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 A083886 E.g.f. exp(3*x)*exp(x^2). 2
 1, 3, 11, 45, 201, 963, 4899, 26253, 147345, 862083, 5238459, 32957037, 214117209, 1433320515, 9867008979, 69734001357, 505212273441, 3747124863747, 28418591888235, 220152270759597, 1740363304031721, 14027180742479043, 115176800996769411, 962726355659386125, 8186311912829551281, 70769800810139187843 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Binomial transform of A000898. Hankel transform is A108400. [From Paul Barry, Jun 13 2009] a(n) is the number of self-inverse signed permutations of length 2n that are equal to their reverse-complements and avoid the pattern (-2,-1).  As a result, a(n) also gives the same thing but for avoiding any one of (-1,-2), (+2,+1) or (+1,+2) instead of (-2,-1).  (See the Hardt and Troyka reference.) - Justin M. Troyka, Aug 05 2011. REFERENCES A. Hardt and J. M. Troyka, Restricted symmetric signed permutations, pre-print. [For more information, email troykaj(AT)carleton.edu.] LINKS FORMULA E.g.f.: exp(3*x+x^2). From Paul Barry, Jun 13 2009: (Start) G.f.: 1/(1-3x-2x^2/(1-3x-4x^2/(1-3x-6x^2/(1-3x-8x^2/(1-... (continued fraction); a(n) = sum{k=0..floor(n/2), C(n,2k)(2k)!3^(n-2k)/k!}. (End) a(n) = i^n*Hermite_H(n, -3i/2), i=sqrt(-1). [Paul Barry, Jun 15 2009] a(0) = 1; a(1) = 3; a(n) = 3*a(n-1) + 2*(n-1)*a(n-2) for n >= 2. - Justin M. Troyka, Aug 05 2011. E.g.f. 1 + (x+3)*x/(G(0)-x^2-3*x) where G(k)= x^2 + 3*x + k + 1 - (x+3)*x*(k+1)/G(k+1); (continued fraction, Euler's 1st kind, 1-step). - Sergei N. Gladkovskii, Jul 12 2012 G.f.: 1/Q(0) where Q(k) = 1 + 2*x*k - 2*x - x/(1 - 2*x*(k+1)/Q(k+1) ); (continued fraction). - Sergei N. Gladkovskii, Mar 07 2013 EXAMPLE Since a(2) = 11, there are 11 self-inverse signed permutations of 4 that are equal to their reverse-complements and avoid (-2,-1).  Some of these are: (+3,+4,+1,+2), (+4,-2,-3,+1), (-1,+3,+2,-4), (-1,-2,-3,-4). - Justin M. Troyka, Aug 05 2011. MATHEMATICA a = {1, 3}; For[n = 2, n < 13, n++, a = Append[a, 3 a[[n]] + 2 (n - 1) a[[n - 1]]]]; a  // Justin M. Troyka, Aug 5 2011. PROG (PARI)  x='x+O('x^66);  Vec(serlaplace(exp(3*x)*exp(x^2))) /* Joerg Arndt, Jul 12 2012 */ CROSSREFS Cf. A047974, A001813. Sequence in context: A187764 A151133 A213333 * A030866 A030941 A030918 Adjacent sequences:  A083883 A083884 A083885 * A083887 A083888 A083889 KEYWORD easy,nonn AUTHOR Paul Barry, May 09 2003 STATUS approved

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