisting of elements with cycle lengths equal to the parts of the
partition µ,
hλ = det
(
1
(λi − i+ j)!
)−1
(1.4)
is the product of the hook lengths in the Young diagram of partition λ and
zµ =
∏
i∈N
imi(µ)(mi(µ))! (1.5)
is the order of the stabilizer under conjugation of any element of the conjugacy class cyc(µ),
with mi(µ) the number of parts of the partition µ equal to i,
There is an alternative interpretation of H
(
µ(1), . . . , µ(k)
)
that is purely combinatorial; it
equals 1
n! times the number of ways in which the identity element I ∈ Sn may be expressed as
a product of k elements belonging to the conjugacy classes of cycle type
{
cyc
(
µ(i)
)}
i=1,...,k
I = g1g2 · · · gk, where gi ∈ cyc
(
µ(i)
)
. (1.6)
The two are related by noting that each such factorization may be understood as defining the
image in Sn of the identity element in the fundamental group of the punctured sphere with
the branch points removed under the monodromy map obtained by lifting closed loops to the
covering surface.
1.2 Weighted geometrical Hurwitz numbers
As defined in [12], given a weight generating function G(z) expressible as an infinite product
G(z) =
∞∏
i=1
(1 + ciz), c = (c1, c2, . . . ), (1.7)
the weight WG
(
µ(1), . . . , µ(k)
)
assigned to a configuration of k+2 branch points with ramification
profiles
{
µ(1), . . . , µ(k), µ, ν
}
is solely determined by the colengths
{
`∗
(
µ(1)
)
, . . . , `∗
(
µ(k)
)}
, and
is given by evaluation of the monomial sum symmetric functions at the parameter values
WG
(
µ(1), . . . , µ(k)
)
:= mλ(c) =
1
|aut(λ)|
∑
σ∈Sk
∑
1≤i1<···<ik
c
`∗(µ(1))
iσ(1)
· · · c`
∗(µ(k))
iσ(k)
. (1.8)
Here λ is the partition of length `(λ) = k whose parts are
{
`∗
(
µ(i)
)}
i=1,...,k
, and |aut(λ)| is the
order of the automorphism group of λ
| aut(λ)| :=
`(λ∏
i=1
(mi(λ))!, (1.9)
where mi(λ) is the number of parts of λ equal to i.
4 J. Harnad
The geometrically defined double weighted Hurwitz numbers Hd
G(µ, ν) give a weighted enu-
meration of n-sheeted branched covers of the Riemann sphere that contain a pair of fixed branch
points, say at (0,∞), with ramification profile types given by the pair of partitions (µ, ν) and
a further set of k branch points with ramification profiles
(
µ(1), . . . , µ(k)
)
. They are defined by
the weighted sums
Hd
G(µ, ν) :=
∞∑
k=0
∑′
µ(1),...,µ(k)
k∑
i=1
`∗(µ(i))=d
WG
(
µ(1), . . . , µ(k)
)
H
(
µ(1), . . . , µ(k), µ, ν
)
, (1.10)
over all k-tuples of nontrivial ramification profiles satisfying the condition
d =
k∑
i=1
`∗
(
µ(i)
)
= |λ|, (1.11)
with weight WG
(
µ(1), . . . , µ(k)
)
given by (1.8).
The Riemann–Hurwitz formula for the genus g of the covering surface is then
2− 2g = `(µ) + `(ν)− d. (1.12)
An alternative is to use the dual weight generating function
G̃(z) =
∞∏
i=1
(1− c̃iz)−1, c̃ = (c̃1, c̃2, . . . ), (1.13)
for which the geometrical weight WG̃
(
µ(1), . . . , µ(k)
)
is given by the “forgotten” symmetric func-
tion fλ(c̃), c̃ = (c̃1, c̃2, . . . ),
WG̃
(
µ(1), . . . , µ(k)
)
:= fλ(c̃) =
(−1)`
∗(λ)
|aut(λ)|
∑
σ∈Sk
∑
1≤i1≤···≤ik
c̃
`∗(µ(1))
iσ(1)
· · · c̃`
∗(µ(k))
iσ(k)
, (1.14)
where the partition λ is again defined as above, with parts consisting of the colengths
{`∗(µ(i))}i=1,...,k. The dually weighted geometrical Hurwitz numbers are similarly defined by
the weighted sum
Hd
G̃
(µ, ν) :=
∞∑
k=0
∑′
µ(1),...,µ(k)
k∑
i=1
`∗(µ(i))=d
WG̃
(
µ(1), . . . , µ(k)
)
H
(
µ(1), . . . , µ(k), µ, ν
)
. (1.15)
1.3 Weighted combinatorial Hurwitz numbers
Following [11, 12], we may alternatively define a combinatorial Hurwitz number F dG(µ, ν) that
gives the weighted enumeration of d-step paths in the Cayley graph of the symmetric group Sn
generated by transpositions (a, b), b > a, starting at an element h ∈ cyc(µ) in the conjugacy
class cyc(µ) consisting of elements with cycle lengths equal to the parts of µ and ending in the
conjugacy class cyc(ν)
(adbd) · · · (a1b1)h ∈ cyc(ν). (1.16)
Every such path has a signature λ, which is defined to be the partition of weight d, whose parts
are, in weakly decreasing order, the number of times any given second element bi, i = 1, . . . , `(λ)
Multispecies Weighted Hurwitz Numbers 5
is repeated. In the case of the weight generating function G(z), we assign to any path with
signature λ a combinatorial weight equal to the product eλ(c) of the elementary symmetric
functions [20], evaluated at the parameters (ca, c2, . . . )
eλ(c) =
`(λ)∏
i=1
eλi(c). (1.17)
In the case of the dual generating functions G̃(z), we assign a combinatorial weight equal to the
product hλ(c) of the complete symmetric functions [20], evaluated at the parameters (c̃1, c̃2, . . . )
hλ(c̃) =
`(λ)∏
i=1
hλi(c̃). (1.18)
Let mλ
µν be the number of d = |λ| step paths of signature λ starting at h ∈ cyc(µ) and ending
in the conjugacy class cyc(ν). Then the combinatorial weighted Hurwitz numbers F dG(µ, ν),
F d
G̃
(µ, ν) are defined to be the weighted sums
F dG(µ, ν) :=
1
n!
