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Apr
27
comment Counting distinct undirected, partially labelled graphs
There's a proof of Pólya's theorem by Gian-Carlo Rota and David Smith using Möbius inversion along the lines that gowers suggests, but the usual proof of Burnside's lemma is much easier. See Rota, Gian-Carlo; Smith, David A. Enumeration under group action. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Sér. 4, 4 no. 4 (1977), p. 637-646 numdam.org/item?id=ASNSP_1977_4_4_4_637_0
Apr
11
comment convergence radius of Pochhammer symbol series
This is not a power series, so it doesn't have a radius of convergence.
Apr
11
revised convergence radius of Pochhammer symbol series
Improved formatting and punctuation.
Feb
20
answered Number of Dyck paths with prescribed number of edges
Feb
13
comment A combinatorial identity generalizing identity (3.111) from Gould's book
If we change the upper limit of the sum to $n$ then the sum is the $n$th difference of a polynomial of degree $n-1$ and is therefore 0. The only nonzero term in the extended sum not in the restricted sum is $-1$, corresponding to $m=n$.
Feb
1
comment Identity for Power Series and Binomial Coefficients
(2) can be derived from (1) by subtracting 1, dividing by $r$ and taking the limit as $r\to 0$.
Jan
26
answered Identity for Power Series and Binomial Coefficients
Jan
20
comment Probability question involving drawing balls from an urn
One way to do this is with the transfer matrix method. (See, e.g., chapter 4 of Richard Stanley's Enumerative Combinatorics, volume 1.) The basic idea is that as you draw the balls, you keep track of the colors of the last two balls drawn. You can represent the possible transitions as edges in a directed graph with weights that keep track of the number of R's, B's, and triples of each type, so that the numbers you want can be obtained by extracting coefficients from powers of a 4 by 4 matrix, and from this matrix you can get a rational generating function for the numbers.
Jan
11
comment Binomial coefficient identity
This identity is also a special case of Vandermonde's theorem.
Dec
26
answered A recurrence relation on Catalan numbers
Dec
8
comment Counting path generating sentences in a specific formal language
For an arbitrary Turing machine there is little or nothing that can be said.
Dec
7
comment Nontrivial question about Fibonacci numbers?
More generally, for $p\ne 2$ or 5, $F_{n+p} \equiv F_{n+1} \pmod p$ if $(p|5)=1$ and $F_{n+p}\equiv -F_{n-1} \pmod p$ if $(p|5) = -1$. (Here $F_{-1}=1$.)
Dec
5
comment A natural sum over multisets (expectation over multinomial)
The generating function $f(x)=\sum_n n^n x^n/n!$ is equal to $1/(1+W(-x))$, where $W$ is the Lambert $W$-function. The denominator has a zero at $x=1/e$ which should enable you to get a good approximation to the coefficients of $f(x)^k$ by standard techniques (see, e.g., Flajolet and Sedgewick's Analytic Combinatorics).
Nov
30
revised Dyck paths on rectangles
improved TeX formatting
Nov
29
comment Number of Dyck paths with k returns and b peaks
See Emeric Deutsch, Dyck Path Enumeration, Discrete Mathematics 204 (1999) 167-202, section 6.5, sciencedirect.com/science/article/pii/S0012365X98003719
Nov
28
comment Inclusion-preserving bijection between subsets of cardinality k and n-k
See also Curtis Greene and Daniel J. Kleitman. Strong versions of Sperner’s theorem. J. Combinatorial Theory Ser. A, 20(1):80–88, 1976. For similar decompositions of other posets, search for "symmetric chain decomposition".
Nov
11
awarded  Yearling
Nov
2
revised Combinatorial Morse functions and random permutations
added 138 characters in body
Nov
1
comment Where can I buy Napier's Bones/Rods?
I put it in back quotes which works, but puts it in a gray box which I couldn't get rid of.
Nov
1
revised Where can I buy Napier's Bones/Rods?
added 6 characters in body