

A001147


Double factorial of odd numbers: a(n) = (2*n1)!! = 1*3*5*...*(2*n1).
(Formerly M3002 N1217)


358



1, 1, 3, 15, 105, 945, 10395, 135135, 2027025, 34459425, 654729075, 13749310575, 316234143225, 7905853580625, 213458046676875, 6190283353629375, 191898783962510625, 6332659870762850625, 221643095476699771875, 8200794532637891559375
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OFFSET

0,3


COMMENTS

The solution to Schröder's third problem.
Number of fixedpointfree involutions in symmetric group S_{2n} (cf. A000085).
a(n+2) is the number of full Steiner topologies on n points with n2 Steiner points.
a(n) is also the number of perfect matchings in the complete graph K(2n).  Ola Veshta (olaveshta(AT)mydeja.com), Mar 25 2001
Number of ways to choose n disjoint pairs of items from 2*n items.  Ron Zeno (rzeno(AT)hotmail.com), Feb 06 2002
Number of ways to choose n1 disjoint pairs of items from 2*n1 items (one item remains unpaired).  Bartosz Zoltak, Oct 16 2012
For n >= 1 a(n) is the number of permutations in the symmetric group S_(2n) whose cycle decomposition is a product of n disjoint transpositions.  Ahmed Fares (ahmedfares(AT)mydeja.com), Apr 21 2001
a(n) is the number of distinct products of n+1 variables with commutative, nonassociative multiplication.  Andrew Walters (awalters3(AT)yahoo.com), Jan 17 2004. For example, a(3)=15 because the product of the four variables w, x, y and z can be constructed in exactly 15 ways, assuming commutativity but not associativity: 1. w(x(yz)) 2. w(y(xz)) 3. w(z(xy)) 4. x(w(yz)) 5. x(y(wz)) 6. x(z(wy)) 7. y(w(xz)) 8. y(x(wz)) 9. y(z(wx)) 10. z(w(xy)) 11. z(x(wy)) 12. z(y(wx)) 13. (wx)(yz) 14. (wy)(xz) 15. (wz)(xy).
a(n) = E(X^(2n)), where X is a standard normal random variable (i.e. X is normal with mean = 0, variance = 1). So for instance a(3) = E(X^6) = 15, etc. See Abramowitz and Stegun or Hoel, Port and Stone.  Jerome Coleman, Apr 06 2004
Second Eulerian transform of 1,1,1,1,1,1... The second Eulerian transform transforms a sequence s to a sequence t by the formula t(n) = Sum[E(n,k)s(k), k=0...n], where E(n,k) is a secondorder Eulerian number [A008517].  Ross La Haye, Feb 13 2005
Integral representation as nth moment of a positive function on the positive axis, in Maple notation: a(n)=int(x^n*exp(x/2)/sqrt(2*Pi*x), x=0..infinity), n=0,1... .  Karol A. Penson, Oct 10 2005
Let PI be the set of all partitions of {1, 2, ..., 2n} into pairs without regard to order. There are (2n1)!! such partitions. An element alpha in PI can be written as alpha = {(i_1, j_1), (i_2, j_2), ..., (i_n, j_n)} with i_k < j_k. Let pi be the corresponding permutation which maps 1 to i_1, 2 to j_1, 3 to i_2, 4 to j_2, ..., 2n to j_n. Define sgn(alpha) to be the signature of pi, which depends only on the partition alpha and not on the particular choice of pi. Let A = {a_ij} be a 2n X 2n skewsymmetric matrix. Given a partition alpha as above define A_alpha = sgn(alpha) a_(i_1,j_1)a_(i_2,j_2)...a_(i_n,j_n). We can then define the Pfaffian of A to be Pf(A) = SUM[alpha in PI]A_alpha. The Pfaffian of an n X n skewsymmetric matrix for n odd is defined to be zero.  Jonathan Vos Post, Mar 12 2006
