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A006480 De Bruijn's S(3,n): (3n)!/(n!)^3.
(Formerly M4284)
55

%I M4284

%S 1,6,90,1680,34650,756756,17153136,399072960,9465511770,227873431500,

%T 5550996791340,136526995463040,3384731762521200,84478098072866400,

%U 2120572665910728000,53494979785374631680,1355345464406015082330

%N De Bruijn's S(3,n): (3n)!/(n!)^3.

%C Number of paths of length 3n in an n X n X n grid from (0,0,0) to (n,n,n), using steps (0,0,1), (0,1,0), and (1,0,0).

%C Appears in Ramanujan's theory of elliptic functions of signature 3.

%C S(s,n) = Sum_{k=0..2n} (-1)^(k+n) * binomial(2n, k)^s. The formula S(3,n) = (3n)!/(n!)^3 is due to Dixon (according to W. N. Bailey 1935). - _Charles R Greathouse IV_, Dec 28 2011

%C a(n) is the number of ballot results that end in a 3-way tie when 3n voters each cast two votes for two out of three candidates vying for 2 slots on a county board; in such a tie, each of the three candidates receives 2n votes. Note there are C(3n,2n) ways to choose the voters who cast a vote for the youngest candidate. The n voters who did note vote for the youngest candidate voted for the two older candidates. Then there are C(2n,n) ways to choose the other n voters who voted for both the youngest and the second youngest candidate. The remaining voters vote for the oldest candidate. Hence there are C(3n,2n)*C(2n,n)=(3n)!/(n!)^3 ballot results. - _Dennis P. Walsh_, May 02 2013

%C a(n) is the constant term of (X+Y+1/(X*Y))^(3*n). - _Mark van Hoeij_, May 07 2013

%C For n > 2 a(n) is divisible by (n+2)*(n+1)^2, a(n) = (n+1)^2*(n+2)*A161581(n). - _Alexander Adamchuk_, Dec 27 2013

%C a(n) is the number of permutations of the multiset {1^n, 2^n, 3^n}, the number of ternary words of length 3*n with n of each letters. - _Joerg Arndt_, Feb 28 2016

%C Diagonal of the rational function 1/(1 - x - y - z). - _Gheorghe Coserea_, Jul 06 2016

%D L. A. Aizenberg and A. P. Yuzhakov, "Integral representations and residues in multidimensional complex analysis", American Mathematical Society, 1983, p. 194.

%D L. Comtet, Advanced Combinatorics, Reidel, 1974, p. 174.

%D N. G. de Bruijn, Asymptotic Methods in Analysis, North-Holland Publishing Co., 1958. See chapters 4 and 6.

%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

%H T. D. Noe, <a href="/A006480/b006480.txt">Table of n, a(n) for n = 0..100</a>

%H G. E. Andrews, <a href="http://dx.doi.org/10.1023/A:1009746303654">The well-poised thread: An Organized Chronicle of Some Amazing Summations and their Implications</a>, Ramanujan J., 1 (1997), 7-23; see Section 8.

%H A. Bostan, S. Boukraa, J.-M. Maillard, J.-A. Weil, <a href="http://arxiv.org/abs/1507.03227">Diagonals of rational functions and selected differential Galois groups</a>, arXiv preprint arXiv:1507.03227 [math-ph], 2015.

%H H. J. Brothers, <a href="http://www.brotherstechnology.com/math/pascals-prism.html">Pascal's Prism: Supplementary Material</a>.

%H R. M. Dickau, <a href="http://mathforum.org/advanced/robertd/path3d.html">3-dimensional shortest-path diagrams</a>.

%H H. W. Gould, <a href="http://www.math.wvu.edu/~gould/">Tables of Combinatorial Identities</a>, Edited by J. Quaintance.

%H B. Hinman, <a href="/A006480/a006480_1.pdf">Letter to N. J. A. Sloane, Aug. 1980</a>.

