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 A067727 a(n) = 7*n^2 + 14*n. 9
 21, 56, 105, 168, 245, 336, 441, 560, 693, 840, 1001, 1176, 1365, 1568, 1785, 2016, 2261, 2520, 2793, 3080, 3381, 3696, 4025, 4368, 4725, 5096, 5481, 5880, 6293, 6720, 7161, 7616, 8085, 8568, 9065, 9576, 10101, 10640, 11193, 11760, 12341, 12936 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Positive numbers k such that 7*(7 + k) is a perfect square. LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..1000 Index entries for linear recurrences with constant coefficients, signature (3,-3,1). FORMULA a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3). - Vincenzo Librandi, Jul 08 2012 G.f.: 7*x*(3-x)/(1-x)^3. - Vincenzo Librandi, Jul 08 2012 E.g.f.: 7*x*(3 + x)*exp(x). - G. C. Greubel, Sep 01 2019 From Amiram Eldar, Feb 25 2022: (Start) Sum_{n>=1} 1/a(n) = 3/28. Sum_{n>=1} (-1)^(n+1)/a(n) = 1/28. (End) MAPLE seq(7*n*(n+2), n=1..45); # G. C. Greubel, Sep 01 2019 MATHEMATICA Select[ Range[15000], IntegerQ[ Sqrt[ 7(7 + # )]] & ] CoefficientList[Series[7*(3-x)/(1-x)^3, {x, 0, 50}], x] (* Vincenzo Librandi, Jul 08 2012 *) 7*(Range[2, 45]^2 -1) (* G. C. Greubel, Sep 01 2019 *) LinearRecurrence[{3, -3, 1}, {21, 56, 105}, 50] (* Harvey P. Dale, Dec 07 2022 *) PROG (PARI) a(n)= 7*n*(n+2) \\ Charles R Greathouse IV, Dec 07 2011 (Magma) [7*n*(n+2): n in [1..50]]; // Vincenzo Librandi, Jul 08 2012 (Sage) [7*n*(n+2) for n in (1..45)] # G. C. Greubel, Sep 01 2019 (GAP) List([1..45], n-> 7*n*(n+2)); # G. C. Greubel, Sep 01 2019 CROSSREFS Cf. A186029. Cf. numbers k such that k*(k + m) is a perfect square: A028560 (k=9), A067728 (k=8), A067726 (k=6), A067724 (k=5), A028347 (k=4), A067725 (k=3), A054000 (k=2), A005563 (k=1). Sequence in context: A301607 A145719 A031963 * A254144 A165237 A271734 Adjacent sequences: A067724 A067725 A067726 * A067728 A067729 A067730 KEYWORD nonn,easy AUTHOR Robert G. Wilson v, Feb 05 2002 EXTENSIONS Edited by Charles R Greathouse IV, Jul 25 2010 STATUS approved

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Last modified April 19 07:38 EDT 2024. Contains 371782 sequences. (Running on oeis4.)