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A093345
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a(n) = n! * {1 + Sum[i=1..n, 1/i*Sum(j=0..i-1, 1/j!)]}.
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2
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1, 2, 6, 23, 108, 605, 3956, 29649, 250892, 2367629, 24662700, 281153801, 3482350724, 46572620757, 668943488084, 10271127486065, 167892667249116, 2911049382788189, 53365747562592092, 1031352659792534169
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OFFSET
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0,2
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COMMENTS
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Number of {12,2*1}-avoiding signed permutations in the hyperoctahedral group B_n.
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LINKS
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FORMULA
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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
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MATHEMATICA
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a[n_] := n! (1+Sum[1/i Sum[1/j!, {j, 0, i-1}], {i, 1, n}])
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PROG
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(PARI) a(n)=n!+n!*sum(i=1, n, 1/i*sum(j=0, i-1, 1/j!))
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CROSSREFS
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(End)
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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