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 A036290 a(n) = n*3^n. 15
 0, 3, 18, 81, 324, 1215, 4374, 15309, 52488, 177147, 590490, 1948617, 6377292, 20726199, 66961566, 215233605, 688747536, 2195382771, 6973568802, 22082967873, 69735688020, 219667417263, 690383311398, 2165293113021, 6778308875544 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS f X_1,X_2,...,X_n is a partition of a 3n-set X into 3-blocks then, for n>0, a(n) is equal to the number of (n+1)-subsets of X intersecting each X_i (i=1,2,...,n). - Milan Janjic, Jul 21 2007. LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..1000 Milan Janjic, Two Enumerative Functions László Németh, The trinomial transform triangle, J. Int. Seqs., Vol. 21 (2018), Article 18.7.3. Also arXiv:1807.07109 [math.NT], 2018. Index entries for linear recurrences with constant coefficients, signature (6,-9). FORMULA From Paul Barry, Feb 06 2004: (Start) A trinomial transform. Differentiate (1+x+x^2)^n and set x=1. a(n) = Sum_{i=0..n} Sum_{j=0..n} (2*n-2*i-j)*n!/(i!*j!*(n-i-j)!). (End) From Paul Barry, Feb 15 2005: (Start) a(n) = Sum_{k=0..2*n} T(n, k)*k, where T(n, k) is given by A027907. a(n) = Sum_{k=0..n} Sum_{j=0..n} C(n, j)*C(j, k)*(j+k). (End) From R. J. Mathar, Jun 19 2011: (Start) G.f.: 3*x / (3*x-1)^2 . a(n) = 3*A027471(n+1). (End) Sum_{n>=1} 1/a(n) = log(3/2) = 0.405465108... = A016578. - Franz Vrabec, Jan 07 2012 Sum_{n>=1} (-1)^(n+1)/a(n) = log(4/3) = A083679. - Amiram Eldar, Jul 20 2020 MAPLE A036290 := proc(n) n*3^n ; end proc: # R. J. Mathar, Jun 18 2011 MATHEMATICA nn=20; a=1/(1-3x); CoefficientList[Series[x D[ a, x] , {x, 0, nn}], x]  (* Geoffrey Critzer, Nov 18 2012 *) Table[n 3^n, {n, 0, 30}] (* Vincenzo Librandi, Jul 06 2017 *) PROG (PARI) a(n)=3^n*n \\ Charles R Greathouse IV, Jun 18, 2011 (MAGMA) [n*3^n: n in [0..30]]; // Vincenzo Librandi, Jul 06 2017 CROSSREFS Cf. A006234, A016578, A083679. Sequence in context: A135371 A086346 A337193 * A078904 A099012 A122069 Adjacent sequences:  A036287 A036288 A036289 * A036291 A036292 A036293 KEYWORD nonn,easy AUTHOR STATUS approved

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Last modified October 20 16:06 EDT 2020. Contains 337905 sequences. (Running on oeis4.)