∑
λ
eλ(c)mλ
µν , (1.19)
F d
G̃
(µ, ν) :=
1
n!
∑
λ
hλ(c̃)mλ
µν . (1.20)
In [12] it is proved that these two notions of weighted Hurwitz numbers in fact coincide:
Theorem 1.1 ([12]).
F dG(µ, ν) = Hd
G(µ, ν), F d
G̃
(µ, ν) = Hd
G̃
(µ, ν). (1.21)
The main idea behind the proof is to define associated elements Gn(w,J ) ∈ Z(C[Sn]) and
G̃n(z,J ) ∈ Z(C[Sn]) in the centre Z(C[Sn]) of the group algebra C[Sn] by
Gn(z,J ) :=
n∏
a=1
G(zJa), G̃(z,J ) :=
n∏
a=1
G̃(zJa), (1.22)
where J := (J1, . . . ,Jn) are the Jucys–Murphy elements [2, 18, 21]
J1 := 0, Jb :=
b−1∑
a=1
(ab), b = 1, . . . , n, (1.23)
which generate an abelian subalgebra of the centre Z(C[Sn]) of the group algebra C[Sn].
The elementsGn(z,J ) and G̃n(z,J ) define endomorphisms of Z(C[Sn]) under multiplication,
which are diagonal in the basis {Fλ} of Z(C[Sn]) consisting of the orthogonal idempotents,
corresponding to irreducible representations, labelled by partitions λ of n
Gn(z,J )Fλ = r
G(z)
λ Fλ, G̃n(z,J )Fλ = r
G̃(z)
λ Fλ, (1.24)
with eigenvalues of the following content product form
r
G(z)
λ (N) :=
∏
(i,j)∈λ
G(z(N + j − i)), r
G̃(z)
λ (N) :=
∏
(i,j)∈λ
G̃(z(N + j − i)). (1.25)
6 J. Harnad
On the other hand, the Cauchy–Littlewood generating function relation [20] and its dual
show that Gn(z,J ) and G̃n(z,J ) may be expanded in terms of dual bases of the algebra of
symmetric functions, evaluated either on the parameters c or on the Jucys–Murphy elements
J = (J1, . . . ,Jn)
Gn(z,J ) =
∑
λ, |λ|=n
eλ(c)mλ(J ) =
∑
λ, |λ|=n
mλ(c)eλ(J ), (1.26)
G̃n(z,J ) =
∑
λ, |λ|=n
hλ(c)mλ(J ) =
∑
λ, |λ|=n
fλ(c)eλ(J ), (1.27)
where eλ, hλ, mλ and fλ are the elementary, complete, monomial and “forgotten” symmetric
functions [20], respectively. Applying (1.26) and (1.27) to the basis for Z(C[Sn]) consisting of
the cycle sums
Cµ :=
∑
h∈cyc(µ)
h, (1.28)
and using the identities
ej(J ) =
∑
µ, `∗(µ)=j
Cµ and mλ(J )Cµ =
∑
ν, |ν|=|µ|
mλ
µνCν (1.29)
leads to (1.21).
The bases {Fλ}|λ|=n and {Cµ}|µ|=n are related by
Fλ = hλ
∑
µ, |µ|=|λ|
χλ(µ)Cµ, (1.30)
where χλ(µ) denotes the irreducible character of the irreducible representation of type λ eva-
luated on the conjugacy class cyc(µ). Under the characteristic map, this is equivalent to the
Frobenius character formula [20]
sλ =
∑
µ, |µ|=|λ|
z−1µ χλ(µ)pµ. (1.31)
1.4 Hypergeometric 2D Toda τ -functions as generating functions
As shown in [12], for any given generating function of type G(z) or G̃(z), there is a naturally
associated 2D Toda τ -function of hypergeometric type, expressible as a diagonal double Schur
function expansion
τG(z)(N, t, s) :=
∑
λ
r
G(z)
λ (N)sλ(t)sλ(s), (1.32)
τ G̃(z)(N, t, s) :=
∑
λ
r
G̃(z)
λ (N)sλ(t)sλ(s), (1.33)
where
t = (t1, t2, . . . ), s = (s1, s2, . . . ) (1.34)
are the 2D Toda flow variables, which may be identified in this notation with the power sums
ti =
pi
i
, si =
p′i
i
, N ∈ Z (1.35)
Multispecies Weighted Hurwitz Numbers 7
in two independent sets of variables. (See [20] for notation and further definitions involving
symmetric functions.) The coefficients have the standard content product form that characterize
such 2D τ -functions of hypergeometric kind
r
G(z)
λ (N) := r
G(z)
0 (N)
∏
(i,j)∈λ
G(z(N + j − i)), (1.36)
where
r
G(z)
0 (N) :=
N−1∏
j=1
G((N − j)z)j , r
G(z)
0 (0) := 1, (1.37)
r
G(z)
0 (−N) :=
N∏
j=1
G((j −N)z)−j , N ≥ 1, (1.38)
and identical formulae for G replaced by G̃.
The other main result of [12] is that the resulting τ -functions (1.32), (1.33), for N = 0
τG(z)(t, s) := τG(z)(0, t, s), τ G̃(z)(t, s) := τ G̃(z)(0, t, s), (1.39)
when expanded in the basis of tensor products of pairs of power sum symmetric functions {pµ},
using the Frobenius character formula (1.31), are generating functions for the weighted double
Hurwitz numbers.
