OVERPARTITIONS AND GENERATING FUNCTIONS FOR GENERALIZEDFROBENIUS PARTITIONSSYLVIE CORTEEL, JEREMY LOVEJOY, AND AE JA YEE
Abstract. Generalized Frobenius partitions, or F -partitions, have recently played an important role in several combinatorial investigations of basic hypergeometric series identities. The
goal of this paper is to use the framework of these investigations to interpret families of infinite
products as generating functions for F -partitions. We employ q-series identities and bijective
combinatorics.

1. Introduction
Let PA,B (n) denote the number of generalized Frobenius partitions of n, i.e., the number of
two-rowed arrays,¶µ
a1 , a2 , . . . , am,
(1.1)
b1 , b2 , . . . , bm
Pin which the top (bottom) row is a partition from a set A (B), and such that (ai + bi ) + m = n[2]. The classical example is the case PD,D (n), where D is the set of partitions into distinct
non-negative parts. Frobenius observed that these objects are in one-to-one correspondencewith the ordinary partitions of n, giving
∞
∞
X
Y
1
n
PD,D (n)q =.
(1.2)(1 − q n )
n=0

n=1

Andrews [2] later made an extensive study of two infinite families of F -partitions that begin
with PD,D (n). He replaced D by Dk or Ck , the set of partitions where parts repeat at most ktimes and the set of partitions into distinct parts with k colors, respectively. The generating
functions are multiple theta series, which in three known cases can be written as an infinite
product.
∞
X
n=0
∞
X

PD2 ,D2 (n)q n =(q 2 ; q 2 )∞ (−q 3 ; q 6 )∞,(q)2∞ (−q; q 2 )∞(1.3)

PD3 ,D3 (n)q n =(q 6 ; q 6 )∞ (q 6 ; q 12 )2∞ (q 2 ; q 2 )∞ (q; q 2 )2∞,(q)3∞ (q 3 ; q 6 )2∞(1.4)

PC2 ,C2 (n)q n =(−q; q 2 )2∞.
(q)∞ (q; q 2 )∞

(1.5)

n=0
∞
X
n=0

Date: September 19, 2004.
The second author was partially supported by the European Commission’s IHRP Programme, grant HPRNCT-2001-00272, “Algebraic Combinatorics in Europe”.
1

2

SYLVIE CORTEEL, JEREMY LOVEJOY, AND AE JA YEEHere we have employed the standard notation(a1 , . . . , aj )∞ := (a1 , . . . , aj ; q)∞ :=∞Y(1 − a1 q k ) · · · (1 − aj q k ).

(1.6)

k=0

While Andrews’ families subsequently received quite a bit of attention [10, 13, 15, 16, 18, 20],other types of Frobenius partitions have recently been turning up as novel interpretations forsome infinite products that figure prominently in basic hypergeometric series identities [6, 7,
8, 21]. The combinatorial setting here is that of overpartitions, which are partitions wherein
the first occurrence of a part may be overlined. Let O denote the set of overpartitions into
non-negative parts. Then it turns out thatX(−bq)∞(1.7)
PD,O (m, n)bm q n =
(q)∞
m,n≥0

and

X

PO,O (`, m, n)a` bm q n =`,m,n≥0(−aq, −bq)∞.
(q, abq)∞(1.8)

Here PD,O (m, n) denotes the number of F -partitions counted by PD,O (n) that have m nonoverlined parts in the bottom row, and PO,O (`, m, n) denotes the number of objects counted by
PO,O (n) that have ` non-overlined parts in the top row and m non-overlined parts in the bottom
row.
Since a thorough combinatorial understanding of (1.7) and (1.8) has been so useful, we givein this paper a variety of other infinite product generating functions for F -partitions and begin
to study them using bijective combinatorics. The first goal is to use restricted overpartitions
and a useful property of the 1 ψ1 summation (see Lemma 2.2) to embed some of (1.2) – (1.8) infamilies of infinite products that generate F -partitions.
Theorem 1.1. Let Ok be the set of overpartitions where the non-overlined parts occur lessthan k times. Let POk ,Ok (m, n) (resp. POk ,O (m, n)) be the number of F -partitions counted by
POk ,Ok (n) (resp. POk ,O (n)) wherein the number of overlined parts on the top minus the numberof overlined parts on the bottom is m. ThenX

