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 A060542 a(n) = (1/6)*multinomial(3*n;n,n,n). 5
 1, 15, 280, 5775, 126126, 2858856, 66512160, 1577585295, 37978905250, 925166131890, 22754499243840, 564121960420200, 14079683012144400, 353428777651788000, 8915829964229105280, 225890910734335847055, 5744976449471863238250, 146603287914300510042750 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Number of ways of dividing 3n labeled items into 3 unlabeled boxes with n items in each box. From Antonio Campello (campello(AT)ime.unicamp.br), Nov 11 2009: (Start) A060542(t) is the number of optimal [n,2,d] binary codes that correct at most t errors, i.e., having Hamming distance 2*t + 1 (achieved on length n = 3*t + 2). These codes are all isometric. It is also the number of optimal [n,2,d] binary codes that detect 2*t + 1 errors, i.e., having Hamming distance 2t+2 (obtained by adding an overall parity check to the n = 3*t + 2 optimal codes). These codes are also all isometric. For t = 0, we have the famous MDS, cyclic, simplex code {(000), (101), (110), (011)}. (End) Also the number of distinct adjacency matrices of the complete tripartite graph K_{n,n,n}. - Eric W. Weisstein, Apr 21 2017 LINKS Harry J. Smith, Table of n, a(n) for n = 1..100 Eric Weisstein's World of Mathematics, Adjacency Matrix Eric Weisstein's World of Mathematics, Complete Tripartite Graph FORMULA a(n) = (3*n)!/((n!)^3*6) = a(n-1)*3*(3*n - 1)*(3*n - 2)/n^2 = A060540(3,n) = A006480(n)/6. - corrected by Vaclav Kotesovec, Sep 23 2013 a(n) ~ 3^(3*n-1/2)/(4*Pi*n). - Vaclav Kotesovec, Sep 23 2013 MATHEMATICA Table[(3*n)!/(n!^3*6), {n, 1, 20}] (* Vaclav Kotesovec, Sep 23 2013 *) Table[Multinomial[n, n, n], {n, 20}]/6 (* Eric W. Weisstein, Apr 21 2017 *) PROG (PARI) { a=1/6; for (n=1, 100, write("b060542.txt", n, " ", a=a*3*(3*n - 1)*(3*n - 2)/n^2); ) } \\ Harry J. Smith, Jul 06 2009 CROSSREFS Row 3 of A060540. Cf. A025035. Sequence in context: A159239 A279976 A199096 * A095654 A279167 A249960 Adjacent sequences:  A060539 A060540 A060541 * A060543 A060544 A060545 KEYWORD nonn AUTHOR Henry Bottomley, Apr 02 2001 EXTENSIONS Definition revised by N. J. A. Sloane, Feb 02 2009 STATUS approved

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Last modified March 20 09:48 EDT 2019. Contains 321345 sequences. (Running on oeis4.)