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 A014635 a(n) = 2*n*(4*n - 1). 20
 0, 6, 28, 66, 120, 190, 276, 378, 496, 630, 780, 946, 1128, 1326, 1540, 1770, 2016, 2278, 2556, 2850, 3160, 3486, 3828, 4186, 4560, 4950, 5356, 5778, 6216, 6670, 7140, 7626, 8128, 8646, 9180, 9730, 10296, 10878, 11476, 12090, 12720, 13366, 14028, 14706 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Even hexagonal numbers. Number of edges in the join of two complete graphs of order 3n and n, K_3n * K_n - Roberto E. Martinez II, Jan 07 2002 Bisection of A000384. Also, this sequence arises from reading the line from 0, in the direction 0, 6, ..., in the square spiral whose vertices are the triangular numbers A000217. Perfect numbers are members of this sequence because a(A134708(n)) = A000396(n). Also, positive members are a bisection of A139596. - Omar E. Pol, May 07 2008 LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..880 Amelia Carolina Sparavigna, The groupoids of Mersenne, Fermat, Cullen, Woodall and other Numbers and their representations by means of integer sequences, Politecnico di Torino, Italy (2019), [math.NT]. Amelia Carolina Sparavigna, The groupoid of the Triangular Numbers and the generation of related integer sequences, Politecnico di Torino, Italy (2019). Index entries for linear recurrences with constant coefficients, signature (3,-3,1). FORMULA a(n) = C(4*n,2), n>=0. - Zerinvary Lajos, Jan 02 2007 O.g.f.: 2*x*(3+5*x)/(1-x)^3. - R. J. Mathar, May 06 2008 a(n) = 8*n^2 - 2*n. - Omar E. Pol, May 07 2008 a(n) = a(n-1) + 16*n - 10 (with a(0)=0). - Vincenzo Librandi, Nov 20 2010 E.g.f.: (8*x^2 + 6*x)*exp(x). - G. C. Greubel, Jul 18 2017 From Vaclav Kotesovec, Aug 18 2018: (Start) Sum_{n>=1} 1/a(n) = 3*log(2)/2 - Pi/4. Sum_{n>=1} (-1)^n / a(n) = log(2)/2 + log(1+sqrt(2))/sqrt(2) - Pi / 2^(3/2). (End) MAPLE [seq(binomial(4*n, 2), n=0..43)]; # Zerinvary Lajos, Jan 02 2007 MATHEMATICA s=0; lst={s}; Do[s+=n++ +6; AppendTo[lst, s], {n, 0, 7!, 16}]; lst (* Vladimir Joseph Stephan Orlovsky, Nov 16 2008 *) Table[2*n*(4*n - 1), {n, 0, 50}] (* G. C. Greubel, Jul 18 2017 *) PROG (MAGMA) [2*n*(4*n-1): n in [0..50]]; // Vincenzo Librandi, Apr 25 2011 (PARI) a(n)=2*n*(4*n-1) \\ Charles R Greathouse IV, Jun 17 2017 CROSSREFS Cf. A000217, A000384, A000396, A134708, A139596. Sequence in context: A081537 A058007 A033588 * A227970 A034955 A117978 Adjacent sequences:  A014632 A014633 A014634 * A014636 A014637 A014638 KEYWORD nonn,easy AUTHOR EXTENSIONS More terms from Erich Friedman STATUS approved

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Last modified August 15 10:12 EDT 2020. Contains 336492 sequences. (Running on oeis4.)