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 A007004 a(n) = (3*n)! / ((n+1)*(n!)^3). (Formerly M3125) 7
 1, 3, 30, 420, 6930, 126126, 2450448, 49884120, 1051723530, 22787343150, 504636071940, 11377249621920, 260363981732400, 6034149862347600, 141371511060715200, 3343436236585914480, 79726203788589122490, 1914992149823954412750, 46295775130831740013500 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Number of walks with steps (0,1)/North, (1,0)/East and (-1,-1)/Southwest from (0,0) to (0,0) of length 3n, and staying above the line y=x (i.e., any point (x,y) along the walk satisfies y>=x ). - Shanzhen Gao, Nov 09 2010 Number of walks in 3-dimensions using steps (1,0,0), (0,1,0), and (0,0,1) from (0,0,0) to (n,n,n) such that after each step we have y<=x. - Eric Werley, Jun 24 2011 Number of possible necklaces consisting of n white beads, n-1 red beads and n-1 black beads, where two necklaces are considered equivalent if they differ by a cyclic permutation. - Thotsaporn Thanatipanonda, Feb 20 2011 REFERENCES N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..150 FORMULA a(n) = C(2*n,n)*C(3*n,n)/(n+1) = A000108(n)*C(3*n,n). - Zerinvary Lajos, May 27 2006 a(n) = A060693(2n,n) = A088617(2n,n). - Philippe Deléham, Nov 23 2011 a(n) = (3*(3*n-1)*(3*n-2)*a(n-1))/(n*(n+1)) for n>0, a(0)=1. - Alois P. Heinz, Aug 13 2013 a(n) ~ 3^(3*n+1/2)/(2*Pi*n^2). - Vaclav Kotesovec, Sep 06 2016 EXAMPLE n=1, three walks: NE(SW), (SW)NE, N(SW)E. - Shanzhen Gao, Nov 09 2010 MAPLE seq(binomial(2*n, n)*binomial(3*n, n)/(n+1), n=0..20); # Zerinvary Lajos, May 27 2006 MATHEMATICA a[n_]:=(3*n)!/((n + 1)*(n!)^3); (* Vladimir Joseph Stephan Orlovsky, Dec 13 2008 *) CoefficientList[Series[Hypergeometric2F1[1/3, 2/3, 2, 27 x], {x, 0, 20}], x] (* Harvey P. Dale, Apr 07 2013 *) Table[Multinomial[n, n, n]/(n + 1), {n, 0, 12}] (* Emanuele Munarini, Oct 25 2016 *) PROG (MAGMA) [Factorial(3*n) / ((n+1)*Factorial(n)^3): n in [0..30]]; // Vincenzo Librandi, May 26 2011 (Maxima) makelist(multinomial_coeff(n, n, n)/(n+1), n, 0, 24); /* Emanuele Munarini, Oct 25 2016 */ CROSSREFS Cf. A000108, A060693. Row n=3 of A215561. Sequence in context: A234506 A212425 A294240 * A276361 A218304 A242005 Adjacent sequences:  A007001 A007002 A007003 * A007005 A007006 A007007 KEYWORD nonn AUTHOR EXTENSIONS More terms from Zerinvary Lajos, May 27 2006 STATUS approved

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Last modified October 15 10:15 EDT 2019. Contains 328026 sequences. (Running on oeis4.)