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 A093345 a(n) = n! * {1 + Sum[i=1..n, 1/i*Sum(j=0..i-1, 1/j!)]}. 2
 1, 2, 6, 23, 108, 605, 3956, 29649, 250892, 2367629, 24662700, 281153801, 3482350724, 46572620757, 668943488084, 10271127486065, 167892667249116, 2911049382788189, 53365747562592092, 1031352659792534169 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Number of {12,2*1}-avoiding signed permutations in the hyperoctahedral group B_n. LINKS D. Daly and L. Pudwell, Pattern avoidance in rook monoids, 2013. - From N. J. A. Sloane, Feb 03 2013 T. Mansour and J. West, Avoiding 2-letter signed patterns, arXiv:math/0207204 [math.CO], 2002. FORMULA E.g.f.: (exp(1)*(Ei(1, 1-x)-Ei(1, 1))+1)/(1-x). a(n) = n!*(1+Sum(A000522(i-1)/i!, i =1..n)). - Vladeta Jovovic, Apr 27 2004 Conjecture: a(n) -2*n*a(n-1) +(n^2-2)*a(n-2) -(n-2)^2*a(n-3)=0. - R. J. Mathar, May 30 2014 MATHEMATICA a[n_] := n! (1+Sum[1/i Sum[1/j!, {j, 0, i-1}], {i, 1, n}]) Table[a[n], {n, 0, 19}] (* Jean-François Alcover, Oct 05 2018 *) PROG (PARI) a(n)=n!+n!*sum(i=1, n, 1/i*sum(j=0, i-1, 1/j!)) CROSSREFS Cf. A000774. Cf. A000522, A093344. Contribution from Johannes W. Meijer, Oct 16 2009: (Start) Equals row sums of A165675. (End) Sequence in context: A071076 A297196 A112501 * A289681 A002136 A328441 Adjacent sequences:  A093342 A093343 A093344 * A093346 A093347 A093348 KEYWORD nonn AUTHOR Ralf Stephan, Apr 26 2004 STATUS approved

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Last modified December 8 18:37 EST 2019. Contains 329865 sequences. (Running on oeis4.)