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 A062741 3 times pentagonal numbers: 3*n*(3*n-1)/2. 25
 0, 3, 15, 36, 66, 105, 153, 210, 276, 351, 435, 528, 630, 741, 861, 990, 1128, 1275, 1431, 1596, 1770, 1953, 2145, 2346, 2556, 2775, 3003, 3240, 3486, 3741, 4005, 4278, 4560, 4851, 5151, 5460, 5778, 6105, 6441, 6786, 7140, 7503, 7875, 8256, 8646, 9045 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Write 0,1,2,3,4,... in a triangular spiral, then a(n) is the sequence found by reading from 0 in the vertical upward direction. Number of edges in the join of two complete graphs of order 2n and n, K_2n * K_n - Roberto E. Martinez II, Jan 07 2002 LINKS Nathaniel Johnston, Table of n, a(n) for n = 0..10000 Index entries for linear recurrences with constant coefficients, signature (3,-3,1). FORMULA a(n) = C(3*n,2). - Zerinvary Lajos, Jan 02 2007 a(n) = (9*n^2-3*n)/2 = 3*n(3*n-1)/2 = A000326(n)*3. - Omar E. Pol, Dec 11 2008 a(n) = 9*n+a(n-1)-6, with n>0, a(0)=0. - Vincenzo Librandi, Aug 07 2010 G.f.: 3*x*(1+2*x)/(1-x)^3. - Bruno Berselli, Jan 21 2011 a(n) = A218470(9n+2). - Philippe Deléham, Mar 27 2013 a(n) = n*A008585(n) + sum( A008585(i), i=0..n-1 ) for n>0. - Bruno Berselli, Dec 19 2013 EXAMPLE The spiral begins: ..........15 ........16..14 ......17..3...13 ....18..4...2...12 ..19..5...0...1...11 20..6...7...8...9...10 MAPLE [seq(binomial(3*n, 2), n=0..45)]; - Zerinvary Lajos, Jan 02 2007 MATHEMATICA s=0; lst={s}; Do[s+=n++ +3; AppendTo[lst, s], {n, 0, 6!, 9}]; lst (* Vladimir Joseph Stephan Orlovsky, Nov 16 2008 *) PROG (PARI) a(n)=3*n*(3*n-1)/2 \\ Charles R Greathouse IV, Sep 24 2015 CROSSREFS Cf. A000326, A008585, A051682. Cf. 3 times n-gonal numbers: A045943, A033428, A094159, A152773, A152751, A152759, A152767, A153783, A153448, A153875. Sequence in context: A162441 A001803 A161738 * A185541 A176661 A117561 Adjacent sequences:  A062738 A062739 A062740 * A062742 A062743 A062744 KEYWORD nonn,easy,changed AUTHOR Floor van Lamoen, Jul 21 2001 EXTENSIONS Better definition and edited by Omar E. Pol, Dec 11 2008 STATUS approved

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