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 A006882 Double factorials n!!: a(n) = n*a(n-2) for n > 1, a(0) = a(1) = 1. (Formerly M0876) 220
 1, 1, 2, 3, 8, 15, 48, 105, 384, 945, 3840, 10395, 46080, 135135, 645120, 2027025, 10321920, 34459425, 185794560, 654729075, 3715891200, 13749310575, 81749606400, 316234143225, 1961990553600, 7905853580625, 51011754393600 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Product of pairs of successive terms gives factorials in increasing order. - Amarnath Murthy, Oct 17 2002 a(n) = number of down-up permutations on [n+1] for which the entries in the even positions are increasing. For example, a(3)=3 counts 2143, 3142, 4132. Also, a(n) = number of down-up permutations on [n+2] for which the entries in the odd positions are decreasing. For example, a(3)=3 counts 51423, 52413, 53412. - David Callan, Nov 29 2007 The double factorial of a positive integer n is the product of the positive integers <= n that have the same parity as n. - Peter Luschny, Jun 23 2011 For n even, a(n) is the number of ways to place n points on an n X n grid with pairwise distinct abscissa, pairwise distinct ordinate, and 180-degree rotational symmetry. For n odd, the number of ways is a(n-1) because the center point can be considered "fixed". For 90-degree rotational symmetry cf. A001813, for mirror symmetry see A000085, A135401, and A297708. - Manfred Scheucher, Dec 29 2017 REFERENCES Putnam Contest, 4 Dec. 2004, Problem A3. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Indranil Ghosh, Table of n, a(n) for n = 0..806 (terms 0..100 from T. D. Noe) Christian Aebi and Grant Cairns, Generalizations of Wilson's Theorem for Double-, Hyper-, Sub-and Superfactorials, The American Mathematical Monthly 122.5 (2015): 433-443. Joseph E. Cooper III, A recurrence for an expression involving double factorials, arXiv:1510.00399 [math.CO], 2015. G. T. Leavens and M. Vermeulen, 3x+1 search programs, Computers and Mathematics with Applications, 24 (1992), 79-99. (Annotated scanned copy) P. Luschny, Multifactorials B. E. Meserve, Double Factorials, American Mathematical Monthly, 55 (1948), 425-426. R. Ondrejka, Tables of double factorials, Math. Comp., 24 (1970), 231. Eric Weisstein, Double Factorial , (MathWorld) Eric Weisstein, Multifactorial, (MathWorld) FORMULA a(n) = Product_{i=0..floor((n-1)/2)} (n - 2*i). E.g.f.: 1+exp(x^2/2)*x*(1+sqrt(Pi/2)*erf(x/sqrt(2))). - Wouter Meeussen, Mar 08 2001 Satisfies a(n+3)*a(n) - a(n+1)*a(n+2) = (n+1)!. [Putnam Contest] n!! = 2^[(n + 1)/2]/sqrt(Pi)*Gamma(n/2 + 1)*{[sqrt(Pi)/2^(1/2) + 1]/2 + (-1)^n*[sqrt(Pi)/2^(1/2)-1]/2}. - Paolo P. Lava, Jul 24 2007 a(n) = 2^((1+2*n-cos(n*Pi))/4)*Pi^((cos(n*Pi)-1)/4)*Gamma(1+(1/2)*n). - Paolo P. Lava, Jul 24 2007 a(n) = n!/a(n-1). - Vaclav Kotesovec, Sep 17 2012 a(n) * a(n+3) = a(n+1) * (a(n+2) + a(n)). a(n) * a(n+1) = (n+1)!. - Michael Somos, Dec 29 2012 a(n) ~ c * n^((n+1)/2) / exp(n/2), where c = sqrt(Pi) if n is even, and c = sqrt(2) if n is odd. - Vaclav Kotesovec, Nov 08 2014 a(2*n) = 2^n*a(n)*a(n-1). a(2^n) = 2^(2^n - 1) * 1!! * 3!! * 7!! * ... * (2^(n-1) - 1)!!. - Peter Bala, Nov 01 2016 EXAMPLE G.f. = 1 + x + 2*x^2 + 3*x^3 + 8*x^4 + 15*x^5 + 48*x^6 + 105*x^7 + 384*x^8 + ... MAPLE A006882 := proc(n) option remember; if n <= 1 then 1 else n*A006882(n-2); fi; end; A006882 := proc(n) doublefactorial(n) ; end proc; seq(A006882(n), n=0..10) ; # R. J. Mathar, Oct 20 2009 A006882 := n -> mul(k, k = select(k -> k mod 2 = n mod 2, [\$1 .. n])):  seq(A006882(n), n = 0 .. 10); # Peter Luschny, Jun 23 2011 A006882 := proc(n) if n=0 then 1 else mul(n-2*k, k=0..floor(n/2)-1); fi; end; # N. J. A. Sloane, May 27 2016 MATHEMATICA Array[ #!!&, 40, 0 ] multiFactorial[n_, k_] := If[n < 1, 1, If[n < k + 1, n, n*multiFactorial[n - k, k]]]; Array[ multiFactorial[#, 2] &, 27, 0] (* Robert G. Wilson v, Apr 23 2011 *) PROG (PARI) {a(n) = prod(i=0, (n-1)\2, n - 2*i )} \\ Improved by M. F. Hasler, Nov 30 2013 (PARI) {a(n) = if( n<2, n>=0, n * a(n-2))}; /* Michael Somos, Apr 06 2003 */ (PARI) {a(n) = if( n<0, 0, my(E); E = exp(x^2 / 2 + x * O(x^n)); n! * polcoeff( 1 + E * x * (1 + intformal(1 / E)), n))}; /* Michael Somos, Apr 06 2003 */ (MAGMA) DoubleFactorial:=func< n | &*[n..2 by -2] >; [ DoubleFactorial(n): n in [0..28] ]; // Klaus Brockhaus, Jan 23 2011 (Haskell) a006882 n = a006882_list !! n a006882_list = 1 : 1 : zipWith (*) [2..] a006882_list -- Reinhard Zumkeller, Oct 23 2014 CROSSREFS Bisections are A000165 and A001147. These two entries have more information. Cf. A052319. A diagonal of A202212. Cf. A000085, A001813, A135401, A297708. - Manfred Scheucher, Jan 07 2018 Sequence in context: A148011 A148012 A161178 * A080498 A148013 A133983 Adjacent sequences:  A006879 A006880 A006881 * A006883 A006884 A006885 KEYWORD nonn,easy,core,nice AUTHOR STATUS approved

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Last modified October 14 11:51 EDT 2019. Contains 327997 sequences. (Running on oeis4.)