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 A003149 a(n) = Sum_{k=0..n} k!(n-k)!. (Formerly M1496) 30
 1, 2, 5, 16, 64, 312, 1812, 12288, 95616, 840960, 8254080, 89441280, 1060369920, 13649610240, 189550368000, 2824077312000, 44927447040000, 760034451456000, 13622700994560000, 257872110354432000, 5140559166898176000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS The sequence is the resistance between opposite corners of an (n+1)-dimensional hypercube of unit resistors, multiplied by (n+1)!. The resistances for n+1 = 1,2,3,... are 1, 1, 5/6, 2/3, 8/15, 13/30, 151/420, 32/105, 83/315, 73/315, 1433/6930 ... (see A046878/A046879). Number of {12,21*,2*1}-avoiding signed permutations in the hyperoctahedral group. a(n) is the sum of the reciprocals of the binomial coefficients C(n,k), multiplied by n!; example: a(4) = 4!*(1/1 + 1/4 + 1/6 + 1/4 + 1/1) = 64. - Philippe Deléham, May 12 2005 a(n) is the number of permutations on [n+1] that avoid the pattern 13-2|. The absence of a dash between 1 and 3 means the "1" and "3" must be consecutive in the permutation; the vertical bar means the "2" must occur at the end of the permutation. For example, 24153 fails to avoid this pattern: 243 is an offending subpermutation. - David Callan, Nov 02 2005 n!/A003149(n) is the probability that a random walk on an (n+1)-dimensional hypercube will visit the diagonally opposite vertex before it returns to its starting point. 2^n*A003149(n)/n! is the expected length of a random walk from one vertex of an (n+1)-dimensional hypercube to the diagonally opposite vertex (a walk which may include one or more passes through the starting point). These "random walk" examples are solutions to IBM's "Ponder This" puzzle for April, 2006. - Graeme McRae, Apr 02 2006 a(n) is the number of strong fixed points in all permutations of {1,2,...,n+1} (a permutation p of {1,2,...,n} is said to have j as a strong fixed point (splitter) if p(k)j for k>j). Example: a(2)=5 because the permutations of {1,2,3}, with marked strong fixed points, are: 1'2'3', 1'32, 312, 213', 231 and 321. - Emeric Deutsch, Oct 28 2008 Coefficients in the asymptotic expansion of exp(-2*x)*Ei(x)^2 for x -> inf, where Ei(x) is the exponential integral. - Vladimir Reshetnikov, Apr 24 2016 REFERENCES I. P. Goulden and D. M. Jackson, Combinatorial Enumeration, Wiley, N.Y., 1983, (1.1.11 b, p.342). N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). R. P. Stanley, Enumerative Combinatorics, Volume 1 (1986), p. 49. [From Emeric Deutsch, Oct 28 2008] LINKS T. D. Noe, Table of n, a(n) for n=0..100 Joerg Arndt, Generating Random Permutations, PhD thesis, Australian National University, Canberra, Australia, (2010). Fred Curtis, Resistance-network Problems. Todd Feil, Gary Kennedy and David Callan, Problem E3467, Amer. Math. Monthly, 100 (1993), 800-801. [From Emeric Deutsch, Oct 28 2008] Henry W. Gould, Letter to N. J. A. Sloane, Nov 1973, and various attachments. T. Mansour and J. West, Avoiding 2-letter signed patterns, arXiv:math/0207204 [math.CO], 2002. F. Nedemeyer and Y. Smorodinsky, Resistance in the multidimensional cube, Quantum 7:1 (1996) 12-15 and 63. R. Sprugnoli, Moments of Reciprocals of Binomial Coefficients, Journal of Integer Sequences, 14 (2011), #11.7.8. V. Strehl, The average number of splitters in a random permutation [Unpublished; included here with the author's permission.] B. Sury, Sum of the reciprocals of the binomial coefficients, Europ. J. Comb., 14 (1993), 351-353. Eric Weisstein's World of Mathematics, Incomplete Beta Function. Eric Weisstein's World of Mathematics, Lerch Transcendent. FORMULA a(n) = n! + ((n+1)/2)*a(n-1), n >= 1. - Leroy Quet, Sep 06 2002 a(n) = ((3n+1)*a(n-1) - n^2*a(n-2))/2, n >= 2. - David W. Wilson, Sep 06 2002; corrected by N. Sato, Jan 27 2010 G.f.: (Sum_{k>=0} k!