%H Gilbert Labelle and Annie Lacasse, <a href="http://www.dmtcs.org/dmtcs-ojs/index.php/proceedings/article/view/dmAO0153">Closed paths whose steps are roots of unity</a>, in FPSAC 2011, Reykjav┬┤k, Iceland DMTCS proc. AO, 2011, 599-610.

%H Pedro J. Miana, Hideyuki Ohtsuka, Natalia Romero, <a href="http://arxiv.org/abs/1602.04347">Sums of powers of Catalan triangle numbers</a>, arXiv:1602.04347 [math.NT], 2016.

%H T. Peck, <a href="/A006480/a006480.pdf">Letter to N. J. A. Sloane, Aug. 1980</a>.

%H K. A. Penson and A. I. Solomon, <a href="http://arXiv.org/abs/quant-ph/0111151">Coherent states from combinatorial sequences</a>, arXiv:quant-ph/0111151, 2001.

%H M. Petkovsek et al., <a href="http://www.math.upenn.edu/~wilf/AeqB.html">A=B</a>, Peters, 1996, p. 22.

%H B. Salvy, <a href="http://algo.inria.fr/libraries/autocomb/AGM-html/AGM1.html">GFUN and the AGM</a>.

%H R. Sprugnoli, <a href="http://www.dsi.unifi.it/~resp/GouldBK.pdf">Riordan array proofs of identities in Gould's book</a>.

%H Dennis Walsh, <a href="http://capone.mtsu.edu/dwalsh/VOTENEW.pdf">The probability of a run-off election when three equally-favored candidates vie for two slots</a>.

%H Jacques-Arthur Weil, <a href="http://www.unilim.fr/pages_perso/jacques-arthur.weil/diagonals/">Supplementary Material for the Paper "Diagonals of rational functions and selected differential Galois groups"</a>.

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/BinomialSums.html">Binomial Sums</a>.

%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Dixon%27s_identity">Dixon's identity</a>.

%F Using Stirling's formula in A000142 it is easy to get the asymptotic expression a(n) ~ 1/2 * sqrt(3) * 27^n / (Pi*n) - Dan Fux (dan.fux(AT)OpenGaia.com or danfux(AT)OpenGaia.com), Apr 07 2001

%F O.g.f.: hypergeom([1/3, 2/3], [1], 27*x); E.g.f.: hypergeom([1/3, 2/3], [1, 1], 27*x). Integral representation as n-th moment of a positive function on [0, 27]: a(n)= int( x^n* (-1/24*(3*sqrt(3)*hypergeom([2/3, 2/3], [4/3], 1/27*x)* GAMMA(2/3)^6*x^(1/3)-8*hypergeom([1/3, 1/3], [2/3], 1/27*x)*Pi^3)/Pi^3/x^(2/3)/GAMMA(2/3)^3), x=0..27), n=0, 1, .... This representation is unique. - _Karol A. Penson_, Nov 21 2001

%F a(n) = Sum_{k=-n..n} (-1)^k*binomial(2*n, n+k)^3. - _Benoit Cloitre_, Mar 02 2005

%F a(n) = C(2n,n)*C(3n,n) = A104684(2n,n). - _Paul Barry_, Mar 14 2006

%F G.f. satisfies: A(x^3) = A( x*(1+3*x+9*x^2)/(1+6*x)^3 )/(1+6*x). - _Paul D. Hanna_, Oct 29 2010

%F n^2*a(n) - 3*(3*n-1)*(3*n-2)*a(n-1) = 0. - _R. J. Mathar_, Dec 04 2012

%F a(n) = (n+1)^2*(n+2)*A161581(n) for n>2. - _Alexander Adamchuk_, Dec 27 2013

%F 0 = a(n)^2*(472392*a(n+1)^2 - 83106*a(n+1)*a(n+2) + 3600*a(n+2)^2) + a(n)*a(n+1)*(-8748*a(n+1)^2 + 1953*a(n+1)*a(n+2) - 120*a(n+2)^2) + a(n+1)^2*(+36*a(n+1)^2 - 12*a(n+1)*a(n+2) + a(n+2)^2 for all n in Z. - _Michael Somos_, Oct 22 2014

%F 0 = x*(27*x-1)*y'' + (54*x-1)*y' + 6*y, where y is g.f. - _Gheorghe Coserea_, Jul 06 2016

%F From _Peter Bala_, Jul 15 2016: (Start)

%F a(n) = 3*binomial(2*n - 1,n)*binomial(3*n - 1,n) = 3*[x^n] 1/(1 - x)^n * [x^n] 1/(1 - x)^(2*n) for n >= 1.

