

A001710


Order of alternating group A_n, or number of even permutations of n letters.
(Formerly M2933 N1179)


204



1, 1, 1, 3, 12, 60, 360, 2520, 20160, 181440, 1814400, 19958400, 239500800, 3113510400, 43589145600, 653837184000, 10461394944000, 177843714048000, 3201186852864000, 60822550204416000, 1216451004088320000, 25545471085854720000, 562000363888803840000
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OFFSET

0,4


COMMENTS

For n >= 3, a(n1) is also the number of ways that a 3cycle in the symmetric group S_n can be written as a product of 2 long cycles (of length n).  Ahmed Fares (ahmedfares(AT)mydeja.com), Aug 14 2001
a(n) is the number of Hamiltonian circuit masks for an n X n adjacency matrix of an undirected graph.  Chad Brewbaker, Jan 31 2003
a(n1) is the number of necklaces one can make with n distinct beads: n! bead permutations, divide by two to represent flipping the necklace over, divide by n to represent rotating the necklace. Related to Stirling numbers of the first kind, Stirling cycles.  Chad Brewbaker, Jan 31 2003
Number of increasing runs in all permutations of [n1] (n>=2). Example: a(4)=12 because we have 12 increasing runs in all the permutations of [3] (shown in parentheses): (123), (13)(2), (3)(12), (2)(13), (23)(1), (3)(2)(1).  Emeric Deutsch, Aug 28 2004
Minimum permanent over all n X n (0,1)matrices with exactly n/2 zeros.  Simone Severini, Oct 15 2004
The number of permutations of 1..n that have 2 following 1 for n >= 1 is 0, 1, 3, 12, 60, 360, 2520, 20160, ... .  Jon Perry, Sep 20 2008
Starting (1, 3, 12, 60, ...) = binomial transform of A000153: (1, 2, 7, 32, 181, ...).  Gary W. Adamson, Dec 25 2008
The asymptotic expansion of the higher order exponential integral E(x,m=1,n=3) ~ exp(x)/x*(1  3/x + 12/x^2  60/x^3 + 360/x^4  2520/x^5 + 20160/x^6  81440/x^7 + ...) leads to the sequence given above. See A163931 and A130534 for more information.  Johannes W. Meijer, Oct 20 2009
Starting (1, 3, 12, 60, ...) = eigensequence of triangle A002260, (a triangle with k terms of (1,2,3,...) in each row given k=1,2,3,...). Example: a(6) = 360, generated from (1, 2, 3, 4, 5) dot (1, 1, 3, 12, 60) = (1 + 2 + 9 + 48 + 300).  Gary W. Adamson, Aug 02 2010
For n>=2: a(n) is the number of connected 2regular labeled graphs on (n+1) nodes (Cf. A001205).  Geoffrey Critzer, Feb 16 2011.
The Fi1 and Fi2 triangle sums of A094638 are given by the terms of this sequence (n>=1). For the definition of these triangle sums see A180662.  Johannes W. Meijer, Apr 20 2011
a(n1) is, for n>=2, also the number of necklaces with n beads (only C_n symmetry, no turnover) with n1 distinct colors and signature c[.]^2 c[.]^(n2). This means that two beads have the same color, and for n=2 the second factor is omitted. Say, cyclic(c[1]c[1]c[2]c[3]..c[n1]), in short 1123...(n1), taken cyclically. E.g., n=2: 11, n=3: 112, n=4: 1123, 1132, 1213, n=5: 11234, 11243, 11324, 11342, 11423, 11432, 12134, 12143, 13124, 13142, 14123, 14132. See the nexttolast entry in line n>=2 of the representative necklace partition array A212359.  Wolfdieter Lang, Jun 26 2012
For m >= 3, a(m1) is the number of distinct Hamiltonian circuits in a complete simple graph with m vertices. See also A001286.  Stanislav Sykora, May 10 2014
In factorial base (A007623) these numbers have a simple pattern: 1, 1, 1, 11, 200, 2200, 30000, 330000, 4000000, 44000000, 500000000, 5500000000, 60000000000, 660000000000, 7000000000000, 77000000000000, 800000000000000, 8800000000000000, 90000000000000000, 990000000000000000, etc. See also the formula based on this observation, given below.  Antti Karttunen, Dec 19 2015
Also (by definition) the independence number of the ntransposition graph.  Eric W. Weisstein, May 21 2017
Number of permutations of n letters containing an even number of even cycles.  Michael Somos, Jul 11 2018
Equivalent to Brewbaker's and Sykora's comments, a(n  1) is the number of undirected cycles covering n labeled vertices, hence the logarithmic transform of A002135.  Gus Wiseman, Oct 20 2018
For n >= 2 and a set of n distinct leaf labels, a(n) is the number of binary, rooted, leaflabeled tree topologies that have a caterpillar shape (column k=1 of A306364).  Noah A Rosenberg, Feb 11 2019
Also the clique covering number of the nBruhat graph.  Eric W. Weisstein, Apr 19 2019
a(n) is the number of lattices of the form [s,w] in the weak order on S_n, for a fixed simple reflection s.  Bridget Tenner, Jan 16 2020
For n > 3, a(n) = p_1^e_1*...*p_m^e_m, where p_1 = 2 and e_m = 1. There exists p_1^x where x <= e_1 such that p_1^x*p_m^e_m is a primitive Zumkeller number (A180332) and p_1^e_1*p_m^e_m is a Zumkeller number (A083207). Therefore, for n > 3, a(n) = p_1^e_1*p_m^e_m*r, where r is relatively prime to p_1*p_m, is also a Zumkeller number.  Ivan N. Ianakiev, Mar 11 2020
For n>1, a(n) is the number of permutations of [n] that have 1 and 2 as cyclemates, that is, 1 and 2 are contained in the same cycle of a cyclic representation of permutations of [n]. For example, a(4) counts the 12 permutations with 1 and 2 as cyclemates, namely, (1 2 3 4), (1 2 4 3), (1 3 2 4), (1 3 4 2), (1 4 2 3), (1 4 3 2), (1 2 3) (4), (1 3 2) (4), (1 2 4 )(3), (1 4 2)(3), (1 2)(3 4), and (1 2)(3)(4). Since a(n+2)=row sums of A162608, our result readily follows.  Dennis P. Walsh, May 28 2020


