All Questions
28 questions
4
votes
1
answer
197
views
Solving a three-parameter recursive sequence
Consider the triple-indexed sequence of integers defined by
\begin{align} \label{coefficientsV} \nonumber
f(\alpha,\beta,\gamma)
&:=(2\alpha+8\beta+12\gamma-1)\cdot f(\alpha-1,\beta,\gamma)...
- 3,671
16
votes
1
answer
471
views
Types of generating functions (ordinary, exponential, ???) closed under substitution
A nice feature of ordinary and exponential generating functions is that they are closed under substitution: if $F(z)$ and $G(z)$ both have integer coefficients, then $F(G(z))$ also has integer ...
- 5,822
3
votes
1
answer
156
views
$q$-series and Stirling of the 1st kind
Denote the (unsigned) Stirling numbers of the $1^{st}$-kind by ${n \brack k}$ and define
$$\mathbf{F}_a(q)=\sum_{m\geq1}\frac{q^{am}}{(1-q^m)^{2a}} \qquad \text{and} \qquad
\mathbf{G}_b(q)=\sum_{m\...
- 7,226
4
votes
0
answers
208
views
Extract this constant term
Given a Laurent polynomial $F$ in the variables $\mathbf{t}=(t_1,\dots,t_n)$, let $CT_{\vec{\mathbf{t}}}\,F$ denote its constant term.
For example, $CT_{t_1,t_2}((8t_1-\frac1{3t_1t_2})(5t_1t_2+t_2^2+\...
- 43.2k
1
vote
1
answer
344
views
Products involving exponents of tribonacci numbers
The Fibonacci numbers $F_n$ can be given by
$$\sum_{k\geq0}F_kx^k=\frac{x}{1-x-x^2}.$$
Among many many properties of this sequence, consider the following two results:
(1) the coefficients of the ...
- 43.2k
12
votes
2
answers
716
views
Alternating sum of hook lengths: Part I
Given $\lambda$ an integer partition of $n$, let $h_{ij}(\lambda)$ denote the hook length of cell $(i,j)$ in the Young diagram of $\lambda$.
Is there a closed formula or a generating function for the ...
- 43.2k
0
votes
1
answer
195
views
Fibonacci and product polynomials
The motivation for my current question arises from this MO post by R. Stanley. Caveat. There's a slight alteration.
With the convention $F_1=F_2=1$ for the Fibonacci numbers, define the polynomials $...
- 50.8k
2
votes
0
answers
110
views
Asking for a generating function for an arithmetic sequence
For fixed integer $n\geq1$, let $c_m(n)$ be the number of divisors $d$ of $m$ such that $n<d\leq 2n$. Here is an experimental generating function for which I ask:
QUESTION. Is this true?
$$\sum_{m\...
- 43.2k
8
votes
2
answers
262
views
Congruences of binomial sums
Let $a_n$ is a binomial sum, for example
$$
a_n := \sum_{k} \binom{n-k}{k} \quad \text{or} \quad \sum_{k=0}^n\binom{n+k}{n-k}\binom{2k}{k}
\quad \text{or} \quad \sum_{k=0}^n\sum_{\ell=0}^k\binom{n}{k}\...
- 17k
3
votes
0
answers
315
views
When does the Taylor coefficient of $e^{\sin x}$ vanish?
If $f(x)=\frac{a_1}{1!}x+\frac{a_2}{2!}x^2+\frac{a_3}{3!}x^3+\frac{a_4}{4!}x^4+\cdots$ is an exponential generating function for $\{a_k\}_{k\geq1}$ then
$$e^{f(x)}=1+\frac{a_1}{1!}x+\frac{a_1^2+a_2}{2!...
- 63.9k
2
votes
0
answers
117
views
A multi-variable "Fibonacci polynomial"?
There is a tremendous literature on the Fibonacci sequence, including its polynomial analogue $F_{-1}=0, F_0=1$ and
$$F_n(x)=xF_{n-1}(x)+F_{n-2} \qquad \text{for $n\geq1$}.$$
What I have found is the ...
- 43.2k
1
vote
1
answer
155
views
Log-concavity of sequence related to overpartitions
The number $p_1(n)$ of overpartitions of $n$ is generated by
$$\sum_{n\geq0}p_1(n)\,q^n=\prod_{k=1}^{\infty}\frac{1+q^k}{1-q^k}.$$
Let $t\in\mathbb{N}$. Now, extend this to construct a family of ...
- 43.2k
2
votes
0
answers
98
views
Two-variable generating functions over coprime pairs
I am studying a sequence $(\alpha_{p,q})$ indexed by a pair of coprime integers; this sequence arises naturally in the study of a particular set of spaces in geometric topology, but unfortunately the ...
