A006882 -id:A006882 - cp's OEIS frontend

A079484 a(n) = (2n-1)!! * (2n+1)!!, where the double factorial is A006882.

1, 3, 45, 1575, 99225, 9823275, 1404728325, 273922023375, 69850115960625, 22561587455281875, 9002073394657468125, 4348001449619557104375, 2500100833531245335015625, 1687568062633590601135546875, 1321365793042101440689133203125

Offset: 0

Benoit Cloitre, Jan 17 2003

nonn

a(n) is the determinant of M(2n+1) where M(k) is the k X k matrix with m(i,j)=j if i+j=k m(i,j)=i otherwise. - Adapted to offset 0, Rainer Rosenthal, Jun 19 2024
In the following two comments on the calculation of the terms using permanents, offset 1 is assumed. In the corresponding PARI code, this is implemented with offset 0. - Hugo Pfoertner, Jun 23 2024
(-1)^n*a(n)/2^(2n-1) is the permanent of the (m X m) matrix {1/(x_i-y_j), 1<=i<=m, 1<=j<=m}, where x_1,x_2,...,x_m are the zeros of x^m-1 and y_1,y_2,...,y_m the zeros of y^m+1 and m=2n-1.
In 1881, R. F. Scott posed a conjecture that the absolute value of permanent of square matrix with elements a(i,j)= (x_i - y_j)^(-1), where x_1,...,x_n are roots of x^n=1, while y_1,...,y_n are roots of y^n=-1, equals a((n-1)/2)/2^n, if n>=1 is odd, and 0, if n>=2 is even. After a century (in 1979), the conjecture was proved by H. Minc. - Vladimir Shevelev, Dec 01 2013
a(n) is the number of permutations in S_{2n+1} in which all cycles have odd length. - José H. Nieto S., Jan 09 2012
Number of 3-bundled increasing bilabeled trees with 2n labels. - Markus Kuba, Nov 18 2014
a(n) is the number of rooted, binary, leaf-labeled topologies with 2n+2 leaves that have n+1 cherry nodes. - Noah A Rosenberg, Feb 12 2019

			G.f. = 1 + 3*x + 45*x^2 + 1575*x^3 + 99225*x^4 + 9823275*x^5 + ...
M(5) =
  [1, 2, 3, 1, 5]
  [1, 2, 2, 4, 5]
  [1, 3, 3, 4, 5]
  [4, 2, 3, 4, 5]
  [1, 2, 3, 4, 5].
Integral_{x=0..oo} x^3*BesselK(1, sqrt(x)) = 1575*Pi. - _Olivier Gérard_, May 20 2009

Miklós Bóna, A walk through combinatorics, World Scientific, 2006.

Alois P. Heinz, Table of n, a(n) for n = 0..224
Cyril Banderier, Markus Kuba, and Michael Wallner, Analytic Combinatorics of Composition schemes and phase transitions with mixed Poisson distributions, arXiv:2103.03751 [math.PR], 2021.
Guo-Niu Han and Christian Krattenthaler, Rectangular Scott-type permanents, arXiv:math/0003072 [math.RA], 2000.
Markus Kuba and Alois Panholzer, Combinatorial families of multilabelled increasing trees and hook-length formulas, arXiv:1411.4587 [math.CO], 17 Nov 2014.
MathOverflow, Geometric / physical / probabilistic interpretations of Riemann zeta(n>1)?, answer by Tom Copeland posted in Aug 2021.
Henryk Minc, On a conjecture of R. F. Scott (1881), Linear Algebra Appl., Vol. 28 (1979), pp. 141-153.
Theodoros Theodoulidis, On the Closed-Form Expression of Carson’s Integral, Period. Polytech. Elec. Eng. Comp. Sci., Vol. 59, No. 1 (2015), pp. 26-29.
Eric Weisstein's World of Mathematics, Struve function.

Cf. A001818, A000165.
Bisection of A000246, A053195, |A013069|, |A046126|. Cf. A000909.
Cf. A001044, A010791, |A129464|, A114779, are also values of similar moments.
Equals the row sums of A162005.
Cf. A316728.
Diagonal elements of A306364 in even-numbered rows.

