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 A135410 Number of permutations of {1,2,3,...,n} each with the same up-down signature as its inverse permutation. 1
 1, 1, 2, 4, 10, 26, 80, 272, 1076, 4848, 24832, 142340, 902440, 6257264, 47042924 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS An "up-down signature" of the permutation (p(1),p(2),p(3),...,p(n)) is the sequence of (n-1) signs (+ or -) where the k-th sign in the sequence is the sign of (p(k+1)-p(k)). All permutations of {1,2,3,...,n} which are their own inverses obviously have the same up-down signature as their inverse permutations. The terms were calculated by R. J. Mathar (see link). LINKS R. J. Mathar, C== program EXAMPLE The up-down signature of the permutation (4,2,6,3,5,1) is -+-+-. This permutation's inverse permutation is (6,2,4,1,5,3). The inverse permutation also has the up-down signature -+-+-. So these two permutations, among others, are included in the count for n = 6. CROSSREFS Cf. A000085. Sequence in context: A003239 A195924 A116673 * A148103 A148104 A179981 Adjacent sequences:  A135407 A135408 A135409 * A135411 A135412 A135413 KEYWORD nonn,more AUTHOR Leroy Quet, Dec 10 2007 EXTENSIONS a(13)-a(14) from Alois P. Heinz, Oct 18 2013 STATUS approved

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Last modified February 16 04:34 EST 2019. Contains 320140 sequences. (Running on oeis4.)