a(n) is the number of binary total partitions (each nonsingleton block must be partitioned into exactly two blocks) or, equivalently, the number of unordered full binary trees with labeled leaves (Stanley, ex 5.2.6).  Mitch Harris, Aug 01 2006
a(n) is the Pfaffian of the skewsymmetric 2n X 2n matrix whose (i,j) entry is i for i<j.  David Callan, Sep 25 2006
a(n) is the number of increasing ordered rooted trees on n+1 vertices where "increasing" means the vertices are labeled 0,1,2,...,n so that each path from the root has increasing labels. Increasing unordered rooted trees are counted by the factorial numbers A000142.  David Callan, Oct 26 2006
Number of perfect multi Skolemtype sequences of order n.  Emeric Deutsch, Nov 24 2006
a(n) = total weight of all Dyck npaths (A000108) when each path is weighted with the product of the heights of the terminal points of its upsteps. For example with n=3, the 5 Dyck 3paths UUUDDD, UUDUDD, UUDDUD, UDUUDD, UDUDUD have weights 1*2*3=6, 1*2*2=4, 1*2*1=2, 1*1*2=2, 1*1*1=1 respectively and 6+4+2+2+1=15. Counting weights by height of last upstep yields A102625.  David Callan, Dec 29 2006
a(n) is the number of increasing ternary trees on n vertices. Increasing binary trees are counted by ordinary factorials (A000142) and increasing quaternary trees by triple factorials (A007559).  David Callan, Mar 30 2007
This sequence is essentially selfreciprocal under the list partition transform and associated operations in A133314. More precisely, A001147 and A001147 with a leading 1 attached are reciprocal. Therefore their e.g.f.'s are reciprocal. See A132382 for an extension of this result.  Tom Copeland, Nov 13 2007
From Ross Drewe, Mar 16 2008: (Start)
This is also the number of ways of arranging the elements of n distinct pairs, assuming the order of elements is significant but the pairs are not distinguishable, i.e., arrangements which are the same after permutations of the labels are equivalent.
If this sequence and A000680 are denoted by a(n) and b(n) respectively, then a(n) = b(n)/n! where n! = the number of ways of permuting the pair labels.
For example, there are 90 ways of arranging the elements of 3 pairs [1 1], [2 2], [3 3] when the pairs are distinguishable: A = { [112233], [112323], .... [332211] }.
By applying the 6 relabeling permutations to A, we can partition A into 90/6 = 15 subsets: B = { {[112233], [113322], [221133], [223311], [331122], [332211]}, {[112323], [113232], [221313], [223131], [331212], [332121]}, ....}
Each subset or equivalence class in B represents a unique pattern of pair relationships. For example, subset B1 above represents {3 disjoint pairs} and subset B2 represents {1 disjoint pair + 2 interleaved pairs}, with the order being significant (contrast A132101). (End)
A139541(n) = a(n) * a(2*n).  Reinhard Zumkeller, Apr 25 2008
a(n+1) = sum(j=0 to n) A074060(n,j) * 2^j.  Tom Copeland, Sep 01 2008
From Emeric Deutsch, Jun 05 2009: (Start)
