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 A193778 Number of signed permutations of length 2n invariant under D and D'bar. 2
 1, 2, 8, 32, 160, 832, 4864, 29696, 195584, 1341440, 9723904, 73105408, 574062592, 4657184768, 39165624320, 339133595648, 3028204650496, 27760959422464, 261439835078656, 2521668748574720, 24912764963127296, 251559029812232192, 2595790316527157248 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS See section 6 in the Hardt reference. LINKS 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. FORMULA 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) EXAMPLE For n = 1, the permutations of length 2 are (-1, 2) and (1, -2). MAPLE exp(2*x*(x + 1)): series(%, x, 23): seq(n!*coeff(%, x, n), n = 0..22); # Peter Luschny, May 30 2021 MATHEMATICA 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 *) PROG (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 CROSSREFS Cf. A193777, A000085. Sequence in context: A131785 A347363 A006669 * A030804 A319364 A030895 Adjacent sequences:  A193775 A193776 A193777 * A193779 A193780 A193781 KEYWORD nonn AUTHOR Andy Hardt, Aug 04 2011 EXTENSIONS Terms a(6) and beyond from Andrew Howroyd, Dec 08 2018 STATUS approved

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Last modified August 14 09:27 EDT 2022. Contains 356110 sequences. (Running on oeis4.)