REFERENCES

J. Riordan, An Introduction to Combinatorial Analysis, Wiley, 1958, pp. 878, 20. (a), c_n^e(t=1).
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

Olivier Bodini, Antoine Genitrini, and Mehdi Naima, Ranked Schröder Trees, arXiv:1808.08376 [cs.DS], 2018.
SZ Song, SG Hwang, SH Rim, and GS Cheon, Extremes of permanents of (0,1)matrices, Special issue on the Combinatorial Matrix Theory Conference (Pohang, 2002). Linear Algebra Appl. 373 (2003), 197210.


FORMULA

a(n) = numerator(n!/2) and A141044(n) = denominator(n!/2).
Dfinite with recurrence: a(0) = a(1) = a(2) = 1; a(n) = n*a(n1) for n>2.  Chad Brewbaker, Jan 31 2003 [Corrected by N. J. A. Sloane, Jul 25 2008]
a(0) = 0, a(1) = 1; a(n) = Sum_{k=1..n1} k*a(k).  Amarnath Murthy, Oct 29 2002
Stirling transform of a(n+1) = [1, 3, 12, 160, ...] is A083410(n) = [1, 4, 22, 154, ...].  Michael Somos, Mar 04 2004
a(n) = 0^n + Sum_{k=0..n} (1)^(nk1)*T(n1, k)*cos(Pi*(nk1)/2)^2.
E.g.f.: (2  x^2)/(2  2*x).
E.g.f. of a(n+2), n>=0, is 1/(1x)^3.
a(n+1) = (1)^n * A136656(n,1), n>=1.
a(n) = n!/2 for n>=2 (proof from the e.g.f).  Wolfdieter Lang, Apr 30 2010
a(n) = (n2)! * t(n1), n>1, where t(n) is the nth triangular number (A000217).  Gary Detlefs, May 21 2010
O.g.f.: 1 + x*Sum_{n>=0} n^n*x^n/(1 + n*x)^n.  Paul D. Hanna, Sep 13 2011
a(n) = if n < 2 then 1, otherwise Pochhammer(n,n)/binomial(2*n,n).  Peter Luschny, Nov 07 2011
a(n) = Sum_{k=0..floor(n/2)} s(n,n2*k) where s(n,k) are Stirling number of the first kind, A048994.  Mircea Merca, Apr 07 2012
a(n1), n>=3, is M_1([2,1^(n2)])/n = (n1)!/2, with the M_1 multinomial numbers for the given n1 part partition of n. See the second to last entry in line n>=3 of A036038, and the above necklace comment by W. Lang.  Wolfdieter Lang, Jun 26 2012
G.f.: A(x) = 1 + x + x^2/(G(0)2*x) where G(k) = 1  (k+1)*x/(1  x*(k+3)/G(k+1)); (continued fraction).  Sergei N. Gladkovskii, Dec 26 2012.
G.f.: 1 + x + (Q(0)1)*x^2/(2*(sqrt(x)+x)), where Q(k) = 1 + (k+2)*sqrt(x)/(1  sqrt(x)/(sqrt(x) + 1/Q(k+1))); (continued fraction).  Sergei N. Gladkovskii, May 15 2013
G.f.: 1 + x + (x*Q(x)x^2)/(2*(sqrt(x)+x)), where Q(x) = Sum_{n>=0} (n+1)!*x^n*sqrt(x)*(sqrt(x) + x*(n+2)).  Sergei N. Gladkovskii, May 15 2013
G.f.: 1 + x/2 + (Q(0)1)*x/(2*(sqrt(x)+x)), where Q(k) = 1 + (k+1)*sqrt(x)/(1  sqrt(x)/(sqrt(x) + 1/Q(k+1))); (continued fraction).  Sergei N. Gladkovskii, May 15 2013
G.f.: 1 + x + x^2*G(0)/2, where G(k) = 1 + 1/(1  x/(x + 1/(k+3)/G(k+1))); (continued fraction).  Sergei N. Gladkovskii, Jun 01 2013