a(n) is the number of adjacent transpositions in all fixedpointfree involutions of {1,2,...,2n}. Example: a(2)=3 because in 2143=(12)(34), 3412=(13)(24), and 4321=(14)(23) we have 2 + 0 + 1 adjacent transpositions.
a(n) = Sum(k*A079267(n,k), k>=0).
(End)
Hankel transform is A137592.  Paul Barry, Sep 18 2009
(1, 3, 15, 105,...) = INVERT transform of A000698 starting (1, 2, 10, 74,...).  Gary W. Adamson, Oct 21 2009
a(n) = (1)^(n+1)*H(2*n,0), where H(n,x) is the probabilists' Hermite polynomial. The generating function for the probabilists' Hermite polynomials is as follows: exp(x*tt^2/2)=sum(H(i,x)*t^i/i!,i=0,1,...).  Leonid Bedratyuk, Oct 31 2009
The Hankel transform of a(n+1) is A168467.  Paul Barry, Dec 04 2009
a(n+1) is the number of ways to form n pairs from 2n people.  Andrew (andrewkirk_17(AT)yahoo.co.uk), Sep 07 2010
Partial products of odd numbers.  JuriStepan Gerasimov, Oct 17 2010
See A094638 for connections to differential operators.  Tom Copeland, Sep 20 2011
a(n) is the number of subsets of {1,...,n^2} that contain exactly k elements from {1,...,k^2} for k=1,...,n. For example, a(3)=15 since there are 15 subsets of {1,2,...,9} that satisfy the conditions, namely, {1,2,5}, {1,2,6}, {1,2,7}, {1,2,8}, {1,2,9}, {1,3,5}, {1,3,6}, {1,3,7}, {1,3,8}, {1,3,9}, {1,4,5}, {1,4,6}, {1,4,7}, {1,4,8}, and {1,4,9}.  Dennis P. Walsh, Dec 02 2011
a(n) is the leading coefficient of the Bessel polynomial y_n(x) (cf. A001498).  Leonid Bedratyuk, Jun 01 2012
For n>0: a(n) is also the determinant of the symmetric n X n matrix M defined by M(i,j) = min(i,j)^2 for 1 <= i,j <= n.  Enrique Pérez Herrero, Jan 14 2013
a(n) is also the numerator of the mean value from 0 to Pi/2 of sin(x)^(2n).  JeanFrançois Alcover, Jun 13 2013
a(n) is the size of the Brauer monoid on 2n points (see A227545).  James Mitchell, Jul 28 2013
For n>1: a(n) is the numerator of M(n)/M(1) where the numbers M(i) have the property that M(n+1)/M(n) ~ n1/2 (for example, large KendellMann numbers, see A000140 or A181609, as n > infinity).  Mikhail Gaichenkov, Jan 14 2014
a(n) = the number of uppertriangular matrix representations required for the symbolic representation of a first order central moment of the multivariate normal distribution of dimension 2(n1), i.e., E[X_1*X_2...*X_(2n2)mu=0, Sigma]. See vignette for symmoments R package on CRAN and Phillips reference below.  Kem Phillips, Aug 10 2014
For n>1: a(n) is the number of Feynman diagrams of order 2n (number of internal vertices) for the vacuum polarization with one charged loop only, in quantum electrodynamics.  Robert Coquereaux, Sep 15 2014
Aerated with intervening zeros (1,0,1,0,3,...)=a(n) (cf. A123023), the e.g.f. is e^(t^2/2), so this is the base for the Appell sequence A099174 with e.g.f. e^(t^2/2) e^(x*t) = exp(P(.,x),t) = unsigned A066325(x,t), the probabilist's (or normalized) Hermite polynomials. P(n,x)= (a. + x)^n with (a.)^n = a_n and comprise the umbral compositional inverses for A066325(x,t) = exp(UP(.,x),t), i.e., UP(n,P(.,t))= x^n = P(n,UP(.,t)), where UP(n,t) are the polynomials of A066325 and, e.g., (P(.,t))^n = P(n,t).  Tom Copeland, Nov 15 2014