3
votes
0
answers
123
views
$q$-series for the number of rectangles in a square lattice
Given a partition $\lambda\vdash n$ of $n$, look at its Young diagram $Y_{\lambda}$. Let $a(\lambda)$ be the number of squares (of all sizes) in $Y_{\lambda}$. For example, if $n=4$ then $a(4)=4, a(3,...
- 43.2k
1
vote
0
answers
43
views
Correspondence between monomer-dimer heaps and words in 2 alphabets
For background and some illustrative pictures, refer to this preprint by A M Grasia and G Ganzberger: Fibonacci polynomials. For the present purpose, it suffices to read into pages 4 and 5.
The part ...
- 43.2k
6
votes
0
answers
207
views
Parameter independence of Stanley's "content formula". Why?
For a cell $\square$ in the Young diagram of a partition $\lambda$, let $h_{\square}$ and $c_{\square}$ denote the hook length and content of $\square$, respectively.
R. Stanley remarked following ...
- 43.2k
3
votes
1
answer
272
views
Generating function for parity in hooks
Let $\lambda\vdash n$ denote an integer partition of $n$ and $\frak{H}_{\lambda}$ be the multiset of hook lengths of $\lambda$. Further, let $o(\lambda)=\#$ of odd entries and $e(\lambda)=\#$ of even ...
- 43.2k
1
vote
0
answers
134
views
Counting unions of unlabelled connected graphs
My question can be stated as follows: let $X$ be a hereditary family of unlabelled graphs closed under disjoint unions. Suppose we know, for each $n$, the number $c_n$ of connected graphs in X on $n$ ...
- 183
14
votes
7
answers
3k
views
A special type of generating function for Fibonacci
Notation. Let $[x^n]G(x)$ be the coefficient of $x^n$ in the Taylor series of $G(x)$.
Consider the sequence of central binomial coefficients $\binom{2n}n$. Then there two ways to recover them:
$$\...
- 13.4k
8
votes
3
answers
536
views
Looking for a "cute" justification for a Catalan-type generating function
The Catalan numbers $C_n=\frac1{n+1}\binom{2n}n$ have the generating function
$$c(x)=\frac{1-\sqrt{1-4x}}{2x}.$$
Let $a\in\mathbb{R}^+$. It seems that the following holds true
$$\frac{c(x)^a}{\sqrt{1-...
- 5,622
7
votes
0
answers
174
views
A diagonal generating function for Fibonacci: Part II
In my earlier MO question, I mentioned although we have for the Fibonacci numbers that
$$F_n=[x^n]\left(\frac1{1-x-x^2}\right),$$
is there a function $F(x)$ such that $F_n=[x^n]\left(F(x)\right)^n$?
...
CommunityBot
- 1
11
votes
2
answers
1k
views
Proofs of some combinatorial identities
Just wondering if anyone knows any references in the literature to bijections corresponding to the following simple generating function identities. Let $B(z)=\dfrac{1}{\sqrt{1-4z}}$ and $C(z)=\dfrac{1-...
- 2,175
2
votes
3
answers
253
views
asymptotics for coefficients of generating functions involving logarithms
I have a generating function that has a closed form like $1/(\log(z-a)+b)$ and I would like to get asymptotics for the size of the coefficients of it.
I was going to use the methods in Chapter 5 of ...
- 41.1k
4
votes
1
answer
122
views
D-finiteness of Hilbert series of non-commutative invariant ring under reductive group
Let $G$ be reductive group over a field of characteristic $0$ ($GL_n$ fine for this question). Let $V$ be a linear representation of $G$. Then $G$ acts on the tensor algebra $T(V) = \bigoplus_{n \ge 0}...
- 5,822
15
votes
4
answers
3k
views
Ordinary Generating Function for Bell Numbers
In the OEIS entry for Bell numbers, there appears a generating function
$$\sum_{k=0}^\infty B_k t^k = \sum_{r=0}^\infty \prod_{i=1}^r \frac{t}{1-it}$$
However, I could not locate any proof of ...
- 10.5k
15
votes
2
answers
363
views
Generating functions for objects with irrational sizes
A problem I'm investigating concerns a combinatorial class in which the 'atoms' have irrational sizes. It seems likely that this is something that has been considered before, but I haven't been able ...
- 674
14
votes
1
answer
754
views
Generating function of the Thue-Morse sequence
Let $T$ be the generating function of the Thue-Morse sequence; thus,
$T(x)=x+x^2+x^4+x^7+\dotsb$. It is known that $T$ satisfies the nice
congruence
$$ (1+x)^3 T^2(x) + (1+x)^2 T(x) + x \equiv 0 \...
- 12.3k
1
vote
0
answers
95
views
Generating series of free PROs
Let
\begin{equation}
G := \biguplus_{p \geq 0} \: \biguplus_{q \geq 0} G(p, q)
\end{equation}
be a bigraded set of generators and $\mathcal{F}(G)$ be the free PRO generated by $G$ (see [1] for a net ...
- 437