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 A024489 a(n) = (1/(9n-3))*M(3n; n,n,n), where M() is a multinomial coefficient. 1
 1, 6, 70, 1050, 18018, 336336, 6651216, 137181330, 2921454250, 63804560820, 1422156202740, 32235540595440, 741035948007600, 17240428178136000, 405264998374050240, 9612379180184504130, 229799057978874529530, 5532199543935868303500, 134014085905039247407500 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS a(n) is also the number of possible necklaces consisting of n white beads, n red beads and n-1 black beads, where two necklaces are considered equivalent if they differ by a cyclic permutation. - Thotsaporn Thanatipanonda, Feb 20 2012 LINKS Alois P. Heinz, Table of n, a(n) for n = 1..300 FORMULA a(n) ~ 3^(3*n-3/2) / (2*Pi*n^2). - Vaclav Kotesovec, Aug 25 2014 a(n) = (3*n)!/(n!^3*(9*n-3)). - Peter Luschny, Sep 30 2018 MAPLE with(combinat): a:= n-> multinomial(3*n, n\$3)/(9*n-3): seq(a(n), n=1..20);  # Alois P. Heinz, Feb 20 2012 CROSSREFS Sequence in context: A104900 A186667 A001448 * A036361 A182563 A211036 Adjacent sequences:  A024486 A024487 A024488 * A024490 A024491 A024492 KEYWORD nonn AUTHOR EXTENSIONS More terms from Alois P. Heinz, Feb 20 2012 STATUS approved

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Last modified January 23 10:54 EST 2019. Contains 319391 sequences. (Running on oeis4.)