I:=[1, 3]; [n le 2 select I[n] else (4*n^2-8*n+3)*Self(n-1): n in [1..20]]; // Vincenzo Librandi, Nov 18 2014

a:= n-> (d-> d(2*n-1)*d(2*n+1))(doublefactorial):
seq(a(n), n=0..15);  # Alois P. Heinz, Jan 30 2013
# second Maple program:
A079484 := n-> LinearAlgebra[Determinant](Matrix(2*n+1, (i, j)-> `if`(i+j=2*n+1, j, i))): seq(A079484(n), n=0..14); # Rainer Rosenthal, Jun 18 2024

a[n_] := (2n - 1)!!*(2n + 1)!!; Table[a[n], {n, 0, 13}] (* Jean-François Alcover, Jan 30 2013 *)

/* Formula using the zeta function and a log integral:*/
L(n)= intnum(t=0, 1, log(1-1/t)^n);
Zetai(n)= -I*I^n*(2*Pi)^(n-1)/(n-1)*L(1-n);
a(m)={my(n=m+1);round(real(-I*2^(2*n-1)*Zetai(1/2-n)*L(-1/2+n)/(Zetai(-1/2+n)*L(1/2-n))))};
/* Gerry Martens, Mar 07 2011, adapted to offset 0 by Hugo Pfoertner, Jun 19 2024 */

{a(n) = if( n<0, -1 / self()(-1-n), (2*n + 1)! * (2*n)! / (n! * 2^n)^2 )}; /* Michael Somos, May 04 2017 */

{a(n) = if( n<0, -1 / self()(-1-n), my(m = 2*n + 1); m! * polcoeff( x / sqrt( 1 - x^2 + x * O(x^m) ), m))}; /* Michael Somos, May 04 2017 */

\\ using the Pochhammer symbol
a(n) = {my(P(x,k)=gamma(x+k)/gamma(x)); 4^n*round(P(1/2,n)*P(3/2,n))} \\ Hugo Pfoertner, Jun 20 2024

\\ Scott's (1881) method
a(n) = {my(m=2*n+1, X = polroots(x^m-1), Y = polroots(x^m+1), M = matrix(m, m, i, j, 1/(X[i]-Y[j]))); (-1)^n * round(2^m * real(matpermanent(M)))}; \\ Hugo Pfoertner, Jun 23 2024

D-finite with recurrence a(n) = (4*n^2 - 1) * a(n-1) for all n in Z.
a(n) = A001147(n)*A001147(n+1).
E.g.f.: 1/(1-x^2)^(3/2) (with interpolated zeros). - Paul Barry, May 26 2003
a(n) = (2n+1)! * C(2n, n) / 2^(2n). - Ralf Stephan, Mar 22 2004.
Alternatingly signed values have e.g.f. sqrt(1+x^2).
a(n) is the value of the n-th moment of (1/Pi)*BesselK(1, sqrt(x)) on the positive part of the real line. - Olivier Gérard, May 20 2009
a(n) = -2^(2*n-1)*exp(i*n*Pi)*gamma(1/2+n)/gamma(3/2-n). - Gerry Martens, Mar 07 2011
E.g.f. (odd powers) tan(arcsin(x)) = Sum_{n>=0} (2n-1)!!*(2n+1)!!*x^(2*n+1)/(2*n+1)!. - Vladimir Kruchinin, Apr 22 2011
G.f.: 1 + x*(G(0) - 1)/(x-1) where G(k) = 1 - ((2*k+2)^2-1)/(1-x/(x - 1/G(k+1))); ( continued fraction ). - Sergei N. Gladkovskii, Jan 15 2013
a(n) = (2^(2*n+3)*Gamma(n+3/2)*Gamma(n+5/2))/Pi. - Jean-François Alcover, Jul 20 2015
Limit_{n->oo} 4^n*(n!)^2/a(n) = Pi/2. - Daniel Suteu, Feb 05 2017
From Michael Somos, May 04 2017: (Start)
a(n) = (2*n + 1) * A001818(n).
E.g.f.: Sum_{n>=0} a(n) * x^(2*n+1) / (2*n+1)! = x / sqrt(1 - x^2) = tan(arcsin(x)).
Given e.g.f. A(x) = y, then x * y' = y + y^3.
a(n) = -1 / a(-1-n) for all n in Z.
0 = +a(n)*(+288*a(n+2) -60*a(n+3) +a(n+4)) +a(n+1)*(-36*a(n+2) -4*a(n+3)) +a(n+2)*(+3*a(n+2)) for all n in Z. (End)
a(n) = Sum_{k=0..2n} (k+1) * A316728(n,k). - Alois P. Heinz, Jul 12 2018
From Amiram Eldar, Mar 18 2022: (Start)
Sum_{n>=0} 1/a(n) = 1 + L_1(1)*Pi/2, where L is the modified Struve function.
Sum_{n>=0} (-1)^n/a(n) = 1 - H_1(1)*Pi/2, where H is the Struve function. (End)