POk ,Ok (m, n)bm q n =(−bq)∞ (−q/b)∞ (q k ; q k )∞,(q)2∞ (−bq k , −q k /b; q k )∞(1.9)

POk ,O (m, n)bm q n =(−bq)∞ (−q/b)∞ (q k ; q k )∞.
(q)2∞ (−q k /b; q k )∞(1.10)

m,n≥0

X
m,n≥0

Notice that in both instances the case k → ∞ is the case a = 1/b of (1.8), while the casek = 1 of (1.9) is Frobenius’ example (1.2), the case b = 1, k = 2 of (1.9) is Andrews’ (1.5), andthe case k = 1 of (1.10) is (1.7).
Our next object is to exhibit more families like those above, but where the base cases arenone of (1.2) – (1.8). We use the notation AB for the set of vector partitions (λA , λB ) ∈ A × B,
and Dk for the set of partitions into non-negative parts where each part occurs 0 or k times.

OVERPARTITIONS AND GENERATING FUNCTIONS FOR GENERALIZED FROBENIUS PARTITIONS

Theorem 1.2.

∞
X

POk ,ODk (n)q n =

n=0
∞
X

POk ,O2k (n)q n =n=0

3(−q)2∞ k k(q ; q )∞ (q 2k ; q 4k )∞ ,(q)2∞

(1.11)(−q)2∞ (q k ; q k )∞ (q k ; q 2k )2∞.
(q)2∞ (q k , q 4k ; q 5k )∞(1.12)

Then, by employing more general q-series identities, we find generating functions with moreparameters, like Theorems 1.3 – 1.6 below. The first two contain the k = 1 case of (1.11) andthe case k = 2 of (1.10), respectively.
Theorem 1.3. Let PD,OD (m, n) be the number of F -partitions counted by PD,OD (n) that havem parts in λD .Then
X(−q; q)∞ (−yq; q 2 )∞PD,OD (m, n)y m q n =.
(1.13)(q; q)∞
m,n≥0

Theorem 1.4. Let O2 denote the number of overpartitions in O where the non-overlined parts
repeat an even number of times. Let PO,O2 D (`, m, n) denote the number of F -symbols counted
by PO,O2 D (n) where ` is the number of non-overlined parts in the top row minus the number ofparts in λD and m is the number of non-overlined parts in the bottom row. Then
X(−aq)∞ (−q/a, −ab2 q 2 ; q 2 )∞PO,O2 D (`, m, n)a` bm q n =.
(1.14)(q)∞ (q, a2 b2 q 2 ; q 2 )∞`,m,n≥0

The next theorem also contains the case k = 2 of (1.10). We are concerned here with D̄,
which denotes the set of overpartitions into distinct parts such that parts have to differ by atleast two if the bigger is overlined and 0 does not occur. These overpartitions have recently
arisen in a number of works [5, 14, 17].
Theorem 1.5. Let POD̄,O (`, m, n) denote the number of overpartitions counted by POD̄,O (n)such that ` is the number of non-overlined parts in λO plus the number of overlined parts in λD̄and such that m is the number of non-overlined parts on the bottom minus the number of partsin λD̄ . Then
X(−aq, −bq)∞ (−q/b; q 2 )∞POD̄,O (`, m, n)a` bm q n =.
(1.15)(q, abq)∞ (q; q 2 )∞`,m,n≥0

The last example contains (1.8) and deals with O0 , the set of overpartitions in O that have
no 0.
Theorem 1.6. Let POO0 ,O (k, `, m, n) denote the number of F -partitions counted by POO0 ,O (n)where k is the number of non-overlined parts in λO plus the number of overlined parts in λO0 ,` is the number of non-overlined parts in the bottom row, and m is the number of parts in λO0 .
ThenX(−aq, −bq, −cq)∞.
(1.16)
POO0 ,O (k, `, m, n)ak b` cm q n =(q, abq, bcq)∞k,`,m,n≥0

Finally, we give bijective proofs for some of the generating functions above. We are able toestablish (1.5), (1.13), and the case k = 2 of (1.10) in this way.

