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 A027480 a(n) = n*(n+1)*(n+2)/2. 50
 0, 3, 12, 30, 60, 105, 168, 252, 360, 495, 660, 858, 1092, 1365, 1680, 2040, 2448, 2907, 3420, 3990, 4620, 5313, 6072, 6900, 7800, 8775, 9828, 10962, 12180, 13485, 14880, 16368, 17952, 19635, 21420, 23310, 25308, 27417, 29640, 31980, 34440 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Write the integers in groups: 0; 1,2; 3,4,5; 6,7,8,9; ... and add the groups: a(n) = Sum_{j=0..n} (A000217(n)+j), row sums of the triangular view of A001477. - Asher Auel (asher.auel(AT)reed.edu), Jan 06 2000 With offset = 2, a(n) is the number of edges of the line graph of the complete graph of order n, L(K_n). - Roberto E. Martinez II, Jan 07 2002 Also the total number of pips on a set of dominoes of type n. (A "3" domino set would have 0-0, 0-1, 0-2, 0-3, 1-1, 1-2, 1-3, 2-2, 2-3, 3-3.) - Gerard Schildberger, Jun 26 2003. See A129533 for generalization to n-armed "dominoes". - N. J. A. Sloane, Jan 06 2016 Common sum in an (n+1) X (n+1) magic square with entries (0..n^2-1). Alternate terms of A057587. - Jeremy Gardiner, Apr 10 2005 If Y is a 3-subset of an n-set X then, for n>=5, a(n-5) is the number of 4-subsets of X which have exactly one element in common with Y. Also, if Y is a 3-subset of an n-set X then, for n>=5, a(n-5) is the number of (n-5)-subsets of X which have exactly one element in common with Y. - Milan Janjic, Dec 28 2007 These numbers, starting with 3, are the denominators of the power series f(x)=(1-x)^2 * log(1/(1-x)), if the numerators are kept at 1. This sequence of denominators starts at the term x^3/3. - Miklos Bona (bona(AT)math.ufl.edu), Feb 18 2009 a(n) is the number of triples (w,x,y) having all terms in {0,,,.n} and satisfying at least one of the inequalities x+y=2 [where numbperm(n, k) = n!/(n-k)!]. - Zerinvary Lajos, Apr 26 2007 a(n) = A007531(n+2)/2. - Zerinvary Lajos, Jul 17 2006 Starting with offset 1 = binomial transform of [3, 9, 9, 3, 0, 0, 0]. - Gary W. Adamson, Oct 25 2007 G.f.: 3*x/(x-1)^4. a(n)=4*a(n-1)-6*a(n-2)+4*a(n-3)-a(n-4). - R. J. Mathar, Apr 07 2009 a(n) = Sum_{i=0..n} n*(n - i) + 2*i. [Bruno Berselli, Jan 13 2016] From Ilya Gutkovskiy, Aug 07 2016: (Start) E.g.f.: x*(6 + 6*x + x^2)*exp(x)/2. a(n) = Sum_{k=0..n} A045943(k). Sum_{n>=1} 1/a(n) = 1/2. Sum_{n>=1) (-1)^(n+1)/a(n) = (8*log(2) - 5)/2 = 0.2725887222397812... (End) a(n-1) = binomial(n^2,2)/n for n > 0. - Jonathan Sondow, Jan 07 2018 EXAMPLE Row sums of n consecutive integers, starting at 0, seen as a triangle: . 0   | 0 3   | 1  2 12  | 3  4  5 30  | 6  7  8 9 60  | 10 11 12 13 14 105 | 15 16 17 18 19 20 MAPLE [seq(3*binomial(n+2, 3), n=0..37)]; # Zerinvary Lajos, Nov 24 2006 a := n -> add((j+n)*(n+2)/3, j=0..n): seq(a(n), n=0..35); # Zerinvary Lajos, Dec 17 2006 MATHEMATICA Table[(m^3 - m)/2, {m, 36}] (* Zerinvary Lajos, Mar 21 2007 *) LinearRecurrence[{4, -6, 4, -1}, {0, 3, 12, 30}, 40] (* Harvey P. Dale, Oct 10 2012 *) CoefficientList[Series[3 x / (x - 1)^4, {x, 0, 40}], x] (* Vincenzo Librandi, Nov 14 2014 *) With[{nn=50}, Total/@TakeList[Range[0, (nn(nn+1))/2-1], Range[nn]]] (* Requires Mathematica version 11 or later *) (* Harvey P. Dale, Jun 02 2019 *) PROG (PARI) a(n)=3*binomial(n+2, 3) \\ Charles R Greathouse IV, May 23, 2011 (MAGMA) [n*(n+1)*(n+2)/2: n in [0..40]]; // Vincenzo Librandi, Nov 14 2014 CROSSREFS 1/beta(n, 3) in A061928. Cf. A057587, A006003, A254407. A row of array in A129533. Cf. similar sequences of the type n*(n+1)*(n+k)/2 listed in A267370. Similar sequences are listed in A316224. Cf. A056923, A281258. Sequence in context: A051408 A164013 A057671 * A135503 A048088 A064181 Adjacent sequences:  A027477 A027478 A027479 * A027481 A027482 A027483 KEYWORD nonn,nice,easy AUTHOR Olivier Gérard and Ken Knowlton (kcknowlton(AT)aol.com) STATUS approved

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Last modified October 14 11:58 EDT 2019. Contains 328000 sequences. (Running on oeis4.)