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 A188662 Binomial coefficients: a(n) = binomial(3*n,n)^2. 11
 1, 9, 225, 7056, 245025, 9018009, 344622096, 13521038400, 540917591841, 21966328580625, 902702926350225, 37456461988358400, 1566697064677290000, 65973795093338597136, 2794203818390077646400, 118933541228935777741056, 5084343623375056062840609 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Even-order terms in the diagonal of rational function 1/(1 - (x^2 + y^2 + z^2 - x*y - y*z - x*z)). - Gheorghe Coserea, Aug 09 2018 LINKS Seiichi Manyama, Table of n, a(n) for n = 0..604 FORMULA a(n) = A005809(n)^2. a(n) = binomial(3*n,n)^2 = ( [x^n](1 + x)^(3*n) )^2 = [x^n](F(x)^(9*n)), where F(x) = 1 + x + 4*x^2 + 49*x^3 + 795*x^4 + 15180*x^5 + 320422*x^6 + ... appears to have integer coefficients. For similar results see A000897, A002894, A002897, A006480, A008977 and A186420. - Peter Bala, Jul 12 2016 a(n) ~ 3^(6*n+1)*4^(-2*n-1)/(Pi*n). - Ilya Gutkovskiy, Jul 13 2016 a(n) = Sum_{k=0..n} binomial(n, k)^2*binomial(3*n+k, 2*n). - Seiichi Manyama, Jan 09 2017 MATHEMATICA Table[Binomial[3 n, n]^2, {n, 0, 16}] PROG (Maxima) makelist(binomial(3*n, n)^2, n, 0, 16); (MAGMA) [Binomial(3*n, n)^2: n in [0..100]]; // Vincenzo Librandi, Apr 08 2011 (PARI) a(n) = binomial(3*n, n)^2; \\ Michel Marcus, Nov 01 2016 CROSSREFS Cf. A005809, A000897, A002894, A002897, A006480, A008977, A186420. Sequence in context: A063392 A012831 A012749 * A079727 A251579 A128492 Adjacent sequences:  A188659 A188660 A188661 * A188663 A188664 A188665 KEYWORD nonn,easy AUTHOR Emanuele Munarini, Apr 07 2011 STATUS approved

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Last modified April 17 05:12 EDT 2021. Contains 343059 sequences. (Running on oeis4.)