4

SYLVIE CORTEEL, JEREMY LOVEJOY, AND AE JA YEE

2. Recollections and Proofs
Given a set A of partitions we denote by PA (n, k) the number of partitions of n from the set
A having k parts. We recall from [2] that
Lemma 2.1. The generating function for Frobenius partitions is given by∞
X
X
X
PA,B (n)q n = [z 0 ]PA (n, k)q n (zq)k
PB (n, k)q n z −k ,where [z k ]

P

n=0

n,k

(2.1)

n,k

An z n = Ak .

We assume enough familiarity with the elementary theory of partitions and overpartitions[1, 8] that we can state generating functions for simple PA (n, k) without explanation. The
following is the key lemma mentioned in the introduction.
Lemma 2.2. If[z 0 ](−bzq, −1/bz)∞(−bq, −q/b)∞G(z, q) =H(q),(zq, 1/z)∞
(q)2∞

(2.2)

then

(−bzq, −1/bz)∞(−bq, −q/b)∞
G(z k , q k ) =H(q k ).
(2.3)
(zq, 1/z)∞
(q)2∞
P
P
Proof. Let H(q) = [z 0 ]F (z, q)G(z, q), with F (z, q) =Aj (q)z j and G(z, q) =Bj (q)z j . If
Akj (q) = Aj (q k ), thenXX
[z 0 ]F (z, q)G(z k , q k ) = [z 0 ]Aj (q)z j
Bj (q k )z kj
X=A−kj (q)Bj (q k )X=
A−j (q k )Bj (q k )[z 0 ]= H(q k ).
The proof is finished when we apply the above observation toX (1 + b)q j
F (z, q) =zj .
1 + bq j(2.4)

Substituting a = 1/b and z = bz in the 1 ψ1 summation,∞X(−1/a)n (azq)n(q, abq, −zq, −1/z)∞=,(−bq)n(−bq, −aq, azq, b/z)∞n=−∞

we have
F (z, q) =(q, q, −bzq, −1/bz)∞.
(−bq, −q/b, zq, 1/z)∞

(2.5)

(2.6)

Then

(−bq, −q/b)∞ 0(−bq, −q/b)∞[z ]F (z, q)G(z k , q k ) =H(q k ),
2
(q)∞
(q)2∞
and the lemma follows.

¤

OVERPARTITIONS AND GENERATING FUNCTIONS FOR GENERALIZED FROBENIUS PARTITIONS

5

Above we have introduced the notation(a)n := (a; q)n :=(a; q)∞.
(aq n ; q)∞(2.7)

Proof of Theorem 1.1. For the first part, take G(z, q) = (zq, 1/z)∞ in Lemma 2.2. By (1.2),
H(q) = (q)∞ /(−bq)∞ (−q/b)∞ . Then∞X

POk ,Ok (m, n)bm q n = [z 0 ]n=0(−bzq, −1/bz)∞ (z k q k , z −k ; q k )∞(zq, 1/z)∞(−bzq, −1/bz)∞
G(z k , q k )(zq, 1/z)∞(−bq, −q/b)∞H(q k )(q)2∞

= [z 0 ]=(−bq)∞ (−q/b)∞ (q k ; q k )∞.
(q)2∞ (−bq k , −q k /b; q k )∞=Similarly, take G(z, q) = (zq)∞ for the second part.
¤In the following we will use the 1 ψ1 summation (2.5) or one of its corollaries for the first stepof each proof.
Proof of Theorem 1.2. For the first part, take G(z, q) = (−1/z, zq)∞ in the case b = 1 ofLemma 2.2. Then
−1
−1
(q)2∞
0 (−zq; q)∞ (−z ; q)∞ (−z ; q)∞[z](−q, q)2∞(z −1 ; q)∞
n+1
n
∞
∞
X
z n q ( 2 ) X z −n q ( 2 )0 (q; q)∞= [z ](−q; q)∞ n=−∞ (−q; q)n(q; q)nH(q) =

n=0=2∞(−q; q)∞ X q n(q; q)∞(q 2 ; q 2 )nn=0=(q; q)∞ (−q; q 2 )∞,(−q; q)∞by the q-binomial theorem,∞
X(a)n z n
n=0