Simpler description from Daniel Flath (deflath(AT)yahoo.com), Mar 05 2004

A004730 Numerator of n!!/(n+1)!! (cf. A006882).

Original entry on oeis.org

1, 1, 2, 3, 8, 5, 16, 35, 128, 63, 256, 231, 1024, 429, 2048, 6435, 32768, 12155, 65536, 46189, 262144, 88179, 524288, 676039, 4194304, 1300075, 8388608, 5014575, 33554432, 9694845, 67108864, 300540195, 2147483648, 583401555, 4294967296, 2268783825

Offset: 0

N. J. A. Sloane

T. D. Noe, Table of n, a(n) for n=0..300
Joseph E. Cooper III, A recurrence for an expression involving double factorials, arXiv:1510.00399 [math.CO], 2015.
Svante Janson, On the traveling fly problem, Graph Theory Notes of New York Vol. XXXI, 17, 1996.

Cf. A004731 (denominator), A006882 (double factorials).

import Data.Ratio ((%), denominator)
a004730 n = a004730_list !! n
a004730_list = map denominator ggs where
   ggs = 1 : 2 : zipWith (+) ggs (map (1 /) $ tail ggs) :: [Rational]
-- Reinhard Zumkeller, Dec 08 2011

DoubleFactorial:=func< n | &*[n..2 by -2] >; [ Numerator(DoubleFactorial(n) / DoubleFactorial(n+1)): n in [0..35]]; // Vincenzo Librandi, Dec 03 2018

Numerator[#[[1]]/#[[2]]&/@Partition[Range[0,40]!!,2,1]] (* Harvey P. Dale, Jan 22 2013 *)
Numerator[CoefficientList[Series[(1 - Sqrt[1 - c^2] + ArcSin[c])/(c Sqrt[1 - c^2]),{c, 0, 39}], c]] (* Eugene d'Eon, Nov 01 2018 *)

f(n) = prod(i=0, (n-1)\2, n - 2*i); \\ A006882
a(n) = numerator(f(n)/f(n+1)); \\ Michel Marcus, Feb 09 2025

from sympy import gcd, factorial2
def A004730(n):
    a, b = factorial2(n), factorial2(n+1)
    return a//gcd(a,b) # Chai Wah Wu, Apr 03 2021

Let y(m) = y(m-2) + 1/y(m-1) for m >= 2, with y(0)=y(1)=1. Then the denominator of y(n+1) equals the numerator of n!!/(n+1)!! for n >= 0, where the double factorials are given by A006882. [Reinhard Zumkeller, Dec 08 2011, as corrected in Cooper (2015)]

A115651 Powerful(1) numbers (A001694) which are the sum of distinct double factorials (A006882).

Original entry on oeis.org

1, 4, 8, 9, 16, 25, 27, 49, 64, 72, 108, 121, 125, 128, 169, 392, 400, 432, 441, 500, 512, 961, 968, 972, 1125, 1331, 1352, 1444, 3844, 3888, 3969, 4225, 4232, 4356, 4900, 4913, 5184, 5292, 5324, 10404, 10800, 10952, 11449, 11881, 14283, 14400

Offset: 1

Giovanni Resta, Jan 28 2006

nonn

Double factorials 0!! and 1!! are not considered distinct. Note that double factorial (n!!) is different from (n!)!.

