Characterizing the number of coloured $m$-ary partitions modulo $m$, with and without gaps
I. P. Goulden, Pavel Shuldiner

TL;DR
This paper extends previous work on the modulo properties of $m$-ary partitions to coloured $m$-ary partitions, providing explicit generating function expansions and new characterizations.
Contribution
It introduces a novel proof method and explicit generating function expansions for coloured $m$-ary partitions modulo $m$, with and without gaps.
Findings
Explicit expansions for generating functions modulo m
Complete characterization of coloured m-ary partitions modulo m
Extension of previous results to coloured partitions
Abstract
In a pair of recent papers, Andrews, Fraenkel and Sellers provide a complete characterization for the number of -ary partitions modulo , with and without gaps. In this paper we extend these results to the case of coloured -ary partitions, with and without gaps. Our method of proof is different, giving explicit expansions for the generating functions modulo
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Taxonomy
TopicsAdvanced Combinatorial Mathematics · Advanced Mathematical Identities · Analytic Number Theory Research
Characterizing the number of coloured ary partitions modulo , with and without gaps
I. P. Goulden
Dept. of Combinatorics and Optimization, University of Waterloo, Canada
[email protected], [email protected]
and
Pavel Shuldiner
Abstract.
In a pair of recent papers, Andrews, Fraenkel and Sellers provide a complete characterization for the number of -ary partitions modulo , with and without gaps. In this paper we extend these results to the case of coloured -ary partitions, with and without gaps. Our method of proof is different, giving explicit expansions for the generating functions modulo .
Key words and phrases:
partition, congruence, generating function
2010 Mathematics Subject Classification:
Primary 05A17, 11P83; Secondary 05A15
The work of IPG was supported by an NSERC Discovery Grant.
1. Introduction
An -ary partition is an integer partition in which each part is a nonnegative integer power of a fixed integer . An -ary partition without gaps is an -ary partition in which must occur as a part whenever occurs as a part, for every nonnegative integer .
Recently, Andrews, Fraenkl and Sellers [AFS15] found an explicit expression that characterizes the number of -ary partitions of a nonnegative integer modulo ; remarkably, this expression depended only on the coefficients in the base representation of . Subsequently Andrews, Fraenkel and Sellers [AFS16] followed this up with a similar result for the number of -ary partitions without gaps, of a nonnegative integer modulo ; again, they were able to obtain a (more complicated) explicit expression, and again this expression depended only on the coefficients in the base representation of . See also Edgar [E16] and Ekhad and Zeilberger [EZ15] for more on these results.
The study of congruences for integer partition numbers has a long history, starting with the work of Ramanujan (see, e.g., [R19]). For the special case of -ary partitions, a number of authors have studied congruence properties, including Churchhouse [C69] for , Rødseth [R70] for a prime, and Andrews [A71] for arbitrary positive integers . The numbers of -ary partitions without gaps had been previously considered by Bessenrodt, Olsson and Sellers [BOS13] for .
In this note, we consider -ary partitions, with and without gaps, in which the parts are coloured. To specify the number of colours for parts of each size, we let for positive integers , and say that an -ary partition is -coloured when there are colours for the part , for . This means that there are different kinds of parts of the same size . Let denote the number of -coloured -ary partitions of , and let denote the number of -coloured -ary partitions of without gaps. For the latter, some part of any colour must occur as a part whenever some part of any colour (not necessarily the same colour) occurs as a part, for every nonnegative integer .
We extend the results of Andrews, Fraenkel and Sellers in [AFS15] and [AFS16] to the case of -coloured -ary partitions, where is relatively prime to and to for . Our method of proof is different, giving explicit expansions for the generating functions modulo . These expansions depend on the following simple result.
Proposition 1.1**.**
For positive integers with relatively prime to , we have
[TABLE]
Proof.
From the binomial theorem we have
[TABLE]
Now using the falling factorial notation we have
[TABLE]
But
[TABLE]
for any integer , and exists in since is relatively prime to , which gives
[TABLE]
and the result follows. ∎
2. Coloured -ary partitions
In this section we consider the following generating function for the numbers of -coloured -ary partitions:
[TABLE]
The following result gives an explicit expansion for modulo .
Theorem 2.1**.**
If is relatively prime to and to for , then we have
[TABLE]
Proof.
