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 A006542 a(n) = binomial(n,3)*binomial(n-1,3)/4. (Formerly M4707) 23
 1, 10, 50, 175, 490, 1176, 2520, 4950, 9075, 15730, 26026, 41405, 63700, 95200, 138720, 197676, 276165, 379050, 512050, 681835, 896126, 1163800, 1495000, 1901250, 2395575, 2992626, 3708810, 4562425, 5573800, 6765440, 8162176, 9791320, 11682825, 13869450 (list; graph; refs; listen; history; text; internal format)
 OFFSET 4,2 COMMENTS Number of permutations of n+4 that avoid the pattern 132 and have exactly 3 descents. - Mike Zabrocki, Aug 26 2004 Kekulé numbers for certain benzenoids. - Emeric Deutsch, Jun 20 2005 a(n) = number of Dyck n-paths with exactly 4 peaks. - David Callan, Jul 03 2006 Six-dimensional figurate numbers for a hyperpyramid with pentagonal base. This corresponds to the sum(sum(sum(sum(1+sum(5*n))))) interpretation, see the Munafo webpage. - Robert Munafo, Jun 18 2009 REFERENCES Isaac Ahern, Sam Cook, Affine Symmetry Tensors in Minkowski Space, American Journal of Undergraduate Research, Volume 13 | Issue 3 | August 2016; http://www.ajuronline.org/uploads/Volume_13_3/AJUR%20Vol%2013%20Issue%203%2008.25.16%20pp.27-32.pdf S. J. Cyvin and I. Gutman, Kekulé structures in benzenoid hydrocarbons, Lecture Notes in Chemistry, No. 46, Springer, New York, 1988 (p. 166, no. 1). S. Mukai, An Introduction to Invariants and Moduli, Cambridge, 2003; see p. 238. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS T. D. Noe, Table of n, a(n) for n = 4..200 P. Aluffi, Degrees of projections of rank loci, arXiv:1408.1702 [math.AG], 2014. ["After compiling the results of many explicit computations, we noticed that many of the numbers d_{n,r,S} appear in the existing literature in contexts far removed from the enumerative geometry of rank conditions; we owe this surprising (to us) observation to perusal of [Slo14]."] Brandy Amanda Barnette, Counting Convex Sets on Products of Totally Ordered Sets, Masters Theses & Specialist Projects, Paper 1484, 2015. V. E. Hoggatt, Jr., Letter to N. J. A. Sloane, Apr 1977 G. Kreweras, Traitement simultané du "Problème de Young" et du "Problème de Simon Newcomb", Cahiers du Bureau Universitaire de Recherche Opérationnelle. Institut de Statistique, Université de Paris, 10 (1967), 23-31. G. Kreweras, Traitement simultané du "Problème de Young" et du "Problème de Simon Newcomb", Cahiers du Bureau Universitaire de Recherche Opérationnelle. Institut de Statistique, Université de Paris, 10 (1967), 23-31. [Annotated scanned copy] R. Munafo, C(n,3)C(n-1,3)/4 [From Robert Munafo, Jun 18 2009] Simon Plouffe, Approximations de séries génératrices et quelques conjectures, Dissertation, Université du Québec à Montréal, 1992. Simon Plouffe, 1031 Generating Functions and Conjectures, Université du Québec à Montréal, 1992. Index entries for linear recurrences with constant coefficients, signature (7,-21,35,-35,21,-7,1). FORMULA a(n) = C(n, 3)C(n-1, 3)/4 = A000292(n-3)*A000292(n-2)/4 = n((n-1)(n-2))^2(n-3)/144. E.g.f.: 1/144*x^4*(6 + 6*x + x^2)*exp(x). - Vladeta Jovovic, Jan 29 2003 a(n) = sum(sum(sum(sum(1 + sum(5*n))))) = sum (A006414). - Xavier Acloque, Oct 08 2003 a(n) = C(n, 6) + 3*C(n+1, 6) + C(n+2, 6). - Mike Zabrocki, Aug 26 2004 G.f.: z^4*(1 + 3z + z^2)/(1-z)^7. - Emeric Deutsch, Jun 20 2005 a(n) = C(n-2, n-4)*C(n-1, n-3)*C(n, n-2)/18. - Zerinvary Lajos, Jul 29 2005 a(n) = C(n,4)C(n,3)/n. - Mitch Harris, Jul 06 2006 a(n+2) = 1/4*sum {1 <= x_1, x_2 <= n} x_1*x_2*(det V(x_1,x_2))^2 = 1/4*sum {1 <= i,j <= n} i*j*(i-j)^2, where V(x_1,x_2} is the Vandermonde matrix of order 2. - Peter Bala, Sep 21 2007 a(n) = C(n-1,3)^2 - C(n-1,2)*C(n-1,4). - Gary Detlefs, Dec 05 2011 a(n) = A000292(A000217(n-1)) - A000217(A000292(n-1)). - Ivan N. Ianakiev, Jun 17 2014 a(n) = Product_{i=1..3} A002378(n-4+i)/A002378(i). - Bruno Berselli, Nov 12 2014 (Rewritten, Set 01 2016.) Sum_{n>=4} 1/a(n) = 238 - 24*Pi^2. - Jaume Oliver Lafont, Jul 10 2017 MAPLE A006542:=-(1+3*z+z**2)/(z-1)**7; # conjectured by Simon Plouffe in his 1992 dissertation A006542:=n->n*((n-1)*(n-2))^2*(n-3)/144; seq(A006542(n), n=4..40); # Wesley Ivan Hurt, Jun 17 2014 MATHEMATICA Table[Binomial[n, 3] * Binomial[n-1, 3]/4, {n, 4, 40}] PROG (PARI) a(n)=n*((n-1)*(n-2))^2*(n-3)/144 (MAGMA) [ n*((n-1)*(n-2))^2*(n-3)/144 : n in [4..40] ]; // Wesley Ivan Hurt, Jun 17 2014 CROSSREFS The expression binomial(m+n-1,n)^2-binomial(m+n,n+1)*binomial(m+n-2,n-1) for the values m = 2 through 14 produces the sequences A000012, A000217, A002415, A006542, A006857, A108679, A134288, A134289, A134290, A134291, A140925, A140935, A169937. Cf. A001263, A002378, A004068, A005585, A005891, A006322, A006414, A047819, A107891, A114242. Fourth column of the table of Narayana numbers A001263. Apart from a scale factor, a column of A124428. Sequence in context: A008531 A051230 A008413 * A237655 A261648 A086462 Adjacent sequences:  A006539 A006540 A006541 * A006543 A006544 A006545 KEYWORD nonn,easy AUTHOR EXTENSIONS Zabroki and Lajos formulas offset corrected by Gary Detlefs, Dec 05 2011 STATUS approved

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Last modified December 13 01:24 EST 2018. Contains 318081 sequences. (Running on oeis4.)