REFERENCES

M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards Applied Math. Series 55, Tenth Printing, 1972, (26.2.28).
Aebi, Christian, and Grant Cairns. "Generalizations of Wilson’s Theorem for Double, Hyper, Suband Superfactorials." The American Mathematical Monthly 122.5 (2015): 433443.
L. Comtet, Advanced Combinatorics, Reidel, 1974, p. 228, #19.
Hoel, Port and Stone, Introduction to Probability Theory, Section 7.3.
F. K. Hwang, D. S. Richards and P. Winter, The Steiner Tree Problem, NorthHolland, 1992, see p. 14.
C. Itzykson and J.B. Zuber, Quantum Field Theory, McGrawHill, 1980, pages 466467.
L. B. W. Jolley, "Summation of Series", Dover Publications, 1961, p. 48.
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).
R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 2, 1999; see Example 5.2.6 and also p. 178.
R. Vein and P. Dale, Determinants and Their Applications in Mathematical Physics, SpringerVerlag, New York, 1999, p. 73.


LINKS

T. D. Noe, Table of n, a(n) for n = 0..101
M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards, Applied Math. Series 55, Tenth Printing, 1972 [alternative scanned copy].Jonathan Burns, Assembly Graph Words  Single Transverse Component (Counts).
D. Arques and J.F. Beraud, Rooted maps on orientable surfaces, Riccati's equation and continued fractions, Discrete Math., 215 (2000), 112.
H. Bottomley, Illustration for A000108, A001147, A002694, A067310 and A067311
Jonathan Burns, Egor Dolzhenko, Natasa Jonoska, Tilahun Muche and Masahico Saito, FourRegular Graphs with Rigid Vertices Associated to DNA Recombination, May 23, 2011.
David Callan, A combinatorial survey of identities for the double factorial, arXiv:0906.1317 [math.CO], 2009.
Peter J. Cameron, Some treelike objects Quart. J. Math. Oxford Ser. 38 (1987), 155183. MR0891613 (89a:05009). See p. 155.  N. J. A. Sloane, Apr 18 2014
P. J. Cameron, Sequences realized by oligomorphic permutation groups, J. Integ. Seqs. Vol. 3 (2000), #00.1.5.
P. Codara, O. M. D'Antona, P. Hell, A simple combinatorial interpretation of certain generalized Bell and Stirling numbers, arXiv preprint arXiv:1308.1700, 2013.
Thierry DanaPicard, Sequences of Definite Integrals, Factorials and Double Factorials, Journal of Integer Sequences, Vol. 8 (2005), Article 05.4.6.
Filippo Disanto and Thomas Wiehe, Some combinatorial problems on binary rooted trees occurring in population genetics, arXiv preprint arXiv:1112.1295, 2011.
Ghislain R. Franssens, On a Number Pyramid Related to the Binomial, Deleham, Eulerian, MacMahon and Stirling number triangles, Journal of Integer Sequences, Vol. 9 (2006), Article 06.4.1.
S. Goodenough, C. Lavault, On subsets of Riordan subgroups and HeisenbergWeyl algebra, arXiv preprint arXiv:1404.1894, 2014.
W. S. Gray and M. Thitsa, System Interconnections and Combinatorial Integer Sequences, in: System Theory (SSST), 2013 45th Southeastern Symposium on, Date of Conference: 1111 March 2013.
GuoNiu Han, Enumeration of Standard Puzzles
GuoNiu Han, Enumeration of Standard Puzzles [Cached copy]
Aoife Hennessy, A Study of Riordan Arrays with Applications to Continued Fractions, Orthogonal Polynomials and Lattice Paths, Ph. D. Thesis, Waterford Institute of Technology, Oct. 2011.
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 23
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 106
M. Kauers and S.L. Ko, Problem 11545, Amer. Math. Monthly, 118 (2011), p. 84.
A. Khruzin, Enumeration of chord diagrams, arXiv:math/0008209 [math.CO], 2000.
M. Klazar, Twelve countings with rooted plane trees, European Journal of Combinatorics 18 (1997), 195210; Addendum, 18 (1997), 739740.
W. Lang, On generalizations of Stirling number triangles, J. Integer Seqs., Vol. 3 (2000), #00.2.4.
F. Larrion, M. A. Pizana, and R. VillarroelFlores, The clique operator on matching and chessboard graphs Discrete Math. 309 (2009), no. 1, 8593.
E. Lucas, Theorie des nombres (annotated scans of a few selected pages)
Robert J. Marsh and Paul Martin, Tiling bijections between paths and Brauer diagrams, Journal of Algebraic Combinatorics, Vol 33, No 3 (2011), p 427453.
B. E. Meserve, Double Factorials, American Mathematical Monthly, 55 (1948), 425426.
T. Motzkin, The hypersurface cross ratio, Bull. Amer. Math. Soc., 51 (1945), 976984.
T. Motzkin, The hypersurface cross ratio, Bull. Amer. Math. Soc., 51 (1945), 976984.
T. S. Motzkin, Relations between hypersurface cross ratios and a combinatorial formula for partitions of a polygon, for permanent preponderance and for nonassociative products, Bull. Amer. Math. Soc., 54 (1948), 352360.
F. Murtagh, Counting dendrograms: a survey, Discrete Applied Mathematics, 7 (1984), 191199.
G. Nordh, Perfect Skolem sequences, arXiv:math/0506155 [math.CO], 2005.
J.C. Novelli, J.Y. Thibon, Hopf Algebras of mpermutations,(m+1)ary trees, and mparking functions, arXiv preprint arXiv:1403.5962, 2014
R. Ondrejka, Tables of double factorials, Math. Comp., 24 (1970), 231.
L. Pachter and B. Sturmfels, The mathematics of phylogenomics, arXiv:math/0409132 [math.ST], 20042005.
K. Phillips, R functions to symbolically compute the central moments of the multivariate normal distribution, Journal of Statistical Software, Feb 2010.
R. A. Proctor, Let's Expand Rota's Twelvefold Way for Counting Partitions!, arXiv math.CO.0606404.
Helmut Prodinger, Descendants in heap ordered trees or a triumph of computer algebra
S. Ramanujan, Question 541, J. Ind. Math. Soc.
E. Schröder, Vier combinatorische Probleme, Z. f. Math. Phys., 15 (1870), 361376.
Y. S. Song, On the combinatorics of rooted binary phylogenetic trees, Annals of Combinatorics, 7, 2003, 365379. See Lemma 2.1.  N. J. A. Sloane, Aug 22 2014
Andrew Vince and Miklos Bona, The Number of Ways to Assemble a Graph, arXiv preprint arXiv:1204.3842, 2012.
Eric Weisstein's World of Mathematics, Double Factorial, Normal Distribution Function, Erf
Wikipedia, Pfaffian
Wikipedia, Hermite polynomials
Index to divisibility sequences
Index entries for related partitioncounting sequences
Index entries for sequences related to factorial numbers
Index entries for sequences related to parenthesizing