G.f.: 1+x + x^2*W(0), where W(k) = 1  x*(k+3)/( x*(k+3)  1/(1  x*(k+1)/( x*(k+1)  1/W(k+1) ))); (continued fraction).  Sergei N. Gladkovskii, Aug 26 2013
a(0)=a(1)=1; after which, for even n: a(n) = (n/2) * (n1)!, and for odd n: a(n) = (n1)/2 * ((n1)! + (n2)!). [The formula was empirically found after viewing these numbers in factorial base, A007623, and is easily proved by considering formulas from Lang (Apr 30 2010) and Detlefs (May 21 2010) shown above.]
For n >= 1, a(2*n+1) = a(2*n) + A153880(a(2*n)). [Follows from above.] (End)
The o.g.f. A(x) satisfies the Riccati equation x^2*A'(x) + (x  1)*A(x) + 1  x^2 = 0.
G.f.: A(x) = 1 + x + x^2/(1  3*x/(1  x/(1  4*x/(1  2*x/(1  5*x/(1  3*x/(1  ...  (n + 2)*x/(1  n*x/(1  ... ))))))))) (apply Stokes, 1982).
A(x) = 1 + x + x^2/(1  2*x  x/(1  3*x/(1  2*x/(1  4*x/(1  3*x/(1  5*x/(1  ...  n*x/(1  (n+2)*x/(1  ... ))))))))). (End)
H(x) = (1  (1 + x)^(2)) / 2 = x  3*x^2/2! + 12*x^3/3!  ..., an e.g.f. for the signed sequence here (n!/2!), ignoring the first two terms, is the compositional inverse of G(x) = (1  2*x)^(1/2)  1 = x + 3*x^2/2! + 15*x^3/3! + ..., an e.g.f. for A001147. Cf. A094638. H(x) is the e.g.f. for the sequence (1)^m * m!/2 for m = 2,3,4,... . Cf. A001715 for n!/3! and A001720 for n!/4!. Cf. columns of A094587, A173333, and A213936 and rows of A138533.  Tom Copeland, Dec 27 2019
Sum_{n>=0} 1/a(n) = 2*(e1).
Sum_{n>=0} (1)^n/a(n) = 2/e. (End)


EXAMPLE

G.f. = 1 + x + x^2 + 3*x^3 + 12*x^4 + 60*x^5 + 360*x^6 + 2520*x^7 + ...


MAPLE



MATHEMATICA

a[n_]:= If[n > 2, n!/2, 1]; Array[a, 21, 0]
a[ n_]:= If[n<0, 0, n! SeriesCoefficient[(2x^2)/(22x), {x, 0, n}]]; (* Michael Somos, May 22 2014 *)
a[ n_]:= If[n<0, 0, n! SeriesCoefficient[1 +Sinh[Log[1x]], {x, 0, n}]]; (* Michael Somos, May 22 2014 *)
Table[GroupOrder[AlternatingGroup[n]], {n, 0, 20}] (* Eric W. Weisstein, May 21 2017 *)


PROG

(PARI) {a(n) = if( n<2, n>=0, n!/2)};
(PARI) a(n)=polcoeff(1+x*sum(m=0, n, m^m*x^m/(1+m*x+x*O(x^n))^m), n) \\ Paul D. Hanna
(Scheme, using memoizationmacro definec for which an implementation can be found in http://oeis.org/wiki/Memoization )
(definec (A001710 n) (cond ((<= n 2) 1) (else (* n (A001710 ( n 1))))))
(Python)
from math import factorial


CROSSREFS

Cf. A000142, A000153, A000255, A001147, A001286, A001720, A002135, A002260, A007623, A007717, A049444, A049459, A093468, A094587, A094638, A138533, A153880, A173333, A213936, A215771, A319225, A319226, A320655.
a(n+1)= A046089(n, 1), n >= 1 (first column of triangle), A161739 (q(n) sequence).


KEYWORD

nonn,easy,nice


AUTHOR



EXTENSIONS

More terms from Larry Reeves (larryr(AT)acm.org), Aug 20 2001


STATUS

approved



