

A002866


a(0) = 1; for n > 0, a(n) = 2^(n1)*n!.
(Formerly M3604 N1463)


76



1, 1, 4, 24, 192, 1920, 23040, 322560, 5160960, 92897280, 1857945600, 40874803200, 980995276800, 25505877196800, 714164561510400, 21424936845312000, 685597979049984000, 23310331287699456000, 839171926357180416000, 31888533201572855808000, 1275541328062914232320000
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OFFSET

0,3


COMMENTS

Consider the set of n1 odd numbers from 3 to 2n1, i.e., {3, 5, ..., 2n1}. There are 2^(n1) subsets from {} to {3, 5, 7, ..., 2n1}; a(n) = the sum of the products of terms of all the subsets. (Product for empty set = 1.) a(4) = 1 + 3 + 5 + 7 + 3*5 + 3*7 + 5*7 + 3*5*7 = 192.  Amarnath Murthy, Sep 06 2002
Also, a(n1) is the number of ways to lace a shoe that has n pairs of eyelets such that there is a straight (horizontal) connection between all adjacent eyelet pairs.  Hugo Pfoertner, Jan 27 2003
This is also the denominator of the integral of ((1x^2)^(n1/2))/(Pi/4) where x ranges from 0 to 1. The numerator is (2*x)!/(x!*2^x). In both cases n starts at 1. E.g., the denominator when n=3 is 24 and the numerator is 15.  Al Hakanson (hawkuu(AT)excite.com), Oct 17 2003
Number of ways to use the elements of {1,...,n} once each to form a sequence of nonempty lists.  Bob Proctor, Apr 18 2005
Number of rotational symmetries of an ncube. The number of all symmetries of an ncube is A000165. See Egan for signed cycle notation, other notes, tables and animation.  Jonathan Vos Post, Nov 28 2007
1, 4, 24, 192, 1920, ... is the exponential (or binomial) convolution of 1, 1, 3, 15, 105, ... and 1, 3, 15, 105, 945 (A001147).  David Callan, Jul 25 2008
The nth term of this sequence is the number of regions into which ndimensional space is partitioned by the 2n hyperplanes of the form x_i=x_j and x_i=x_j (for 1 <= i < j <= n).  Edward Scheinerman (ers(AT)jhu.edu), May 04 2008
a(n) is the number of ways to seat n churchgoers into pews and then linearly order the nonempty pews.  Geoffrey Critzer, Mar 16 2009
Next term in the series = (1, 3, 5, 7, ...) dot (1, 1, 4, 24, ...);
e.g., a(5) = 1920 = (1, 3, 5, 7, 9) dot (1, 1, 4, 24, 192) = (1 + 3 + 20 + 168 + 1728). (End)
a(n) is the number of ways to represent the permutations of {1,2,...,n} in cycle notation, taking into account that we can permute the order of all cycles and also have k ways to write a lengthk cycle.
For positive n, a(n) equals the permanent of the n X n matrix with consecutive integers 1 to n along the main diagonal, consecutive integers 2 to n along the subdiagonal, and 1's everywhere else.  John M. Campbell, Jul 10 2011
Number of ways to arrange n books on consecutive bookshelves.
To derive a(n) = n!2^(n1), we note that there are n! ways to arrange the books in a row. Then there are 2^(n1) ways to place the arranged books on consecutive shelves since there are 2^(n1) ordered partitions of n. Hence a(n) = n!2^(n1).
Also, a(n) is the number of ways to stack n different alphabet blocks in contiguous stacks.
Furthermore, a(n) is the number of labeled, rooted forests that have (i) each root labeled larger than any nonroot, (ii) each root having exactly one child node, (iii) n nonroot nodes, and (iv) each node in the forest with at most one child node.
Example: a(3)=24 since there are 24 arrangements of books b1, b2, and b3 on consecutive shelves, namely, b1 b2 b3, b1 b3 b2, b2 b1 b3, b2 b3 b1, b3 b1 b2, b3 b2 b1, b1 b2b3, b2 b1 b3, b1 b3b2, b3 b1b2, b2 b3b1, b3 b2b1, b1b2 b3,b1b3 b2, b2b1 b3, b2b3 b1, b3b1 b2, b3b2 b1, b1b2b3, b1b3b2, b2b1b3, b2b3b1, b3b1b2, and b3b2b1.
(End)
For n > 3, a(n) is the order of the Coxeter group (also called Weyl group) of type D_n.  Tom Edgar, Nov 05 2013


REFERENCES

N. Bourbaki, Groupes et alg. de Lie, Chap. 4, 5, 6, p. 257.
A. P. Prudnikov, Yu. A. Brychkov and O.I. Marichev, "Integrals and Series", Volume 1: "Elementary Functions", Chapter 4: "Finite Sums", New York, Gordon and Breach Science Publishers, 19861992, Eq. (4.2.2.26)
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).