Theorem 1.2 ([12]).
τG(z)(t, s) =
∞∑
d=0
∑
µ,ν,
|µ|=|ν|
zdHd
G(µ, ν)pµ(t)pν(s), (1.40)
τ G̃(z)(t, s) =
∞∑
d=0
∑
µ,ν,
|µ|=|ν|
zdHd
G̃
(µ, ν)pµ(t)pν(s). (1.41)
2 2D Toda τ -functions as generating functions
for multispecies weighted Hurwitz numbers
For any choice of weight generating functions G1(w1), G
2(w2), . . . , G̃
1(z1), G̃
2(z2), . . . , we may
form composites by using the product
∏
αG
α(wα)
∏
β G̃
β(zβ) as generating function for multiple
weighting types. The resulting content product coefficients r
∏
αG
α(wα)
∏
β G̃
β(wβ
λ are just the
product
∏
α r
Gα(wα)
λ
∏
β r
G̃β(zβ)
λ of the individual ones
r
Gα(wα)
λ :=
∏
(i,j)∈λ
Gα(wα(N + j − i)), r
G̃β(zβ)
λ :=
∏
(i,j)∈λ
G̃β(zβ(N + j − i)). (2.1)
We may also include weight factors in which some or all of the parameters (z1, z2, . . . ),
(w1, w2, . . . ) are repeated in the product. This only affects the constraints on the sums of the
colengths in the weighted multispecies Hurwitz numbers. (See, e.g., Example 3.3 in [12], in
which the weights are uniform, but the linear generating function that gives Hurwitz numbers
for Belyi curves and strictly monotonic paths is replaced by a power of the latter, resulting in
multiple branch points, with the total colength fixed, and multimononic paths.)
8 J. Harnad
2.1 The multiparameter family of τ -functions τG(l,m)(w,z)(t, s)
The multispecies partitions are divided into two classes: those corresponding to the weight
factors of type Gα(w), labelled
{
µ(α,uα)
}
1≤uα≤kα and those corresponding to dual type G̃β(z),
labelled
{
ν(β,vβ
}
1≤vβ≤k̃β
. These are further subdivided into l “colours”, or “species” for the first
class, denoted by the label α = 1, . . . , l and m in the second, denoted by β = 1, . . . ,m. Any
given configuration
{{
µ(α,uα)
}
1≤uα≤kα ,
{
ν(β,vβ
}
1≤vβ≤k̃β
}
has kα elements of colour α in the first
class and k̃β elements of colour β in the second class, for a total of
k =
l∑
α=1
kα +
m∑
β=1
k̃β (2.2)
partitions.
Denoting the l +m expansion parameters as
w = (w1, . . . , wl), z = (z1, . . . , zm), (2.3)
the multispecies weight generating function is formed from the product
G(l,m)(w, z) :=
l∏
α=1
Gα(wα)
m∏
β=1
G̃β(zβ), (2.4)
where each factor has an infinite product representation that is of one of the two types
Gα(w) =
∞∏
i=1
(
1 + cαi w
)
, α = 1, . . . , l, (2.5)
G̃β(w) =
∞∏
i=1
(
1− c̃βi w
)−1
, β = 1, . . . ,m, (2.6)
for l +m infinite sequences of parameters
cα =
(
cα1 , c
α
2 , . . .
)
, α = 1, . . . , l, c̃β =
(
c̃β1 , c̃
β
2 , . . .
)
, β = 1, . . . ,m. (2.7)
Following the approach developed in [12], we define an associated element G
(l,m)
n (w, z,J ) of
the center Z(C[Sn]) of the group algebra C[Sn] by
G(l,m)
n (w, z,J ) :=
n∏
a=1
(
l∏
α=1
Gα(wαJa)
) m∏
β=1
G̃β(zβJa)
, (2.8)
where J := (J1, . . . ,Jn) are the Jucys–Murphy elements (1.23) of C[Sn]. The element
G
(l,m)
n (w, z,J ) defines an endomorphism of Z(C[Sn]) under multiplication that is diagonal in
the basis {Fλ} of Z(C[Sn]) consisting of the orthogonal idempotents corresponding to irreducible
representations, labelled by partitions λ of n
G(l,m)
n (w, z,J )Fλ = r
G(l,m)(w,z)
λ Fλ. (2.9)
The eigenvalues are
r
G(l,m)(w,z)
λ =
l∏
α=1
r
Gα(wα)
λ
m∏
β=1
r
G̃β(zβ)
λ , (2.10)
Multispecies Weighted Hurwitz Numbers 9
where, as before,
r
Gα(wα)
λ :=
∏
(ij)∈λ
Gα(wα(j − i)), r
G̃(zβ)
λ :=
∏
(ij)∈λ
G̃β(zβ(j − i)). (2.11)
The associated hypergeometric τ -function is
τG
(l,m)(w,z)(t, s) =
∑
λ
r
G(l,m)(w,z)
λ sλ(t)sλ(s). (2.12)
2.2 Multispecies geometric weighted Hurwitz numbers
The weight assigned to a multispecies n-sheeted branched covering of the Riemann sphere with
a pair of branch points at, say, (0,∞) having ramification profiles (µ, ν) and a further
l∑
α=1
kα +
l∑
β=1
k̃β (2.13)
“coloured” branch points of two classes:
{
µ(α,uα)
}
α=1,...,l
uα=1,...,kα
(“class I” ) and
{
ν(β,vβ)
}
β=1,...,l
vβ=1,...,k̃β
(“class II”) and l + m colours {α = 1, . . . , l}, {β = 1, . . . ,m}, with {kα}α=1,...,l, {k̃β}β=1,...,m
points of the various colours (or “species”), is defined to be the product of those for single