FORMULA

E.g.f.: 1 / sqrt(1  2*x).
a(n) = a(n1)*(2*n1) = (2*n)!/(n!*2^n) = A010050(n)/A000165(n).
a(n) ~ sqrt(2) * 2^n * (n/e)^n.
Rational part of numerator of Gamma(n+1/2): a(n) * sqrt(Pi) / 2^n = Gamma(n+1/2).  Yuriy Brun, Ewa Dominowska (brun(AT)mit.edu), May 12 2001
With interpolated zeros, the sequence has e.g.f. exp(x^2/2).  Paul Barry, Jun 27 2003
The Ramanujan polynomial psi(n+1, n) has value a(n).  Ralf Stephan, Apr 16 2004
a(n) = Sum_{k=0..n} (2)^(nk)*A048994(n, k).  Philippe Deléham, Oct 29 2005
log(1+x+3*x^2+15*x^3+105*x^4+945*x^5+10395*x^6+...) = x+5/2*x^2+37/3*x^3+353/4*x^4+4081/5*x^5+55205/6*x^6+..., where [1, 5, 37, 353, 4081, 55205,...] = A004208.  Philippe Deléham, Jun 20 2006
1/3 + 2/15 + 3/105 +...= 1/2. [Jolley eq. 216]
sum{j=1..n} j/a(j+1) = (11/a(n+1))/2. [Jolley eq. 216]
1/1 + 1/3 + 2/15 + 6/105 + 24/945 +...= Pi/2.  Gary W. Adamson, Dec 21 2006
a(n) = (1/sqrt(2*Pi))*int(x^n*exp(x/2)/sqrt(x),x,0,infty).  Paul Barry, Jan 28 2008
a(n) = A006882(2n1).  R. J. Mathar, Jul 04 2009
G.f.: 1/(1x2x^2/(15x12x^2/(19x30x^2/(113x56x^2/(1 ... (continued fraction).  Paul Barry, Sep 18 2009
a(n) = (1)^n*subs({log(e)=1,x=0},coeff(simplify(series(e^(x*tt^2/2),t,2*n+1)),t^(2*n))*(2*n)!).  Leonid Bedratyuk, Oct 31 2009
a(n) = 2^n*gamma(n+1/2)/gamma(1/2).  Jaume Oliver Lafont, Nov 09 2009
G.f.: 1/(1x/(12x/(13x/(14x/(15x/(1 ...(continued fraction).  Aoife Hennessy (aoife.hennessy(AT)gmail.com), Dec 02 2009
The g.f. of a(n+1) is 1/(13x/(12x/(15x/(14x/(17x/(16x/(1.... (continued fraction).  Paul Barry, Dec 04 2009
a(n) = sum(i=1..n) C(n,i)*a(i1)*a(ni).  Vladimir Shevelev, Sep 30 2010
E.g.f.: A(x)=1sqrt(12*x) satisfies the differential equation A'(x)A'(x)*A(x)1=0.  Vladimir Kruchinin, Jan 17 2011
a(n) = A123023(2*n + 1).  Michael Somos, Jul 24 2011
a(n) = (1/2)*Sum{i=1,...,n} C(n+1,i)*a(i1)*a(ni). See link above.  Dennis P. Walsh, Dec 02 2011
a(n) = sum((1)^k*binomial(2*n,n+k)*Stirling_1(n+k,k),k=0..n) [Kauers and Ko].
a(n) = A035342(n, 1), n >= 1 (first column of triangle).
a(n) = A001497(n, 0) = A001498(n, n), first column, resp. main diagonal, of Bessel triangle.
From Gary W. Adamson, Jul 19 2011: (Start)
a(n) = upper left term of M^n and sum of top row terms of M^(n1), where M = a variant of the (1,2) Pascal triangle (Cf. A029635) as the following production matrix:
1, 2, 0, 0, 0,...
