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 A000366 Genocchi numbers of second kind (A005439) divided by 2^(n-1). 12
 1, 1, 2, 7, 38, 295, 3098, 42271, 726734, 15366679, 391888514, 11860602415, 420258768950, 17233254330343, 809698074358250, 43212125903877439, 2599512037272630686, 175079893678534943287, 13122303354155987156306 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS The earliest known reference to these numbers is the Dellac Marseille memoir. - Don Knuth, Jul 11 2007 According to Ira Gessel, Dellac's interpretation is the following: start with a 2n X n array of cells and consider the set D of cells in rows i through i+n of column i, for i from 1 to n. Then a(n) is the number of subsets of D containing two cells in each column and one cell in each row. Barsky proved that for even n>1, a(n) is congruent to 3 mod 4 and for odd n>1, congruent to 2 mod 4. Gessel shows that for even n>5, a(n) is congruent to 4n-1 mod 16 and for odd n>2 that a(n)/2 is congruent to 2-n mod 8. The entry for A005439 has further information. The number of sequences (I_1,...,I_{n-1}) consisting of subsets of the set {1,...,n} such that the number of elements in I_k is exactly k and I_k\subset I_{k+1}\cup {k+1}. The Euler characteristics of the degenerate flag varieties of type A. - Evgeny Feigin, Dec 15 2011 Kreweras proved that for n>2, a(n) is alternatively congruent to 2 and to 7 mod 36. - Michel Marcus, Nov 06 2012 REFERENCES Anonymous, L'Intermédiaire des Mathématiciens, 7 (1900), p. 328. D. Barsky, Congruences pour les nombres de Genocchi de 2e espèce, Groupe d'étude d'Analyse ultramétrique, 8e année, no. 34, 1980/81, 13 pp. Hippolyte Dellac, Note sur l'élimination, méthode de parallélogramme, Annales de la Faculté des Sciences de Marseille, XI (1901), 141-164. Hippolyte Dellac, Problem 1735, L'Intermédiaire des Mathématiciens, Vol. 7 (1900), 9- E. Lemoine, L'Intermédiaire des Mathématiciens, 8 (1901), 168-169. L. Seidel, Über eine einfache Entstehungsweise der Bernoulli'schen Zahlen und einiger verwandten Reihen, Sitzungsberichte der mathematisch-physikalischen Classe der königlich bayerischen Akademie der Wissenschaften zu München, volume 7 (1877), 157-187. LINKS T. D. Noe, Table of n, a(n) for n=1..100 Ange Bigeni, Enumerating the symplectic Dellac configurations, arXiv:1705.03804 [math.CO], 2017. E. Feigin, Degenerate flag varieties and the median Genocchi numbers, arXiv:1101.1898 [math.AG], 2011. E. Feigin, The median Genocchi numbers, Q-analogues and continued fractions, arXiv:1111.0740 [math.CO], 2011-2012. I. M. Gessel, Applications of the classical umbral calculus, arXiv:math/0108121 [math.CO], 2001. G. Han and J. Zeng, On a q-sequence that generalizes the median Genocchi numbers, Annal Sci. Math. Quebec, 23(1999), no. 1, 63-72 G. Kreweras, Sur les permutations comptées par les nombres de Genocchi de 1-ière et 2-ième espèce, Europ. J. Comb., vol. 18, pp. 49-58, 1997. G. Kreweras and D. Dumont, Sur les anagrammes alternés, Discrete Mathematics, Volume 211, Issues 1-3, 28 January 2000, Pages 103-110. FORMULA From Don Knuth, Jul 11 2007: (Start) The anonymous 1900 note in Interm. Math. gives a formula that is equivalent to a nice generating function: For example, the first four terms on the right are 1 ... 2x - 2x^2 + 2x^3 + ... ........ 9x^2 - 36x^3 + ... ............... 72x^3 + ... summing to 1 + 2x + 7x^2 + 38x^3 + ... . Of course one can replace x by 2x and get a generating function for A005439. (End) (-2)^(2-n) * sum_{k=0..n} C(n, k)*(1-2^(n+k+1))*B(n+k+1), with B(n) the Bernoulli numbers. O.g.f.: A(x) = x/(1-x/(1-x/(1-3*x/(1-3*x/(1-6*x/(1-6*x/(... -[n/2+1]*[n/2+2]/2*x/(1- ...)))))))) (continued fraction). - Paul D. Hanna, Oct 07 2005 Sum_{n>0} a(n)x^n = Sum_{n>0} (n!^2/2^{n-1}) (x^n/((1+x)(1+3x)...(1+binomial(n,2)x))). a(n+1) = Sum_{k=0..n} A211183(n,k). - Philippe Deléham, Feb 03 2013 G.f.: Q(0)*2 - 2, where Q(k) = 1 - x*(k+1)^2/( x*(k+1)^2 - 2/(1 - x*(k+1)^2/( x*(k+1)^2 - 2/Q(k+1) ))); (continued fraction). - Sergei N. Gladkovskii, Oct 22 2013 a(n) ~ 2^(n+5) * n^(2*n+3/2) / (exp(2*n) * Pi^(2*n+1/2)). - Vaclav Kotesovec, Oct 28 2014 EXAMPLE G.f. = x + x^2 + 2*x^3 + 7*x^4 + 38*x^5 + 295*x^6 + 3098*x^7 + ... MATHEMATICA a[n_] = (-2^(-1))^(n-2)* Sum[ Binomial[n, k]*(1 - 2^(n+k+1))*BernoulliB[n+k+1], {k, 0, n}]; Table[a[n], {n, 19}] (* Jean-François Alcover, Jul 18 2011, after PARI prog. *) PROG (PARI) a(n)=(-1/2)^(n-2)*sum(k=0, n, binomial(n, k)*(1-2^(n+k+1))*bernfrac(n+k+1)) (PARI) {a(n)=local(CF=1+x*O(x^n)); if(n<1, return(0), for(k=1, n, CF=1/(1-((n-k)\2+1)*((n-k)\2+2)/2*x*CF)); return(Vec(CF)[n]))} (Hanna) (PARI) {a(n)=polcoeff( x*sum(m=0, n, m!*(m+1)!*(x/2)^m / prod(k=1, m, 1 + k*(k+1)*x/2 +x*O(x^n)) ), n)} \\ Paul D. Hanna, Feb 03 2013 (Sage) # Algorithm of L. Seidel (1877) # n -> [a(1), ..., a(n)] for n >= 1. def A000366_list(n) :     D = [0]*(n+2); D[1] = 1     R = []; z = 1/2; b = False     for i in(0..2*n-1) :         h = i//2 + 1         if b :             for k in range(h-1, 0, -1) : D[k] += D[k+1]             z *= 2         else :             for k in range(1, h+1, 1) :  D[k] += D[k-1]         b = not b         if not b : R.append(D[1]/z)     return R A000366_list(19) # Peter Luschny, Jun 29 2012 CROSSREFS Cf. A001469, A005439, A130168, A130169. First column, first diagonal and row sums of triangle A014784. Also row sums of triangle A239894. Sequence in context: A094664 A001858 A233335 * A106211 A222034 A014058 Adjacent sequences:  A000363 A000364 A000365 * A000367 A000368 A000369 KEYWORD nonn,easy,nice AUTHOR EXTENSIONS More terms from David W. Wilson, Jan 11 2001 Edited by Ralf Stephan, Apr 17 2004 STATUS approved

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