species
WG(l,m)
({
µ(α,uα)
}
,
{
ν(β,vβ
})
=
l∏
α=1
mλ(α)
(
c(α)
) m∏
β=1
mλ̃(β)
(
c̃β
)
. (2.14)
Here the partitions
{
λ(α)
}
α=1,...,l
, and
{
λ̃
(β)
β=1,...,m
}
have parts equal to the colengths `∗
(
µ(α,uα)
)
and `∗
(
µ(β,vβ)
)
, for λ(α) and λ̃(β) respectively. Their lengths are
`
(
λ(α)
)
= kα, `
(
λ̃(β)
)
= k̃β (2.15)
and their weights∣∣λ(α))∣∣ = dα,
∣∣λ̃(β)∣∣ = d̃β (2.16)
are equal to the specified total colengths
d = (d1, . . . , dl), d̃ = (d1, . . . , d̃m), (2.17)
dα =
kα∑
uα=1
`∗
(
µ(α,uα)
)
, d̃β =
k̃β∑
vβ=1
`∗
(
ν(βvβ)
)
. (2.18)
The geometrically defined multispecies weighted Hurwitz numbers are
H
(d,d̃)
G(l,m)(µ, ν) :=
∑
k1,...,kl
∑
k̃1,...,k̃m
∑′
{µ(α,uα)}
|µ(α,uα)|=n
kα∑
uα=1
`∗(µ(α,uα))=dα
∑′
{ν(β,vβ)}
|ν(β,uβ)|=n
k̃β∑
vβ=1
`∗(ν
(β,vβ)
)=d̃β
×WG(l,m)
({
µ(α,uα)
}
,
{
ν(β,vβ)
})
H
({
µ(α,uα)
}
,
{
ν(β,vβ)
}
, µ, ν
)
, (2.19)
10 J. Harnad
which gives the weighted sum of the Hurwitz numbers of n-sheeted branched coverings with
l +m branch points of type
{{
µ(α,uα)
}
1≤uα≤kα ,
{
ν(β,vβ
}
1≤vβ≤k̃β
}
and (µ, ν) at (0,∞),
Substituting the Frobenius–Schur formula (1.3) and the Frobenius character formula (1.31)
into (2.12), it follows that τG
(l,m)(w,z)(t, s) is the generating function for H
(d,d̃)
G(l,m)(µ, ν). Using
multi-index notion to denote
l∏
α=1
wdαα
m∏
β=1
z
d̃β
β =: wdzd̃, (2.20)
we then have
Theorem 2.1.
τG
(l,m)(w,z)(t, s) =
∑
d∈N
wd
∑
d̃∈N
zd̃
∑
µ,ν
H
(d,d̃)
G(l,m)(µ, ν)pµ(t)pν(s). (2.21)
Proof. This follows, as in the single species case, by combining the eigenvalue formula (2.9)
with the Frobenius character formula. �
2.3 Multispecies combinatorial weighted Hurwitz numbers
The combinatorial multispecies weighted Hurwitz number F
(d,d̃)
G(l,m)(µ, ν) is defined as follows.
Let Dn be the number of partitions of n and let FdαGα and F
d̃β
G̃β
denote the Dn × Dn matrices
whose elements are F dαGα(µ, ν) and F
d̃β
G̃β
(µ, ν), respectively, as defined in (1.19), (1.20),
F dGα(µ, ν) :=
1
n!
∑
λ
eλ
(
cα
)
mλ
µν , F d
G̃β
(µ, ν) :=
1
n!
∑
λ
hλ
(
c̃β
)
mλ
µν , (2.22)
for each generating function Gα(wα) or G̃β(zβ). Since the central elements
{
Gαn(wα,J ),
G̃βn(zβ,J )
}
all commute, it follows that so do the matrices
{
FdαGα ,F
d̃β
G̃β
}
. Denoting their product,
in any order,
F
(d,d̃)
G(l,m) :=
l∏
α=1
FdαGα
m∏
β=1
F
d̃β
G̃β
, (2.23)
the (µ, ν) matrix element F
(d,d̃)
G(l,m)(µ, ν) is the combinatorial multispecies weighted Hurwitz num-
ber.
The combinatorial meaning of F
(d,d̃)
G(l,m)(µ, ν) is as follows. Let
d :=
l∑
α=1
dα +
m∑
β=1
d̃β. (2.24)
Then F
(d,d̃)
G(l,m)(µ, ν), may be interpreted as the weighted sum over all sequences of d step paths
in the Cayley graph from an element h ∈ cyc(µ) in the conjugacy class of cycle type µ to
one (adbd) · · · (a1b1)h ∈ cyc(ν) in the class cyc(ν), in which the transpositions appearing are
subdivided into subsets consisting of (d1, . . . , dl, d̃1, . . . , d̃m) transpositions in all d!
l∏
α=1
dα!
l∏
β=1
d̃β !
possible ways. All paths are divided into equivalence classes, according to their multisignatures
Multispecies Weighted Hurwitz Numbers 11{
λ(α), λ̃(β)
}
α=1,...,l
β=1,...,m
. These consist of a partition of the d steps into l + m parts, each of which
is a subsequence assigned a “colour” and a “class” with l of them of the first class and m of
the second. The number of partitions of first class with colour α is dα while the number of
second class with colour β is d̃β. The partitions λ(α) of weights dα are defined to have parts{
λ
(α)
uα
}
uα=1,...,kα
equal to the number of transpositions appearing within that subsequence having
the same second element, and similarly for
{
λ̃
(β)
vβ
}
vβ=1,...,k̃β
with
kα = `
(
λ(α)
)
, k̃β = `
(
λ̃(β)
)
(2.25)
the number of such parts.
The weight given to any such multisignatured path is the product
l∏
α=1
eλ(α)
(
cα
) m∏
β=1
hλ̃(β)
(
c̃β
)
(2.26)
of the weights along each segment, and F
(d,d̃)
G(l,m)(µ, ν) is the sum of these, each multiplied by the
number of elements of the equivalence class of paths with the given multisignature.