			392 = 2^3*7^2 = 8!! + 4!!.

Index entries for sequences related to powerful numbers

Cf. A001694, A006882, A115652, A115653, A115654.

A232618 a(n) = (2n)!! mod (2n-1)!! where k!! = A006882(k).

Original entry on oeis.org

0, 2, 3, 69, 60, 4500, 104580, 186795, 13497435, 442245825, 13003053525, 64585694250, 3576632909850, 147580842959550, 5708173568847750, 27904470362393625, 2292043480058957625, 126842184377462428875, 6371504674680470700375, 312265748715684068930625

Offset: 1

Alex Ratushnyak, Nov 27 2013

nonn

(2n)!! is the product of first n even numbers, (2n-1)!! is the product of first n odd numbers.

			a(3) = A006882(6) mod A006882(5) = 2*4*6 mod 1*3*5 = 48 mod 15 = 3.

Cf. A006882, A122649, A129890, A232617.

Table[Mod[(2n)!!,(2n-1)!!],{n,20}] (* Harvey P. Dale, Sep 23 2020 *)

o=e=1
for n in range(1,99,2):
  o*=n
  e*=n+1
  print(str(e%o), end=',')

a(n) = A006882(2*n) mod A006882(2*n-1).

A095987 a(n) = gcd(n!!, (n-1)!!) where n!! = A006882.

Original entry on oeis.org

1, 1, 1, 1, 1, 1, 3, 3, 3, 3, 15, 15, 45, 45, 315, 315, 315, 315, 2835, 2835, 14175, 14175, 155925, 155925, 467775, 467775, 6081075, 6081075, 42567525, 42567525, 638512875, 638512875, 638512875, 638512875, 10854718875, 10854718875, 97692469875, 97692469875

Offset: 0

Leroy Quet, Jul 18 2004

nonn

Let f_n(m) be a multifactorial: for m = positive integer, f_n(m) = Product_{k=0..floor((m-1)/n)} (m - k*n). E.g., f_2(m) = m!!. f_n(0) is defined as 1.

a(2n) gives A049606.

a:= n-> (d-> gcd(d(n), d(n-1)))(doublefactorial):
seq(a(n), n=0..40);  # Alois P. Heinz, Oct 26 2019

f[n_] := GCD[n!!, (n - 1)!! ]; Table[ f[n], {n, 35}]
GCD@@#&/@Partition[Range[0,40]!!,2,1] (* Harvey P. Dale, May 04 2015 *)

a(2m) = a(2m+1) = A049606(m).

Edited and extended by Robert G. Wilson v, Jul 19 2004

Missing a(0)=1 inserted by Alois P. Heinz, Oct 26 2019

A115652 Brilliant numbers (A078972) which are the sum of distinct double factorials (A006882).

Original entry on oeis.org

4, 6, 9, 10, 14, 15, 21, 25, 49, 121, 169, 403, 407, 437, 451, 493, 517, 551, 949, 961, 1003, 1007, 1067, 1073, 1079, 1121, 1333, 1343, 1349, 1357, 1387, 1403, 1457, 1501, 3869, 3901, 3953, 4331, 4891, 5183, 5293, 10403, 10807, 11413, 11449

Offset: 1

Giovanni Resta, Jan 28 2006

Double factorials 0!! and 1!! are not considered distinct. Note that double factorial (n!!) is different from (n!)!.

			949 = 13*73 = 9!! + 3!! + 1!!.

Cf. A006882, A078972, A115651, A115653, A115654.

A115653 Prime numbers which are the sum of distinct double factorials (A006882).

Original entry on oeis.org

2, 3, 5, 11, 13, 17, 19, 23, 29, 53, 59, 61, 67, 71, 73, 107, 109, 113, 131, 157, 163, 167, 173, 179, 181, 389, 397, 401, 409, 433, 443, 449, 457, 461, 491, 499, 503, 509, 541, 547, 557, 563, 947, 953, 971, 997, 1009, 1013, 1019, 1021, 1051, 1061, 1063

Offset: 1

Giovanni Resta, Jan 28 2006

nonn

Double factorials 0!! and 1!! are not considered distinct. Note that double factorial (n!!) is different from (n!)!.

