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 A010844 a(n) = 2*n*a(n-1) + 1 with a(0) = 1. 31
 1, 3, 13, 79, 633, 6331, 75973, 1063623, 17017969, 306323443, 6126468861, 134782314943, 3234775558633, 84104164524459, 2354916606684853, 70647498200545591, 2260719942417458913, 76864478042193603043, 2767121209518969709549, 105150605961720848962863 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Related to Incomplete Gamma Function at 1/2. - Michael Somos, Mar 26 1999 For positive n, a(n) is equal to 2^n times the permanent of the n X n matrix with 3/2's along the main diagonal, and 1's everywhere else. - John M. Campbell, Jul 09 2011 Number of ways to sort a spreadsheet with n columns. (A subset of columns is chosen to sort on. These columns are ordered from major to minor, and each designated as to whether to sort by ascending or descending order. For example a spreadsheet with columns A,B,C,D could be sorted by column D ascending, then by column B descending, or any of 632 other ways.) - Marc LeBrun, Dec 07 2013 a(n) is a specific instance of sequences having the form b(0) = x, b(n) = a*n*b(n-1) + k for n >= 1. (Here x = 1, a = 2, and k = 1). Sequences of this form have a closed form of b(n) = n!*a^n*x + k*Sum_{j=1..n} n!*a^(n-j)/j!. - Gary Detlefs, Mar 26 2018 REFERENCES M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards Applied Math. Series 55, Tenth Printing, 1972, p. 262. LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..400 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]. M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards Applied Math. Series 55, Tenth Printing, 1972, p. 262. Roland Bacher, Counting Packings of Generic Subsets in Finite Groups, Electr. J. Combinatorics, 19 (2012), #P7. - From N. J. A. Sloane, Feb 06 2013 Paul Barry, Eulerian-Dowling Polynomials as Moments, Using Riordan Arrays, arXiv:1702.04007 [math.CO], 2017. Mathieu Guay-Paquet and Jeffrey Shallit, Avoiding Squares and Overlaps Over the Natural Numbers, Discrete Math., 309 (2009), 6245-6254. [From N. J. A. Sloane, Nov 27 2009] Mathieu Guay-Paquet and Jeffrey Shallit, Avoiding Squares and Overlaps Over the Natural Numbers, arXiv:0901.1397 [math.CO], 2009. Guo-Niu Han, Enumeration of Standard Puzzles [Wayback Machine link added by Felix Fröhlich, Mar 26 2018] Guo-Niu Han, Enumeration of Standard Puzzles. [Cached copy] Guo-Niu Han, Enumeration of Standard Puzzles, arXiv:2006.14070 [math.CO], 2020. Michael Z. Spivey and Laura L. Steil, The k-Binomial Transforms and the Hankel Transform, Journal of Integer Sequences, Vol. 9 (2006), Article 06.1.1. FORMULA a(n) = floor(n! * e^(1/2) * 2^n) = n! * Sum_{k=0..n} 2^(n-k) / k! (i.e. binomial transform of (2n)!! = n!*2^n) = n! * (e^(1/2) * 2^n - Sum_{k >= n+1} 2^(n-k) / k!)). - Michael Somos, Mar 26 1999 a(n) = A056541(n) + A000165(n). - Henry Bottomley, Jun 20 2000 E.g.f.: exp(x)/(1 - 2*x). - Vladeta Jovovic, Aug 11 2002 Sum_{n >= 1} 1/a(n) = 0.4246665348160769533082551230... - Cino Hilliard, Aug 19 2003 a(n) = Sum_{k=0..n} P(n, k)*2^k, where P(n,k) = n!/(n-k)!. - Ross La Haye, Aug 29 2005 G.f.: 1/(1 - x - 2*x/(1 - 2*x/(1 - x - 4*x/(1 - 4*x/(1 - x - 6*x/(1 - 6*x/(1 - x - 8*x/(1 - 8*x/(1 - x - 10*x/(1 - ... (continued fraction). G.f.: 1/Q(0), where Q(k) = 1 - x*(4*k+3) - 4*x^2*(k+1)^2/Q(k+1); (continued fraction). - Sergei N. Gladkovskii, Sep 30 2013 a(n) = Sum_{k=0..n} C(n,k)*k!*2^k. - Marc LeBrun, Dec 07 2013 0 = a(n)*(2*a(n+1) - 5*a(n+2) + a(n+3)) + a(n+1)*(a(n+1) + a(n+2) - a(n+3)) + a(n+2)*a(n+2) if n > -2. - Michael Somos, Jan 02 2014 a(n) + (-2*n-1)*a(n-1) + 2*(n-1)*a(n-2) = 0. - R. J. Mathar, Jan 31 2014 a(n) = hypergeometric_U(1, n+2, 1/2)/2. - Peter Luschny, Nov 26 2014 From Peter Bala, Jan 30 2015: (Start) a(n) = Integral_{x >= 0} (2*x + 1)^n*exp(-x) dx. (Cf. A000354.) The e.g.f. y = exp(x)/(1 - 2*x) satisfies the differential equation (1 - 2*x)*y' = (3 - 2*x)*y. R. J. Mathar's recurrence above follows easily from this. The sequence b(n) := 2^n*n! also satisfies R. J. Mathar's recurrence with b(0) = 1 and b(1) = 2. This leads to the continued fraction representation a(n) = 2^n*n!*( 1 + 1/(2 - 2/(5 - 4/(7 - ... - (2*n - 2)/(2*n + 1) )))) for n >= 2. Taking the limit gives the continued fraction representation exp(1/2) = 1 + 1/(2 - 2/(5 - 4/(7 - ... - (2*n - 2)/((2*n + 1) - ... )))). (End) EXAMPLE a(3) = 2*3*a(2) + 1 = 6*13 + 1 = 79. G.f. = 1 + 3*x + 13*x^2 + 79*x^3 + 633*x^4 + 6331*x^5 + 75973*x^6 + 1063623*x^7 + ... MAPLE G:=(x, a, k, n)-> n!*a^n*x + k*sum(n!*a^(n-j)/j!, j=1..n); seq(G(1, 2, 1, n), n = 0..20) # Gary Detlefs, Mar 26 2018 a := n -> 2^n*add((n!/k!)*(1/2)^k, k=0..n): seq(a(n), n=0..19); # Peter Luschny, Jan 06 2020 MATHEMATICA Table[ Gamma[ n, 1/2 ]*Exp[ 1/2 ]*2^(n-1), {n, 1, 24} ] ...and/or... s=1; lst={}; Do[s+=s++n; AppendTo[lst, s], {n, 1, 5!, 2}]; lst (* Vladimir Joseph Stephan Orlovsky, Oct 23 2008 *) a[ n_] := If[ n<0, 0, Floor[ n! E^(1/2) 2^n ]] (* Michael Somos, Sep 04 2013 *) PROG (PARI) {a(n) = if( n<0, 0, n! * sum(k=0, n, 2^(n-k) / k!))} /* Michael Somos, Sep 04 2013 */ CROSSREFS Cf. A000165, A000354, A000522, A007566, A010845, A019774, A097817, A097819. Sequence in context: A213527 A261601 A125659 * A090364 A112935 A258377 Adjacent sequences:  A010841 A010842 A010843 * A010845 A010846 A010847 KEYWORD easy,nonn AUTHOR EXTENSIONS Better description and formulas from Michael Somos STATUS approved

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Last modified April 22 19:11 EDT 2021. Contains 343177 sequences. (Running on oeis4.)