The multispecies generalization of (1.21) is equality of the geometric and combinatorial Hur-
witz numbers:
Theorem 2.2.
F
(d,d̃)
G(l,m)(µ, ν) = H
(d,d̃)
G(l,m)(µ, ν), F
(d,d̃)
G̃(l,m)
(µ, ν) = H
(d,d̃)
G̃(l,m)
(µ, ν). (2.27)
Proof. Applying the central element G
(l,m)
n (w, z,J ) defined in (2.8) to the cycle sum Cµ and
applying (1.26) for each factor in the product gives
G(l,m)
n (w, z,J )Cµ =
∑
ν, |ν|=|µ|
F
(d,d̃)
G(l,m)(µ, ν)Cν =
∑
ν, |ν|=|µ|
H
(d,d̃)
G(l,m)(µ, ν)Cν , (2.28)
and the similar formula for G̃. Equation (2.21), together with the Frobenius character formula,
shows that the generating τ -function can be expressed as
τG
(l,m)(w,z)(t, s) =
∑
d∈N
wd
∑
d̃∈N
zd̃
∑
µ,ν
F
(d,d̃)
G(l,m)(µ, ν)pµ(t)pν(s). (2.29)
Comparing this with equation (2.21) proves the result. �
3 Multispecies quantum Hurwitz numbers
3.1 Quantum Hurwitz numbers
Amongst the examples of weighted Hurwitz numbers studied in [12], four special classes were
introduced in which the generating functions G(z), G̃(z) were chosen as a variant of the quantum
dilogarithm [6]. This meant that the parameters {ci} were chosen as powers of a quantum
deformation parameter q. As shown in [12] , a suitable interpretation of the parameter q in terms
of Planck’s constant ~ and Boltzmann factors for a Bosonic gas with linear energy spectrum
leads to a relation between the resulting weighted counting of branched cover and the energy
distribution for a Bosonic gas, which further justifies terming these “quantum” Hurwitz numbers.
12 J. Harnad
In the first case, the weight generating function is
E(q, z) :=
∞∏
i=0
(
1 + qiz
)
= 1 +
∞∑
i=1
Ei(q)z
i, Ei(q) :=
i∏
j=1
qj−1
1− qj
, (3.1)
and hence the parameters ci are identified as {ci := qi−1}i∈N+ . The second is a slight modifi-
cation of this, with weight generating function
E′(q, z) :=
∞∏
i=1
(
1 + qiz
)
= 1 +
∞∑
i=1
E′i(q)z
i, E′i(q) :=
i∏
j=1
qi
1− qj
, (3.2)
i.e., the zero power q0 is omitted, and {ci := qi}i∈N+ .
The third case is based on the weight generating function
H(q, z) :=
∞∏
i=0
(
1− qiz
)−1
= 1 +
∞∑
i=1
Hi(q)z
i, Hi(q) :=
i∏
j=1
1
1− qj
, (3.3)
and hence is the dual of the first case, with {c̃i := qi−1}i∈N+ . The final case is a hybrid, formed
from the product of the first and third for two distinct quantum deformation parameters q and p,
with weight generating function
Q(q, p, z) :=
∞∏
k=0
(
1 + qkz
)(
1− pkz
)−1
=
∞∑
i=0
Qi(q, p)z
i, (3.4)
Qi(q, p) :=
i∑
m=0
q
1
2
m(m−1)
m∏
j=1
(
1− qj
) i−m∏
j=1
(1− pj)
−1, Qλ(q, p) =
`(λ)∏
i=1
Qλi(q, p). (3.5)
These are all expressible as exponentials of the quantum dilogarithm function
Li2(q, z) :=
∞∑
k=1
zk
k(1− qk)
, (3.6)
E(q, z) = e−Li2(q,−z), E′(q, z) = (1 + z)−1e−Li2(q,−z), (3.7)
H(q, z) = eLi2(q,z), Q(q, p, z) = eLi2(p,z)−Li2(q,−z). (3.8)
The content product formulae for the first and third of these are
r
E(q,z)
λ (N) :=
∏
(ij)∈λ
E(q, (N + j − i)z), r
H(q,z)
λ (N) :=
∏
(ij)∈λ
H(q, (N + j − i)z). (3.9)
The associated hypergeometric 2D Toda τ -functions have diagonal double Schur function ex-
pansions with these as coefficients
τE(q,z)(N, t, s) =
∑
λ
r
E(q,z)
λ (N)Sλ(t)Sλ(s), (3.10)
τH(q,z)(N, t, s) =
∑
λ
r
H(q,z)
λ (N)Sλ(t)Sλ(s)). (3.11)
Using the Frobenius character formula (1.31), and setting N = 0, they may be rewritten as
double expansions in the power sum symmetric functions [12]
τE(q,z)(t, s) := τE(q,z)(0, t, s) =
∞∑
d=0
zd
∑
µ,ν, |µ|=|ν|
Hd
E(q)(µ, ν)pµ(t)pν(s), (3.12)
Multispecies Weighted Hurwitz Numbers 13
τH(q,z)(t, s) := τH(q,z)(0, t, s) =
∞∑
d=0
zd
∑
µ,ν, |µ|=|ν|
Hd
H(q)(µ, ν)pµ(t)pν(s). (3.13)
The coefficients are the corresponding quantum Hurwitz numbers Hd
E(q)(µ, ν), Hd
H(q)(µ, ν),
which count weighted n-sheeted branched coverings of the Riemann sphere, defined by
Hd
E(q)(µ, ν) :=
∞∑
k=0
∑′
µ(1),...,µ(k)
k∑
i=1
`∗(µ(i))=d
WE(q)
(
µ(1), . . . , µ(k)
)
H
(
µ(1), . . . , µ(k), µ, ν
)
, (3.14)
Hd
H(q)(µ, ν) :=
∞∑
k=0
(−1)k+d
∑′
µ(1),...,µ(k)
k∑
i=1