1, 3, 2, 0, 0,...
1, 4, 5, 2, 0,...
1, 5, 9, 7, 2,...
...
For example, a(3) = 15 is the left term in top row of M^3: (15, 46, 36, 8) and a(4) = 105 = (15 + 46 + 36 + 8).
(End)
G.f.: A(x)=1+x/(W(0)x); W(k)=1+x+x*2kx*(2k+3)/W(k+1); (continued fraction).  Sergei N. Gladkovskii, Nov 17 2011
a(n) = sum{i=1,...,n} C(n,i1)*a(i1)*a(ni).  Dennis P. Walsh, Dec 02 2011
a(n) = A009445(n) / A014481(n).  Reinhard Zumkeller, Dec 03 2011
a(n) = (1)^n*sum_{k=0..n} 2^(nk)*s(n+1,k+1), where s(n,k) are the Stirling numbers of the first kind, A048994.  Mircea Merca, May 03 2012
a(n) = (2*n)_4! = Gauss_factorial(2*n,4) = product_{1<=j<=2*n, GCD(j,4)=1} j.  Peter Luschny, Oct 01 2012
G.f.: (1  1/Q(0))/x where Q(k) = 1  x*(2*k1)/(1  x*(2*k+2)/Q(k+1) ); (continued fraction).  Sergei N. Gladkovskii, Mar 19 2013
G.f.: 1 + x/Q(0), where Q(k)= 1 + (2*k1)*x  2*x*(k+1)/Q(k+1); (continued fraction).  Sergei N. Gladkovskii, May 01 2013
G.f.: 2/G(0), where G(k)= 1 + 1/(1  2*x*(2*k+1)/(2*x*(2*k+1)  1 + 2*x*(2*k+2)/G(k+1))); (continued fraction).  Sergei N. Gladkovskii, May 31 2013
G.f.: G(0)/2, where G(k)= 1 + 1/(1  x/(x + 1/(2*k+1)/G(k+1))); (continued fraction).  Sergei N. Gladkovskii, Jun 01 2013
G.f.: G(0), where G(k)= 1 + 2*x*(4*k+1)/(4*k+2  2*x*(2*k+1)*(4*k+3)/(x*(4*k+3) + 2*(k+1)/G(k+1))); (continued fraction).  Sergei N. Gladkovskii, Jun 22 2013
a(n) = (2n3)*a(n2) + (2n2)*a(n1), n>1.  Ivan N. Ianakiev, Jul 08 2013
G.f.: G(0), where G(k)= 1  x*(k+1)/(x*(k+1)  1/G(k+1) ); (continued fraction).  Sergei N. Gladkovskii, Aug 04 2013
a(n) = 2*a(n1) + (2n3)^2*a(n2), a(0) = a(1) = 1.  Philippe Deléham, Oct 27 2013
G.f. of reciprocals: sum_{n>=0} x^n/a(n) = 1F1(1;1/2;x/2), confluent hypergeometric Function.  R. J. Mathar, Jul 25 2014
0 = a(n)*(+2*a(n+1)  a(n+2)) + a(n+1)*(+a(n+1)) for all n in Z.  Michael Somos, Sep 18 2014
a(n) = (1)^n / a(n) = 2*a(n1) + a(n1)^2 / a(n2) for all n in Z.  Michael Somos, Sep 18 2014
From Peter Bala, Feb 18 2015: (Start)
Recurrence equation: a(n) = (3*n  2)*a(n1)  (n  1)*(2*n  3)*a(n2) with a(1) = 1 and a(2) = 3.
The sequence b(n) = A087547(n), beginning [1, 4, 52, 608, 12624, ... ], satisfies the same secondorder recurrence equation. This leads to the generalized continued fraction expansion limit {n > inf} b(n)/a(n) = Pi/2 = 1 + 1/(3  6/(7  15/(10  ...  n*(2*n  1)/((3*n + 1)  ... )))). (End)


