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 A085001 a(n) = (3*n+1)*(3*n+4). 3
 4, 28, 70, 130, 208, 304, 418, 550, 700, 868, 1054, 1258, 1480, 1720, 1978, 2254, 2548, 2860, 3190, 3538, 3904, 4288, 4690, 5110, 5548, 6004, 6478, 6970, 7480, 8008, 8554, 9118, 9700, 10300, 10918, 11554, 12208, 12880, 13570, 14278, 15004 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS 1/4 + 1/28 + 1/70 + ... = 1/3; 1/4 + 1/28 + 1/70 + ... n terms = (n+1)/(3n+4). [Jolley]. - Gary W. Adamson, Jan 03 2007 [Corrected by Gary Detlefs, Mar 14 2018] REFERENCES L. B. W. Jolley, "Summation of Series", Dover Publications, 1961, p. 38 LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..1000 Index entries for linear recurrences with constant coefficients, signature (3,-3,1). FORMULA Sum_{k=0..n} 3/a(k) = 3*(n+1)/(3n+4). [Corrected by Gary Detlefs, Mar 14 2018] Sum_{k>=0} 3/a(k) = 1. G.f.: 2*(2+8*x-x^2)/(1-x)^3. - R. J. Mathar, Sep 17 2008 a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3). - Vincenzo Librandi, Jul 08 2012 MATHEMATICA CoefficientList[Series[2*(2+8x-x^2)/(1-x)^3, {x, 0, 50}], x] (* Vincenzo Librandi, Jul 08 2012 *) Table[(3n+1)(3n+4), {n, 0, 40}] (* or *) LinearRecurrence[{3, -3, 1}, {4, 28, 70}, 50] (* Harvey P. Dale, Apr 07 2019 *) PROG (MAGMA) [(3*n+1)*(3*n+4): n in [0..50]]; // Vincenzo Librandi, Jul 08 2012 (PARI) a(n)=(3*n+1)*(3*n+4) \\ Charles R Greathouse IV, Jun 17 2017 CROSSREFS Sequence in context: A197542 A203280 A085024 * A153784 A030117 A005634 Adjacent sequences:  A084998 A084999 A085000 * A085002 A085003 A085004 KEYWORD nonn,easy AUTHOR Gary W. Adamson, Jun 17 2003 EXTENSIONS Edited by Don Reble, Nov 13 2005 STATUS approved

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Last modified November 29 15:46 EST 2021. Contains 349416 sequences. (Running on oeis4.)