*x^k)^2. - Vladeta Jovovic, Aug 30 2002 E.g.f: log(1-x)/(x/2 - 1) if offset 1. Convolution of A000142 [factorial numbers] with itself. - Ross La Haye, Oct 29 2004 a(n) = Sum){k=0..n+1} k*A145878(n+1,k). - Emeric Deutsch, Oct 28 2008 a(n) = A084938(n+2,2). - Philippe Deléham, Dec 17 2008 a(n) = 2*int(Ei(t)*exp(-2*t)*t^(n+1),t=0..infinity) where Ei is the exponential integral function. - Groux Roland, Dec 09 2010 Empirical: a(n-1)=2^(-n)*(A103213(n)+n!H(n)) with H(n) harmonic number of order n. - Groux Roland, Dec 18 2010; offset fixed by Vladimir Reshetnikov, Apr 24 2016 O.g.f.: 1/(1-I(x))^2 where I(x) is o.g.f. for A003319. - Geoffrey Critzer, Apr 27 2012 a(n) ~ 2*n!. - Vaclav Kotesovec, Oct 04 2012 a(n) = (n+1)!/2^n * Sum_{k=0..n} 2^k/(k+1). - Vaclav Kotesovec, Oct 27 2012 E.g.f.: 2/((x-1)*(x-2)) + 2*x/(x-2)^2*G(0) where G(k) = 1 + x*(2*k+1)/(2*(k+1) - 4*x*(k+1)^2/(2*x*(k+1) + (2*k+3)/G(k+1) )); (recursively defined continued fraction). - Sergei N. Gladkovskii, Dec 14 2012 a(n) = 2 * n! * (1 + Sum_{k>=1} A005649(k-1)/n^k). - Vaclav Kotesovec, Aug 01 2015 From Vladimir Reshetnikov, Nov 12 2015: (Start) a(n) = -(n+1)!*Re(Beta(2; n+2, 0))/2^(n+1), where Beta(z; a, b) is the incomplete Beta function. a(n) = -2*(n+1)!*Re(LerchPhi(2, 1, n+2)), where LerchPhi(z, s, a) is the Lerch transcendent. (End) a(n) = (n+1)!*(H(n+1) + (n+1)*hypergeom([1, 1, -n], [2, 2], -1))/2^(n+1), where H(n) is the harmonic number. - Vladimir Reshetnikov, Apr 24 2016 Expansion of square of continued fraction 1/(1 - x/(1 - x/(1 - 2*x/(1 - 2*x/(1 - 3*x/(1 - 3*x/(1 - ...))))))). - Ilya Gutkovskiy, Apr 19 2017 MAPLE seq( add(k!*(n-k)!, k=0..n), n=0..20); # G. C. Greubel, Dec 29 2019 MATHEMATICA Table[Sum[k!(n-k)!, {k, 0, n}], {n, 0, 20}] (* Harvey P. Dale, Mar 28 2012 *) Table[(n+1)!/2^n*Sum[2^k/(k+1), {k, 0, n}], {n, 0, 20}] (* Vaclav Kotesovec, Oct 27 2012 *) Round@Table[-2 (n+1)! Re[LerchPhi[2, 1, n+2]], {n, 0, 20}] (* Vladimir Reshetnikov, Nov 12 2015 *) Table[(n+1)!*Sum[Binomial[n+1, 2*j+1]/(2*j+1), {j, 0, n}]/2^n, {n, 0, 20}] (* Vaclav Kotesovec, Dec 04 2015 *) Series[Exp[-2x] ExpIntegralEi[x]^2, {x, Infinity, 20}][] (* Vladimir Reshetnikov, Apr 24 2016 *) PROG (PARI) a(n)=sum(k=0, n, k!*(n-k)!) (PARI) a(n)=if(n<0, 0, (n+1)!*polcoeff(log(1-x+x^2*O(x^n))/(x/2-1), n+1)) (MAGMA) F:=Factorial; [ (&+[F(k)*F(n-k): k in [0..n]]): n in [0..20]]; // G. C. Greubel, Dec 29 2019 (Sage) f=factorial; [sum(f(k)*f(n-k) for k in (0..n)) for n in (0..20)] # G. C. Greubel, Dec 29 2019 (GAP) F:=Factorial;; List([0..20], n-> Sum([0..n], k-> F(k)*F(n-k)) ); # G. C. Greubel, Dec 29 2019 CROSSREFS Cf. A046825, A046878, A046879. Cf. A052186, A006932, A145878. - Emeric Deutsch, Oct 28 2008 Sequence in context: A185998 A127083 A131178 * A027046 A268170 A000522 Adjacent sequences:  A003146 A003147 A003148 * A003150 A003151 A003152 KEYWORD nonn,easy,nice AUTHOR EXTENSIONS More terms from Michel ten Voorde, Apr 11 2001 Additional comments from Michael Somos, Feb 14 2002 STATUS approved

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Last modified October 17 17:21 EDT 2021. Contains 348065 sequences. (Running on oeis4.)