(q)n

=

(az)∞
.
(z)∞

(2.8)

6

SYLVIE CORTEEL, JEREMY LOVEJOY, AND AE JA YEE

For the second part we again apply the case b = 1 of Lemma 2.2, this time with G(z, q) =(z −1 , zq)∞ . Then
H(q) ==(q)2∞[z 0 ](−zq; q)∞ (−z −1 ; q)∞ (−z −1 ; q)∞(−q, q)2∞(q)∞ 0 X n n(n+1)/2 X z −n q n(n−1)/2[z ]z q(−q)2∞
(q)n
n∈

=
=

(q)∞
(−q)2∞

Z

n≥0

2
∞
X
qn

n=0

(q)n

(q)∞
,
(−q)2∞ (q, q 4 ; q 5 )∞by the first Rogers-Ramanujan identity,
2
∞
X
qn

n=0

(q)n

=

1
(q, q 4 ; q 5 )∞

.

(2.9)
¤

Proof of Theorem 1.3.
X(−zq; q)∞ (−z −1 ; q)∞ (−yz −1 ; q)∞
PD,OD (m, n)y m q n = [z 0 ](z −1 ; q)∞m,n≥0

0 (−q; q)∞= [z ]=(q; q)∞

n+1
n
∞
∞
X
z n q ( 2 ) X y n z −n q ( 2 )(−q; q)n(q; q)n
n=−∞

n=0

2
∞
(−q; q)∞ X y n q n(q; q)∞(q 2 ; q 2 )nn=0=(−q; q)∞ (−yq; q 2 )∞,(q; q)∞

the final equality being the case q = q 2 , z = −q/a, and a → ∞ of (2.8).
Proof of Theorem 1.4.
X(−zq, −1/z, −1/az)∞
PO,O2 D (`, m, n)a` bm q n = [z 0 ](azq)∞ (b2 /z 2 ; q 2 )∞¤`,m,n≥0∞= [z 0 ](−aq, −bq)∞ X (−1/a)n (azq)n X (1/ab)n (−b/z)n(q, abq)∞(−bq)n
(q)n

Z

n∈

=

n=0

=

n=0

∞
(−aq, −bq)∞ X (−1/a, 1/ab)n (−abq)n(q, abq)∞(−bq, q)n(−aq)∞ (−q/a, −ab2 q 2 ; q 2 )∞,(q)∞ (q, a2 b2 q 2 ; q 2 )∞

OVERPARTITIONS AND GENERATING FUNCTIONS FOR GENERALIZED FROBENIUS PARTITIONS

by the q-Kummer identity,∞X(a, b)n (−q/b)nn=0(q, aq/b)n=(aq, aq 2 /b2 ; q 2 )∞.
(−q/b, aq/b)∞ (q; q 2 )∞7

(2.10)

Proof of Theorem 1.5.
∞
XX(−aq)n q n(n+1)/2 (z/b)n` m n
0 (−zq, −1/z)∞
POD̄,O (`, m, n)a b q = [z ](azq, b/z)∞
(q)n
n=0

