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1d
comment Finite series with reciprocal factorials
This hypergeometric expression says no more and no less than that the coefficient of $x^{k+2}$ in the sum is $$\frac{1}{(n-k)!\,(k+2)}.$$
1d
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Oct
22
comment Representing numbers in a non-integer base with few (but possibly negative) nonzero digits
Alpert's theorem was proved earlier by Clemens Heuberger, "Minimal expansions in redundant number systems: Fibonacci bases and greedy algorithms", Periodica Mathematica Hungarica 49 (2), 2004, 65–89.
Oct
22
comment K-Permutations with forbidden numbers
Search for "permutations with forbidden positions" and "rook polynomials".
Oct
21
revised A number array related to colored necklaces and the primes
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Oct
21
revised A number array related to colored necklaces and the primes
edited body
Oct
21
revised A number array related to colored necklaces and the primes
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Oct
21
revised A number array related to colored necklaces and the primes
added 246 characters in body
Oct
21
answered A number array related to colored necklaces and the primes
Oct
15
comment A generalisation of Narayana-like numbers (walks on the 2D lattice)
See mat.univie.ac.at/~kratt/artikel/encystat.pdf. A similar question is mathoverflow.net/questions/164389/number-of-walks.
Oct
13
comment Name for series $\sum f_n x^n / (n! (n+k)!)$
It's unlikely that these will have any combinatorial applications since the product of two "Bessel generating functions" with integer coefficients won't in general have integer coefficients (by which I mean the coefficients of $x^n/n!(n+k)!$).
Oct
13
answered Name for series $\sum f_n x^n / (n! (n+k)!)$
Oct
7
comment Counting Problems where Labeled is Known but Unlabeled is Not
I would say that a lot is known about the count of unlabeled trees; it's just more complicated than for labeled trees, and this is very typical of graphical enumeration. You can find lots of examples in Harary and Palmer's book "Graphical Enumeration." There is even an explicit though complicated formula (as a sum over partitions) for the number of unlabeled trees. It might be noted that there are some problems, such as counting self-complementary graphs, in which the unlabeled version has been solved, but not the labeled version.
Oct
7
answered Is there a formula for the number of labeled forests with $k$ components on $n$ vertices?
Oct
2
comment Is there a bijection of permutations onto mathematical objects that preserve information about descents?
The Robinson-Schensted correspondence restricted to FPFI is a bijection onto standard Young tableaux with every column of even length that preserves descents (i is a descent of a standard Young tableau if i+1 is to the left of i).
Sep
30
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Aug
11
awarded  Nice Answer
Aug
11
revised Combinatorial Morse functions and random permutations
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Jul
23
comment Combinatorial Proof of Real Analysis Identity
I believe that this identity is a limit of a finite sum that can be proved using the WZ method. Whether that makes it combinatorial is a matter of opinion.