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A193776
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Number of signed permutations of length n invariant under the reverse complement and avoiding (-2, -1), (-2, 1), (2, -1).
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0
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1, 2, 3, 5, 12, 17, 65, 80, 473, 527, 4444, 4679, 51391, 52628, 703659, 711449, 11098896, 11156477, 197809793, 198299024, 3927270089, 3931960343, 85908742132, 85958728847, 2052375195679, 2052960568556, 53160174898371, 53167638586121, 1483752628890840, 1483855482962885, 44391655829672177
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OFFSET
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0,2
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LINKS
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FORMULA
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a(2k) = 2^k k! + \sum_{j=0}^{k-1}(k-j-1)! a(2j)
a(2k+1) = (2^k + 1)k! + \sum_{j=0}^{k-1}(k-j-1)! a(2j+1)
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EXAMPLE
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For n = 2, the permutations are (1, 2), (2, 1), (-1, -2), (-2, -1).
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MATHEMATICA
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a[n_] := a[n] = If[EvenQ[n],
Sum[a[2j]*(n/2 - j - 1)!, {j, 0, n/2 - 1}] + 2^(n/2)*(n/2)!,
Sum[a[2j+1]*((1/2)*(n - 2j - 3))!, {j, 0, (n - 3)/2}] +
(2^((n - 1)/2) + 1)*((n - 1)/2)!];
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CROSSREFS
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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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