`,m,n≥0∞= [z 0 ](−aq, −bq)∞ X (−1/b)n (b/z)n X (−aq)n q n(n+1)/2 (z/b)n(q, abq)∞(−aq)n
(q)n
n=0

n∈Z

∞
X

(−1/b)n q n(n+1)/2(q)n=(−aq, −bq)∞(q, abq)∞=(−aq, −bq)∞ (−q/b; q 2 )∞,(q, abq)∞ (q; q 2 )∞n=0

the final equality being Lebesgue’s identity which is the case b → ∞ and a = −1/b of (2.10). ¤Proof of Theorem 1.6.
X(−zq, −1/z, −aczq 2 )∞POO0 ,O (k, `, m, n)ak b` cm q n = [z 0 ](azq, b/z, czq)∞k,`,m,n≥00 (−aq, −bq)∞= [z ](q, abq)∞∞
X (−1/b)n (b/z)n X(−aq)n (czq)n

∞

=

Z

n∈

(−aq)nn=0(q)n(−aq, −bq)∞ X (−1/b)n (bcq)n(q, abq)∞
(q)n
n=0

=

(−aq, −bq, −cq)∞,(q, abq, bcq)∞

by the q-binomial theorem (2.8).

¤

It should be noted that Theorems 1.4 – 1.6 have k-generalizations like Theorems 1.1 and 1.2
but the combinatorial definition of the F-partitions are less palatable.
3. Bijections
In this section we establish some partitions that explains some of the first cases.
Bijection for (1.5) We will here give a combinatorial proof of∞X
n=0

PO2 ,O2 (n)q n =(−q; q)∞ (−q; q 2 )2∞.
(q; q)∞

We start with a F-partition and add one to each entry of the first row. Let k be the number ofoverlined parts in the first row minus the number of overlined parts of the second row. Suppose
without loss of generality that k ≥ 0. Then we split the F-partition into two F-partitions :SYLVIE CORTEEL, JEREMY LOVEJOY, AND AE JA YEE
top not overlinedn parts

overlined < n8Overpartition
into odd parts

bottom overlined at least n
top overlined
k parts
Overpartition

bottom nonoverlined

Figure 1. Bijection for 1.10
one that contains the overlined parts and one that contains the non-overlined parts. Apply
Wright’s bijection (see [20]) to both F-partitions and get two ordinary partitions and two triangles (k, k − 1, . . . , 1) and (k − 1, . . . , 1). We keep the first partition, which gives 1/(q; q)∞ andthe odd parts of the second partition, which gives 1/(q; q 2 )∞ . Then we divide the even partsof the second partition by 2. To the left of these parts we put k, k − 1, . . . , 1. By applying
the reverse of Wright’s bijection, we obtain two partitions into distinct parts. We multiply by
two and decrease by 1 the parts of the first partition and we multiply by 2 and increase by 1
the parts of the second partition. We get two partitions into distinct odd parts, which is generated by (−q; q 2 )2 . It is easy to check that the weight is preserved and that every step is reversible.
Bijection for (1.10). Now let us prove combinatorially that∞X
n=0

PO2 ,O (n)q n =(−q; q)∞ (−q; q 2 )∞(q; q)∞ (q; q 2 )∞This proof is inspired by some ideas of [21]. We start with the Frobenius partition and add one
to each entry of the first row. The top row is an overpartition in O2 . Therefore the non-overlined
parts form a partition into distinct parts. Let us suppose that this top row has n non-overlined
parts and k overlined parts. We separate these into partitions α and β. The bottom row is an
overpartition into n + k non-negative parts. Using algorithm Z [4, 22], we can decompose it intoa partition δ into parts at most n + k and a partition γ into non-negative distinct parts less thann + k.
We take the parts of γ that are less than n, conjugate them, and add them to α to create
an overpartition into distinct parts (a part is overlined if the difference with the previous partis at least 2). Then we change α into an overpartition into odd parts generalizing Sylvester’s
bijection [19]. First we take off 2b(n − 1)/2c + 2 − i from the ith part and change it to η a(2b(n − 1)/2c + 1) × dn/2e rectangle. Then we look at the conjugate of what is left. Every
odd part is inserted in η and every even part 2i is changed to two i parts that are inserted in
the conjugate of η. The overlines follow. Note that this part is a bijective proof of Lebesgue’s
identity.
Now we take the parts of γ that are equal to or greater than n. If the part n + i − 1 occurs we
add it to the ith part of β and take off the overline. After sorting, that creates an overpartition
where the non-overlined parts are greater than n + k. We add then δ and get an overpartition,
which is generated by (−q; q)∞ /(q; q)∞ .