%F a(n) = binomial(2*n,n)*binomial(3*n,n) = ([x^n](1 + x)^(2*n)) *([x^n](1 + x)^(3*n)) = [x^n](F(x)^(6*n)), where F(x) = 1 + x + 2*x^2 + 14*x^3 + 127*x^4 + 1364*x^5 + 16219*x^6 + ... appears to have integer coefficients. Cf. A002894.

%F This sequence occurs as the right-hand side of several binomial sums:

%F Sum_{k = 0..2*n} (-1)^(n+k)*binomial(2*n,k)^3 = a(n) (Dixon's identity).

%F Sum_{k = 0..n} binomial(n,k)*binomial(2*n,n - k)*binomial(3*n + k,k) = a(n) (Gould, Vol. 4, 6.86)

%F Sum_{k = 0..n} (-1)^(n+k)*binomial(n,k)*binomial(2*n + k,n)*binomial(3*n + k,n) = a(n).

%F Sum_{k = 0..n} binomial(n,k)*binomial(2*n + k,k)*binomial(3*n,n - k) = a(n).

%F Sum_{k = 0..n} (-1)^(k)*binomial(n,k)*binomial(3*n - k,n)*binomial(4*n - k,n) = a(n).

%F Sum_{k = 0..2*n} (-1)^(n+k)*binomial(2*n + k,2*n - k)*binomial(2*k,k)*binomial(4*n - k,2*n) = a(n) (see Gould, Vol.5, 9.23).

%F Sum_{k = 0..2*n} (-1)^k*binomial(3*n,k)*binomial(3*n - k,n)^3 = a(n) (Sprugnoli, Section 2.9, Table 10, p. 123). (End)

%e G.f.: 1 + 6*x + 90*x^2 + 1680*x^3 + 34650*x^4 + 756756*x^5 + 17153136*x^6 + ...

%p seq((3*n)!/(n!)^3, n=0..16); # _Zerinvary Lajos_, Jun 28 2007

%t Sum [ (-1)^(k+n) Binomial[ 2n, k ]^3, {k, 0, 2n} ]

%t a[ n_] := If[ n < 0, 0, (-1)^n HypergeometricPFQ[ {-2 n, -2 n, -2 n}, {1, 1}, 1]]; (* _Michael Somos_, Oct 22 2014 *)

%t Table[Multinomial[n, n, n], {n, 0, 100}] (* _Emanuele Munarini_, Oct 25 2016 *)

%o (PARI) {a(n) = if( n<0, 0, (3*n)! / n!^3)};

%o (PARI) {a(n) = local(A, m); if( n<1, n==0, m=1; A = 1 + O(x); while( m<=n, m*=3; A = subst( (1 + 2*x) * subst(A, x, (x/3)^3), x, serreverse(x * (1 + x + x^2) / (1 + 2*x)^3 / 3 + O(x^m)))); polcoeff(A, n))};

%o (MAGMA) [Factorial(3*n)/(Factorial(n))^3: n in [0..20] ]; // _Vincenzo Librandi_, Aug 20 2011

%o (Maxima) makelist(multinomial_coeff(n,n,n),n,0,24); /* _Emanuele Munarini_, Oct 25 2016 */

%Y Cf. A000984, A008977, A050983, A050984, A161581, A181545.

%Y Related to diagonal of rational functions: A268545-A268555.

%K nonn,easy,nice

%O 0,2

%A _N. J. A. Sloane_

%E More terms from _Eric W. Weisstein_

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Last modified February 20 18:41 EST 2018. Contains 299381 sequences. (Running on oeis4.)