EXAMPLE

a(3) = 1*3*5 = 15.
From Joerg Arndt, Sep 10 2013: (Start)
There are a(3)=15 involutions of 6 elements without fixed points:
#: permutation transpositions
01: [ 1 0 3 2 5 4 ] (0, 1) (2, 3) (4, 5)
02: [ 1 0 4 5 2 3 ] (0, 1) (2, 4) (3, 5)
03: [ 1 0 5 4 3 2 ] (0, 1) (2, 5) (3, 4)
04: [ 2 3 0 1 5 4 ] (0, 2) (1, 3) (4, 5)
05: [ 2 4 0 5 1 3 ] (0, 2) (1, 4) (3, 5)
06: [ 2 5 0 4 3 1 ] (0, 2) (1, 5) (3, 4)
07: [ 3 2 1 0 5 4 ] (0, 3) (1, 2) (4, 5)
08: [ 3 4 5 0 1 2 ] (0, 3) (1, 4) (2, 5)
09: [ 3 5 4 0 2 1 ] (0, 3) (1, 5) (2, 4)
10: [ 4 2 1 5 0 3 ] (0, 4) (1, 2) (3, 5)
11: [ 4 3 5 1 0 2 ] (0, 4) (1, 3) (2, 5)
12: [ 4 5 3 2 0 1 ] (0, 4) (1, 5) (2, 3)
13: [ 5 2 1 4 3 0 ] (0, 5) (1, 2) (3, 4)
14: [ 5 3 4 1 2 0 ] (0, 5) (1, 3) (2, 4)
15: [ 5 4 3 2 1 0 ] (0, 5) (1, 4) (2, 3)
(End)
G.f. = 1 + x + 3*x^2 + 15*x^3 + 105*x^4 + 945*x^5 + 10395*x^6 + 135135*x^7 + ...