			947 is prime and 947 = 9!! + 2!!.

Cf. A006882, A115651, A115652, A115654.

A115654 Semiprimes (A001358) which are the sum of distinct double factorials (A006882).

Original entry on oeis.org

4, 6, 9, 10, 14, 15, 21, 25, 26, 49, 51, 57, 58, 62, 65, 69, 74, 77, 106, 111, 115, 118, 119, 121, 122, 123, 129, 133, 134, 155, 158, 159, 161, 166, 169, 177, 178, 386, 393, 394, 395, 398, 403, 407, 411, 413, 437, 445, 446, 447, 451, 453, 458, 489, 493, 497

Offset: 1

Giovanni Resta, Jan 28 2006

nonn

Double factorials 0!! and 1!! are not considered distinct. Note that double factorial (n!!) is different from (n!)!.

			384 = 2*19 = 8!!+2!!.

Cf. A006882, A001358, A115651, A115652, A115653.

Union[Select[Total/@Subsets[Range[10]!!,10],PrimeOmega[#]==2&]] (* Harvey P. Dale, Aug 24 2012 *)

A167367 a(n) = sigma(n!!) where n!! is A006882(n).

Original entry on oeis.org

1, 1, 3, 4, 15, 24, 124, 192, 1020, 1920, 12264, 23040, 159666, 322560, 2555280, 5041344, 40893840, 90744192, 761260368, 1814883840, 15732804296, 38900010240, 377587663200, 933600245760, 9087075973248, 23520702965760, 254438142416640

Offset: 0

Vladimir Joseph Stephan Orlovsky, Nov 01 2009

nonn

G. C. Greubel, Table of n, a(n) for n = 0..500

Cf. A062569.

Join[{1},Array[DivisorSigma[1,#!! ]&,50,1]]

a(n)=sigma(prod(i=0, (n-1)\2, n - 2*i )) \\ Charles R Greathouse IV, May 01 2016

a(n) = A000203(A006882(n)). - R. J. Mathar, Feb 07 2011

A232617 Product of first n odd numbers plus product of first n even numbers: (2n-1)!! + (2n)!!, where k!! = A006882(k).

Original entry on oeis.org

3, 11, 63, 489, 4785, 56475, 780255, 12348945, 220253985, 4370620275, 95498916975, 2278224696825, 58917607974225, 1641787169697675, 49040157044253375, 1563094742062478625, 52953322446161762625, 1899986948191060603875, 71977860935783603175375, 2870913642898706235455625

Offset: 1

Alex Ratushnyak, Nov 27 2013

nonn

			a(3) = 1*3*5 + 2*4*6 = 15 + 48 = 63.

Cf. A006882, A122649, A129890.

Table[n!!+(n+1)!!,{n,1,41,2}] (* Harvey P. Dale, Jan 22 2019 *)

a(n)=prod(i=1,n,2*i-1)+prod(i=1,n,2*i) \\ Ralf Stephan, Nov 28 2013

o=e=1
for n in range(1,99,2):
  o*=n
  e*=n+1
  print(str(e+o), end=',')

a(n) = A006882(2*n-1) + A006882(2*n).
a(n) = A001147(n) + A000165(n).
a(n) +(-4*n+3)*a(n-1) +2*(n-1)*(2*n-3)*a(n-2)=0. - R. J. Mathar, Nov 23 2014

cp's OEIS Frontend

A079484 a(n) = (2n-1)!! * (2n+1)!!, where the double factorial is A006882.

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A004730 Numerator of n!!/(n+1)!! (cf. A006882).

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A115651 Powerful(1) numbers (A001694) which are the sum of distinct double factorials (A006882).

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A232618 a(n) = (2n)!! mod (2n-1)!! where k!! = A006882(k).

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A095987 a(n) = gcd(n!!, (n-1)!!) where n!! = A006882.

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A115652 Brilliant numbers (A078972) which are the sum of distinct double factorials (A006882).

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A115653 Prime numbers which are the sum of distinct double factorials (A006882).

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A115654 Semiprimes (A001358) which are the sum of distinct double factorials (A006882).

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