LINKS



FORMULA

E.g.f.: (1  x)/(1  2*x).  Paul Barry, May 26 2003, corrected Jun 18 2007
For n >= 1, a(n) = Sum_{i=0..m/2} (1)^i * binomial(n, i) * (n2*i)^n.  Yong Kong (ykong(AT)curagen.com), Dec 28 2000
a(n) ~ 2^(1/2) * Pi^(1/2) * n^(3/2) * 2^n * e^(n) * n^n*{1 + 13/12*n^(1) + ...}.  Joe Keane (jgk(AT)jgk.org), Nov 23 2001
E.g.f. is B(A(x)), where B(x) = 1/(1  x) and A(x) = x/(1  x).  Geoffrey Critzer, Mar 16 2009
G.f.: 1 + x/(1  4*x/(1  2*x/(1  6*x/(1  4*x/(1  8*x/(1  6*x/(1  10*x/(1  ... (continued fraction).  Philippe Deléham, Nov 29 2011
G.f.: (1 + 1/G(0))/2, where G(k)= 1 + 2*x*k  2*x*(k + 1)/G(k+1); (continued fraction, Euler's 1st kind, 1step).  Sergei N. Gladkovskii, Aug 02 2012
G.f.: 1 + x/Q(0), m=4, where Q(k) = 1  m*x*(2*k + 1)  m*x^2*(2*k + 1)*(2*k + 2)/(1  m*x*(2*k + 2)  m*x^2*(2*k + 2)*(2*k + 3)/Q(k+1)) ; (continued fraction).  Sergei N. Gladkovskii, Sep 23 2013
G.f.: 1 + x/(G(0)  x), where G(k) = 1 + x*(k+1)  4*x*(k + 1)/(1  x*(k + 2)/G(k+1)); (continued fraction).  Sergei N. Gladkovskii, Dec 24 2013
a(n) = Sum_{k=0..n} L(n,k)*k!; L(n,k) are the unsigned Lah numbers.  Peter Luschny, Oct 18 2014
a(n) = round(Sum_{k >= 1} log(k)^n/k^(3/2))/4, for n >= 1, which is related to the nth derivative of zeta(x) evaluated at x = 3/2.  Richard R. Forberg, Jan 02 2015
a(n) = n!*hypergeom([n+1], [], 1)) for n>=1.  Peter Luschny, Apr 08 2015
Sum_{n >= 0} 1/a(n) = 2*sqrt(e)  1.
Sum_{n >= 0} (1)^n/a(n) = 2/sqrt(e)  1. (End)


EXAMPLE

For the shoe lacing: with the notation introduced in A078602 the a(31) = 4 "straight" lacings for 3 pairs of eyelets are: 125346, 125436, 134526, 143526. Their mirror images 134256, 143256, 152346, 152436 are not counted.
a(3) = 24 because the 24 rotations of a threedimensional cube fall into four distinct classes:
(i) the identity, which leaves everything fixed;
(ii) 9 rotations which leave the centers of two faces fixed, comprising rotations of 90, 180 and 270 degrees for each of 3 pairs of faces;
(iii) 6 rotations which leave the centers of two edges fixed, comprising rotations of 180 degrees for each of 6 pairs of edges;
(iv) 8 rotations which leave two vertices fixed, comprising rotations of 120 and 240 degrees for each of 4 pairs of vertices. For an ncube, rotations can be more complex. For example, in 4 dimensions a rotation can either act in a single plane, such as the xy plane, while leaving any vectors orthogonal to that plane unchanged, or it can act in two orthogonal planes, performing rotations in both and leaving no vectors fixed. In higher dimensions, there will be room for more planes and more choices as to the number of planes in which a given rotation acts.


MAPLE

A002866 := n> `if`(n=0, 1, 2^(n1)*n!):
with(combstruct); SeqSeqL := [S, {S=Sequence(U, card >= 1), U=Sequence(Z, card >=1)}, labeled];
seq(ceil(count(Subset(n))*count(Permutation(n))/2), n=0..17); # Zerinvary Lajos, Oct 16 2006
G(x):=(1x)/(12*x): f[0]:=G(x): for n from 1 to 26 do f[n]:=diff(f[n1], x) od:x:=0:seq(f[n], n=0..17); # Zerinvary Lajos, Apr 04 2009


MATHEMATICA

Join[{1}, Table[2^(n1) n!, {n, 25}]] (* Harvey P. Dale, Sep 27 2013 *)
a[n_] := (1)^n Hypergeometric2F1Regularized[1, n, 2  n, 2];


PROG

(FORTRAN) See Pfoertner link.
(PARI) a(n) = if(n == 0, 1, 2^(n1)*n!);
(Magma) [1] cat [2^(n1)*Factorial(n): n in [1..25]]; // G. C. Greubel, Jun 13 2019
(Sage) [1] + [2^(n1)*factorial(n) for n in (1..25)] # G. C. Greubel, Jun 13 2019


CROSSREFS



KEYWORD

nonn,easy,nice


AUTHOR



STATUS

approved