MAPLE

f := n>(2*n)!/(n!*2^n);
A001147 := proc(n) doublefactorial(2*n1); end: # R. J. Mathar, Jul 04 2009
A001147 := n > 2^n*pochhammer(1/2, n); # Peter Luschny, Aug 09 2009
G(x):=(12*x)^(1/2): f[0]:=G(x): for n from 1 to 29 do f[n]:=diff(f[n1], x) od: x:=0: seq(f[n], n=0..19); # Zerinvary Lajos, Apr 03 2009; aligned with offset by Johannes W. Meijer, Aug 11 2009
series(hypergeom([1, 1/2], [], 2*x), x=0, 20); # Mark van Hoeij, Apr 07 2013


MATHEMATICA

Table[(2n  1)!!, {n, 0, 19}] (* Robert G. Wilson v, Oct 12 2005 *)
a[ n_] := 2^n Gamma[n + 1/2] / Gamma[1/2]; (* Michael Somos, Sep 18 2014 *)


PROG

(PARI) {a(n) = if( n<0, (1)^n / a(n), (2*n)! / n! / 2^n)}; /* Michael Somos, Sep 18 2014 */
(PARI) x='x+O('x^33); Vec(serlaplace((12*x)^(1/2))) /* Joerg Arndt, Apr 24 2011 */
(MAGMA) A001147:=func< n  n eq 0 select 1 else &*[ k: k in [1..2*n1 by 2] ] >; [ A001147(n): n in [0..20] ]; // Klaus Brockhaus, Jun 22 2011
(MAGMA) I:=[1, 3]; [1] cat [n le 2 select I[n] else (3*n2)*Self(n1)(n1)*(2*n3)*Self(n2): n in [1..25] ]; // Vincenzo Librandi, Feb 19 2015
(Haskell)
a001147 n = product [1, 3 .. 2 * n  1]
a001147_list = 1 : zipWith (*) [1, 3 ..] a001147_list
 Reinhard Zumkeller, Feb 15 2015, Dec 03 2011
(Sage) [rising_factorial(n+1, n)/2^n for n in (0..15)] # Peter Luschny, Jun 26 2012


CROSSREFS

Cf. A000085, A006882, A000165 ((2n)!!), A001818, A009445, A039683, A102992, A001190 (no labels), A000680, A132101.
Cf. A086677; A055142 (for this sequence, a(n+1) + 1 is the number of distinct products which can be formed using commutative, nonassociative multiplication and a nonempty subset of n given variables).
Constant terms of polynomials in A098503. First row of array A099020.
Cf. A079267, A000698, A029635, A161198, A076795, A123023.
Cf. A161124, A051125, A181983.
Cf. A099174, A087547.
Subsequence of A248652.
Sequence in context: A189919 A251598 A247304 * A000268 A207818 A118750
Adjacent sequences: A001144 A001145 A001146 * A001148 A001149 A001150


KEYWORD

nonn,easy,nice,core


AUTHOR

N. J. A. Sloane


EXTENSIONS

More terms from Reinhard Zumkeller, Apr 25 2008
Removed erroneous comments: neither the number of n X n binary matrices A such that A^2 = 0 nor the number of simple directed graphs on n vertices with no directed path of length two are counted by this sequence (for n = 3, both are 13).  Dan Drake, Jun 02 2009


STATUS

approved