`∗(µ(i))=d
WH(q)
(
µ(1), . . . , µ(k)
)
H
(
µ(1), . . . , µ(k), µ, ν
)
, (3.15)
where the weightings for such covers with k additional branch points are [12]
WE(q)
(
µ(1), . . . , µ(k)
)
=
1
|aut(λ)|
∑
σ∈Sk
q(k−1)`
∗(µ(1)) · · · q`∗(µ(k−1))(
1− q`∗(µ(σ(1)))
)
· · ·
(
1− q`∗(µ(σ(1)))
)
· · · q`∗(µ(σ(k)))
, (3.16)
WH(q)
(
µ(1), . . . , µ(k)
)
=
(−1)`
∗(λ)
|aut(λ)|
∑
σ∈Sk
1(
1− q`∗(µ(σ(1)))
)
· · ·
(
1− q`∗(µ(σ(1)))
)
· · · q`∗(µ(σ(k)))
. (3.17)
Here, as in (1.8) and (1.14), λ is the partition of length `(λ) = k whose parts are
{
`∗
(
µ(i)
)}
i=1,...,k
,
and |aut(λ)| is the order of the automorphism group of λ . The sum
∑′
µ(1),...,µ(k)
k∑
i=1
`∗(µ(i))=d
is over
all k-tuples of partitions having nontrivial ramification profiles that satisfy the constraint
k∑
i=1
`∗(µ(i)) = d, and H
(
µ(1), . . . , µ(k), µ, ν
)
is the number of branched n-sheeted coverings, up to
isomorphism, having k + 2 branch points with ramification profiles
(
µ(1), . . . , µ(k), µ, ν
)
.
These thus count weighted covers with a pair of branch points, say (0,∞), having ramification
profiles of type (µ, ν) and an arbitrary number of further branch points, whose profiles (µ(1),
. . . , µ(k)) are constrained only by the requirement that the sum of the colengths, which is related
to the genus by the Riemann–Hurwitz formula
k∑
i=1
`∗(µ(i)) = 2g − 2 + `(µ) + `(ν) = d, (3.18)
be fixed to equal d.
The combinatorial interpretation of the quantum Hurwitz numbers F dE(q)(µ, ν) and F dH(q)(µ, ν)
appearing in (3.13) is as follows. Let (a1b1) · · · (adbd) be a product of d transpositions (aibi) ∈ Sn
in the symmetric group Sn with ai < bi, i = 1, . . . , d. If h ∈ Sn is in the conjugacy class
cyc(µ) ⊂ Sn, we may view the successive steps in the product
(a1b1) · · · (adbd)h (3.19)
as a path in the Cayley graph generated by all transpositions, whose signature is the partition λ
of d, |λ| = d, whose parts λi consist of the number of transpositions (aibi) sharing the same
14 J. Harnad
second element. If we further require that the ones with equal second elements be grouped
together into consecutive subsequences in which these second elements are constant, with the
consecutive subsequences strictly increasing in their second elements, then the number Ñλ of
elements with signature λ is related to the number Nλ of such ordered sequences by
Ñλ =
|λ|!
`(λ)∏
i=1
λi!
Nλ. (3.20)
Denote the number of such paths from the conjugacy class of cycle type cyc(µ) to the one of
type cyc(ν) having signature λ as m̃λ
µν , and the number of ordered sequences of type λ as mλ
µν .
Thus
m̃λ
µν =
|λ|!
`(λ)∏
i=1
λi!
mλ
µν . (3.21)
For all paths of signature λ we assign the weights
Eλ(q) :=
`(λ)∏
i=1
Eλi(q) =
`(λ)∏
i=1
q
1
2
λi(λi−1)
λi∏
j=1
(1− qj)
, Hλ(q) :=
`(λ)∏
i=1
Hλi(q) =
`(λ)∏
i=1
1
λi∏
j=1
(1− qj)
(3.22)
to paths in the Cayley graph, and obtain the pair of corresponding combinatorial weighted
Hurwitz numbers
F dE(q)(µ, ν) :=
1
n!
∑
λ, |λ|=d
Eλ(q)mλ
µν , (3.23)
F dH(p)(µ, ν) :=
1
n!
∑
λ, |λ|=d
Hλ(q)mλ
µν , (3.24)
that give the weighted enumeration of paths, using the weighting factors Eλ(q) and Hλ(q)
respectively for all paths of signature λ.
As shown in general in [12], the enumerative geometrical and combinatorial definitions of
these quantum weighted Hurwitz numbers coincide:
Hd
E(q)(µ, ν) = F dE(q)(µ, ν), Hd
H(q)(µ, ν) = F dH(q)(µ, ν). (3.25)
A similar result holds for weights generated by the function E′(q, z), with corresponding quantum
Hurwitz numbers defined by
Hd
E′(q)(µ, ν) :=
∞∑
k=0
∑′
µ(1),...,µ(k)
k∑
i=1
`∗(µ(i))=d
WE′(q)
(
µ(1), . . . , µ(k)
)
H
(
µ(1), . . . , µ(k), µ, ν
)
, (3.26)
where the weights WE′(q)(µ
(1), . . . , µ(k)) are given by
WE′(q)
(
µ(1), . . . , µ(k)
)
:=
1
k!
∑
σ∈Sk
q(k)`
∗(µ(1)) · · · q`∗(µ(k))(
1− q`∗(µ(σ(1)))
)
· · ·
(
1− q`∗(µ(σ(1)))
)
· · · q`∗(µ(σ(k)))
(3.27)
:=
1
k!
∑
σ∈Sk
1(
q−`∗(µ
(σ(1))) − 1
)
· · ·
(
q−`∗(µ
(σ(1))) · · · q−`∗(µ(σ(k))) − 1
) .