OVERPARTITIONS AND GENERATING FUNCTIONS FOR GENERALIZED FROBENIUS PARTITIONS

9bottom notoverlined
distinct
all parts from
1 to n occur

topdistinctordinary
ordinarybottomoverlinedbottom distinct
n parts

distinctn parts

distinct
n odd parts

Figure 2. Bijection for (1.13)µ¶
8̄, 6, 4̄, 4, 3, 2, 1̄, 0̄, 0For example, we start with. We get α = (7, 5, 4, 3, 1), β = (9, 5, 2, 1),
10, 9̄, 5̄, 3, 3, 1̄, 1, 0, 0
¯ 9, 5̄, 1, 1) and
γ = (7, 6, 4, 1) and δ = (6, 4, 4). Then we apply the mapping and get η = (11,µ = (11, 9̄, 9, 6, 4, 4, 1̄). See Fig 1.
Bijection for (1.13) We will here give a combinatorial proof of∞X

PD,OD (n)q n =n=0(−q; q)∞ (−q; q 2 )∞.
(q; q)∞

We start with a Frobenius partition and add one to each entry of the first row. The bottom
row is made of an overpartition β and a partition into distinct parts α. Let n be the number ofparts of α. We apply a generalization of Wright’s bijection to the top row and β. We draw the
Ferrers diagram of the top row and we shift the ith part by i − 1. We draw the Ferrers diagramof the non overlined parts of β and put it at the left of the diagram of the top row. We draw
the Ferrers diagram of the overlined parts of β, conjugate it and put it at the bottom right ofthe diagram of the top row, as shown on the left of Figure 2. Then we break the diagram into
two parts : the left and the right of the largest overlined part. On the right we get an ordinary
partition, which gives 1/(q; q)∞ and on the left we get a partition λ into distinct parts where all
the parts from 1 to n occur.
Let i be the index of the smallest even part in α. Then we take off i from λ and add it to
the conjugate of α. We do that until α has only odd parts. We finally get λ a partition into
distinct parts, which is generated by (−q; q)∞ and α a partition into distinct odd parts, whichis generated by (−q; q 2 )∞ . Eachµ step is easily reversible.¶
11, 10, 8, 7, 6, 4, 3, 2, 1
For example starting with
we get β = (5, 5, 4, 4, 4, 3, 3, 3, 3, 2, 2),(4̄, 3, 3, 3̄, 0̄), (7, 4, 2, 1)
λ = (9, 8, 4, 2) and α = (9, 5, 3, 1). See Fig 2.
Bijection for (1.15). A combinatorial proof of∞X
n=0

POO20 ,O (n)q n =(−q; q)2∞ (−q; q 2 )∞,(q; q)2∞ (q; q 2 )∞

10

SYLVIE CORTEEL, JEREMY LOVEJOY, AND AE JA YEE

can be easily done mixing the combinatorial proof of the 1 ψ1 summation of [21] and a combinatorial proof of the Lebesgue’s identity for example the one used in the previous bijection.
Bijection for (1.16). A combinatorial proof of∞X
n=0

POO0 ,O (n)q n =(−q; q)3∞,(q; q)3∞

can be easily done mixing the combinatorial proof of the 1 ψ1 summation of [21] and a combinatorial proof of the q-binomial identity (see for example [12]).

Acknowledgments
We would like to thank Bruce Berndt for making [15] available to us.
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CNRS, PRiSM, UVSQ, 45 Avenue des Etats Unis, 78035 Versailles Cedex, FRANCE
E-mail address: Sylvie.Corteel@prism.uvsq.fr
CNRS, LABRI, Université Bordeaux I, 351 Cours de la libération, 33405 Talence Cedex, FRANCE
E-mail address: lovejoy@labri.fr
Dept. of Mathematics, The Pennsylvania State University, University Park, PA 16802, USA
E-mail address: yee@math.psu.edu