Consider the finite product
[TABLE]
We prove that
[TABLE]
by induction on . As a base case, the result for follows immediately from Proposition 1.1 with . Now assume that (2) holds for some choice of , and we obtain
[TABLE]
where the second last equivalence follows from the induction hypothesis, and the last equivalence follows from Proposition 1.1 with , .
This completes the proof of (2) by induction on , and the result follows immediately since
[TABLE]
∎
Now we give the explicit expression for the coefficients modulo that follows from the above expansion of the generating function .
Corollary 2.2**.**
For , suppose that the base representation of is given by
[TABLE]
If is relatively prime to and to for , then we have
[TABLE]
Proof.
In the expansion of the series given in Theorem 2.1, the monomial arises uniquely with the specializations , and , . But with these specializations, we have , and the result follows immediately. ∎
Specializing the expression given in Corollary 2.2 to the case for provides an alternative proof to Andrews, Fraenkel and Sellers’ characterization of ary partitions modulo , which was given as Theorem 1 of [AFS15].
3. Coloured -ary partitions without gaps
In this section we consider the following generating function for the numbers of -coloured -ary partitions without gaps:
[TABLE]
The following result gives an explicit expansion for modulo .
Theorem 3.1**.**
If is relatively prime to and to for , then we have
[TABLE]
Proof.
Consider the finite product
[TABLE]
We prove that
[TABLE]
by induction on . As a base case, the result for follows immediately from Proposition 1.1 with . Now assume that (3) holds for some choice of , and we obtain
[TABLE]
where the second last equivalence follows from the induction hypothesis, and the last equivalence follows from Proposition 1.1 with , and , .
This completes the proof of (3) by induction on , and the result follows immediately since
[TABLE]
∎
Corollary 3.2**.**
For , suppose that is divisible by , with base representation given by
[TABLE]
where , and . If is relatively prime to and to for , then for we have
[TABLE]
where if is even, and if is odd.
Proof.
First note that we have
[TABLE]
Now consider the following specializations: , , , , , , and , . Then, in the expansion of the series given in Theorem 3.1, the monomial arises once for each , in particular with the above specializations truncated to . But with these specializations we have
- •
for :
[TABLE]
- •
for :
[TABLE]
and
[TABLE]
- •
for :
[TABLE]
- •
for :
[TABLE]
- •
for :
[TABLE]
The result follows straightforwardly from Theorem 3.1. ∎
Specializing the expression given in Corollary 3.2 to the case for provides an alternative proof to Andrews, Fraenkel and Sellers’ characterization of ary partitions modulo without gaps, which was given as Theorem 2.1 of [AFS16].
The reference list from the paper itself. Each links out to its DOI / PubMed record.
- 1[A 71] G. E. Andrews, Congruence properties of the m 𝑚 m -ary partition function , J. Number Theory 3 (1971), 104–110.
- 2[AFS 15] George E. Andrews, Aviezri S. Fraenkel, James A. Sellers, Characterizing the Number of m-ary Partitions Modulo m , American Mathematical Monthly 122 (2015), 880–885.
- 3[AFS 16] George E. Andrews, Aviezri S. Fraenkel, James A. Sellers, m-ary partitions with no gaps: A characterization modulo m , Discrete Mathematics 339 (2016), 283–287.
- 4[BOS 13] C. Bessenrodt, J. B. Olsson, J. A. Sellers, Unique path partitions: characterization and congruences , Annals Comb. 17 (2013), 591–602.
- 5[C 69] R. F. Churchhouse, Congruence properties of the binary partition function , Proc. Cambridge Philos. Soc. 66 (1969), 371–376.
- 6[E 16] Tom Edgar, The distribution of the number of parts of m 𝑚 m -ary partitions modulo m 𝑚 m , Rocky Mountain J. Math. (to appear), ar Xiv 1603.00085 math.CO
- 7[EZ 15] Shalosh B. Ekhad and Doron Zeilberger, Computerizing the Andrews-Fraenkel-Sellers Proofs on the Number of m 𝑚 m -ary partitions mod m 𝑚 m (and doing MUCH more!) , ar Xiv 1511.06791 math.CO
- 8[R 19] S. Ramanujan, Some properties of p ( n ) 𝑝 𝑛 p(n) , the number of partitions of n 𝑛 n , Proc. Cambridge Philos. Soc. 19 (1919), 207–210.
