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A193778
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Number of signed permutations of length 2n invariant under D and D'bar.
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2
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1, 2, 8, 32, 160, 832, 4864, 29696, 195584, 1341440, 9723904, 73105408, 574062592, 4657184768, 39165624320, 339133595648, 3028204650496, 27760959422464, 261439835078656, 2521668748574720, 24912764963127296, 251559029812232192, 2595790316527157248
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OFFSET
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0,2
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COMMENTS
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See section 6 in the Hardt reference.
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LINKS
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Harvey P. Dale, Table of n, a(n) for n = 0..661
Andy Hardt and Justin M. Troyka, Restricted Symmetric Signed Permutations, Department of Mathematics, Carleton College, 2013.
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FORMULA
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a(n) = 2*a(n-1) + 4*(n - 1)*a(n-2).
From Peter Luschny, May 30 2021: (Start)
a(n) = 2^n * hypergeom([-n/2, (1 - n)/2], [], 2).
a(n) = 2^n * A000085(n).
a(n) = n! * [x^n] exp(2*x*(x + 1)). (End)
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EXAMPLE
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For n = 1, the permutations of length 2 are (-1, 2) and (1, -2).
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MAPLE
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exp(2*x*(x + 1)): series(%, x, 23):
seq(n!*coeff(%, x, n), n = 0..22); # Peter Luschny, May 30 2021
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MATHEMATICA
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RecurrenceTable[{a[0]==1, a[1]==2, a[n]==2a[n-1]+4(n-1)a[n-2]}, a, {n, 30}] (* Harvey P. Dale, Mar 14 2021 *)
a[n_] := 2^n HypergeometricPFQ[{-n/2, (1-n)/2}, {}, 2];
Table[a[n], {n, 0, 22}] (* Peter Luschny, May 30 2021 *)
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PROG
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(PARI) seq(n)={my(v=vector(n+1)); v[1]=1; v[2]=2; for(n=2, n, v[n+1] = 2*v[n] + 4*(n-1)*v[n-1]); v} \\ Andrew Howroyd, Dec 08 2018
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CROSSREFS
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Cf. A193777, A000085.
Sequence in context: A131785 A347363 A006669 * A030804 A319364 A030895
Adjacent sequences: A193775 A193776 A193777 * A193779 A193780 A193781
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KEYWORD
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nonn
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AUTHOR
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Andy Hardt, Aug 04 2011
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EXTENSIONS
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Terms a(6) and beyond from Andrew Howroyd, Dec 08 2018
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STATUS
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approved
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