Multispecies Weighted Hurwitz Numbers 15
Choosing q as a positive real number, parametrizing it as
q = e−β~ω0 , β =
1
kT
(3.28)
and identifying the energy levels εk as those for a Bose gas with linear spectrum in the integers,
as for a 1−D harmonic oscillator
εk := k~ω0, k ∈ N, (3.29)
it follows that if we assign the energy
ε(µ) := ε`∗(µ) = `∗(µ)~ω (3.30)
to each branch point with ramification profile of type µ, it contributes a factor
n(µ) =
1
eβε(µ) − 1
(3.31)
to the weighting distributions, the same as that for a bosonic gas.
3.2 The multiparameter family of τ -functions τQ(q;w;p,z)(N, t, s)
We now consider the multiparameter family of weight generating functionsQ(q,w;p, z) obtained
by taking the product of any number of the generating functions E(qi, zi) and H(pj , wj) for
distinct sets of generating function parameters w = (w1, . . . , wl), z = (z1, . . . , zm), and quantum
parameters q = (q1, . . . , ql), p = (p1, . . . , pm)
Q(q,w;p, z) :=
l∏
α=1
E(qα, wα)
m∏
β=1
H(pβ, zβ). (3.32)
Following the approach developed in [12], we define an associated element of the center
Z(C[Sn]) of the group algebra C[Sn] by
Qn(q,w;p, z,J ) :=
n∏
a=1
Q(q,Jaw;p,Jaz), (3.33)
where J := (J1, . . . ,Jn) are again the Jucys–Murphy elements (1.23) of C[Sn]. The element
Qn(q,w;p, z,J ) defines an endomorphism of Z(C[Sn]) under multiplication, which is diago-
nal in the basis {Fλ} of Z(C[Sn]) consisting of the orthogonal idempotents, corresponding to
irreducible representations, labelled by partitions λ of n
Qn(q,w;p, z,J )Fλ = r
Q(q,w;p,z)
λ Fλ, (3.34)
where
r
Q(q,w;p,z)
λ =
l∏
α=1
r
E(qα)
λ (wα)
m∏
β=1
r
H(pβ)
λ (zβ). (3.35)
More generally, defining
r
Q(q,w;p,z)
λ (N) =
l∏
α=1
r
E(qα,wα)
λ (N)
m∏
β=1
r
H(pβ ,,zβ)
λ (N), (3.36)
16 J. Harnad
where
r
E(qα,wα)
λ (N) :=
∏
(ij)∈λ
E(qα, (N + j − i)wα), (3.37)
r
H(pβ ,zβ)
λ (N) :=
∏
(ij)∈λ
H(pβ, (N + j − i)zβ), (3.38)
we have
r
Q(q,z;p,w)
λ = r
Q(q,z;p,w)
λ (0). (3.39)
The double Schur function series
τQ(q,z;p,w)(N, t, s) :=
∑
λ
r
Q(q,z;p,w)
λ (N)Sλ(t)Sλ(s) (3.40)
then defines a family of 2D Toda τ -functions of hypergeometric type.
3.3 Multispecies geometric quantum Hurwitz numbers
We now consider coverings in which the branch points are divided, as above, into two different
classes,
{
µ(α,uα)
}
α=1,...,l;uα=1,...,kα
and
{
ν(β,vβ)
}
β=1,...,m; vβ=1,...,k̃β
, corresponding to weight gene-
rating functions of type E(qα) and H(pβ) respectively, each of which is further divided into l
and m distinct species (or “colours”), of which there are
{
kα
}
and
{
k̃β
}
branch points of types E
and H and colours α and β respectively. The weighted number of such coverings with specified
total colengths d = (d1, . . . , dl), d̃ = (d1, . . . , d̃m), dα, d̃β ∈ N
dα =
kα∑
uα=1
`∗
(
µ(α,uα)
)
, d̃β =
k̃β∑
vβ=1
`∗
(
ν(βvβ)
)
(3.41)
for each class and colour is the multispecies quantum Hurwitz number
H
(d,d̃)
(q,p) (µ, ν) :=
∞∑
{kα=1; k̃β=1}
α=1,...,l; β=1,...,m
∑{
µ(α,uα); ν
(β,vβ)
}
kα∑
uα=1
`∗
(
µ(α,uα)
)
=dα,
k̃β∑
vβ=1
`∗
(
ν
(β,vβ)
)
=d̃β
×
l∏
α=1
WE(qα)
(
µ(α,1), . . . , µ(α,kα)
)
×
m∏
β=1
WH(pβ)
(
ν(β,1), . . . , ν(β,k̃β)
)
H
({
µ(α,uα); ν(β,vβ)
}
, µ, ν
)
. (3.42)
Substituting the Frobenius–Schur formula (1.3) and the Frobenius character formula (1.31)
into (3.40), it follows that
τQ(q,z;p,w)(t, s) := τQ(q,z;p,w)(0, t, s) (3.43)
is the generating function for H
(c,d)
(q,p)(µ, ν). Using multi-index notion to denote
l∏
α=1
wdαα
m∏
β=1
z
d̃β
β =: wdzd̃, (3.44)
we have
Multispecies Weighted Hurwitz Numbers 17
Theorem 3.1.
τQ(q,w;p,z)(t, s) :=
∑
d∈N
wd
∑
d̃∈N
zd̃
∑
µ,ν
H
(d,d̃)
(q,p) (µ, ν)pµ(t)pν(s). (3.45)
3.4 Multispecies combinatorial quantum Hurwitz numbers
Another basis for Z(C[Sn]) consists of the cycle sums
Cµ :=
∑
h∈cyc(µ)
h, (3.46)
where cyc(µ) denotes the conjugacy class of elements h ∈ cyc(µ) with cycle lengths equal to the
parts of µ. The two are related by
Fλ = h−1λ
∑
µ, |µ|=|λ|
χλ(µ)Cµ, (3.47)
where χλ(µ) denotes the irreducible character of the irreducible representation of type λ eva-
luated on the conjugacy class cyc(µ). Under the characteristic map, this is equivalent to the
Frobenius character formula (1.31).
Let Dn denote the number of partitions of n and F
(n,dα)
E(qα)
, F
(n,d̃β)
H(pβ)
the Dn×Dn matrices whose
elements are
(
F dαE(qα)
(µ, ν)
)
|µ|=|ν|=n and
(
F
d̃β
H(pβ)
(µ, ν)
)
|µ|=|ν|=n, respectively, for α = 1, . . . , l,
β = 1, . . . ,m as defined in (3.23), (3.24). Since these represent central elements of the group
algebra C[Sn], they commute amongst themselves. Defining the matrix product
F
(d,d̃)
(q,p) =
l∏
α=1
F
(n,dα)
E(qi)
m∏
β=1
F
(n,d̃β)
H(pβ)
, (3.48)
its matrix elements, denoted F
(c,d)
(q,p)(µ, ν), may be interpreted as the weighted number of
d :=
α∑
α=1
dα +
m∑
β=1
d̃β (3.49)
step paths in the Cayley graph from the conjugacy class of cycle type µ to the one of type ν,
where all paths are divided into equivalence classes, according to their multisignatures{
λ(α), λ̃(β)
}
α=1,...,l
β=1,...,m
.
These consist of a partition of the d steps into l + m parts, each of which is a subsequence
assigned a “colour” and a “class” with l of them of the first class and m of the second. The
number of partitions of first class with colour α is dα while the number of second class with
colour β is d̃β. The partitions λ(α) of weights dα are defined to have parts
{
λ
(α)
uα
}
uα=1,...,kα
equal
to the number of a transposition appears within that subsequence, having the same second
element, and similarly for
{
λ̃
(β)
vβ
}
vβ=1,...,k̃β
with
kα = `
(
λ(α)
)
, k̃β = `
(
λ̃(β)
)
(3.50)
the number of such parts.
For each such subpartition, the weight assigned is the product of the weights for each sub-
segment
l∏
α=1
Eλ(α)(qα, wα)
m∏
β=1
H ˜(λ)
β (pβ, zβ) (3.51)
18 J. Harnad
and F
(c,d)
(q,p)(µ, ν) is the sum of these, each multiplied by the number of elements of the equivalence
class of paths with the given multisignature
F
(c,d)
(q,p)(µ, ν) = (n!)−l−m
∑
µ(2),...,µ(m+l)
l∏
α=1
∑
{λ(α)}
Eλ(α)(qα, wα)mλ(α)
µαµα+1
×
m∏
β=1
∑
{λ̃(β)}
Hλ̃(β)(pβ, zβ)mλ(β)
µβ+l,µβ+l+1
, (3.52)
where (µ, ν) :=
(
µ(1), µ(l+m+1)
)
.
The multispecies generalization of (3.25) is equality of the geometric and combinatorial Hur-
witz numbers:
Theorem 3.2.
F
(c,d)
(q,p)(µ, ν) = H
(c,d)
(q,p)(µ, ν). (3.53)
Proof. Applying the central element (3.34) to the cycle sum Cµ gives
Q̂(q, z;p,w,J )Cµ =
∑
ν, |ν|=|µ|
F
(c,d)
(q,p)(µ, ν)Cν (3.54)
and the orthogonality of group characters implies that
τQ(q,z;p,w)(t, s) :=
(∞,...,∞)∑
c=(0,...,0);d=(0,...,0)
zcwd
∑
µ,ν
F
(c,d)
(q,p)(µ, ν)pµ(t)pν(s). (3.55)
Comparing this with equation (3.45) proves the result. �
Remark 3.3 (multispecies Bose gases). Returning to the interpretation of the quantum Hurwitz
weighting distributions in terms of Bose gases, if we choose each qi to be a positive real number
with qi < 1, and parametrize it, as before,
qi = e−β~ωi (3.56)
for some ground state energy ~ωi, and again choose a linear energy spectrum, with energy
assigned to the branchpoint µ of type i with profile type µ to be
ε(i)(µ) := `∗(µ)~ωi, (3.57)
we see that the resulting contributions to the weighting distributions distributions from each
species of branch points of ramification type µ(j) are given by
n
(i)
B (µ) =
1
eβε
(i)(µ) − 1
, (3.58)
which are those of a multispecies mixture of Bose gases.
Acknowledgments
This work is an extension of a joint project [11, 12] with M. Guay-Paquet, in which the notion of
infinite parametric families of weighted Hurwitz numbers was first introduced, combined with
the notion of signed multispecies Hurwitz numbers as introduced in [15] with A.Yu. Orlov. The
author would like to thank both these co-authors for helpful discussions. Work supported by
the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Fonds de
recherche du Québec – Nature et technologies (FRQNT).
Multispecies Weighted Hurwitz Numbers 19
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1 Introduction
1.1 Hurwitz numbers
1.2 Weighted geometrical Hurwitz numbers
1.3 Weighted combinatorial Hurwitz numbers
1.4 Hypergeometric 2D Toda -functions as generating functions
2 2D Toda -functions as generating functions for multispecies weighted Hurwitz numbers
2.1 The multiparameter family of -functions G(l,m)(w,z)(t,s)
2.2 Multispecies geometric weighted Hurwitz numbers
2.3 Multispecies combinatorial weighted Hurwitz numbers
3 Multispecies quantum Hurwitz numbers
3.1 Quantum Hurwitz numbers
3.2 The multiparameter family of -functions Q(q; w; p, z) (N,t, s)
3.3 Multispecies geometric quantum Hurwitz numbers
3.4 Multispecies combinatorial quantum Hurwitz numbers
References
|