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 A139278 a(n) = n*(8*n+7). 14
 0, 15, 46, 93, 156, 235, 330, 441, 568, 711, 870, 1045, 1236, 1443, 1666, 1905, 2160, 2431, 2718, 3021, 3340, 3675, 4026, 4393, 4776, 5175, 5590, 6021, 6468, 6931, 7410, 7905, 8416, 8943, 9486, 10045, 10620, 11211, 11818, 12441, 13080 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Sequence found by reading the segment (0, 15) together with the line from 15, in the direction 15, 46, ..., in the square spiral whose vertices are the triangular numbers A000217. LINKS G. C. Greubel, Table of n, a(n) for n = 0..5000 Omar E. Pol, Determinacion geometrica de los numeros primos y perfectos. Index entries for linear recurrences with constant coefficients, signature (3,-3,1). FORMULA a(n) = 8*n^2 + 7*n. Sequences of the form a(n)=8*n^2+c*n have generating functions x{c+8+(8-c)x} / (1-x)^3 and recurrence a(n)= 3a(n-1)-3a(n-2)+a(n-3). The inverse binomial transform is 0, c+8, 16, 0, 0, ... (0 continued). This applies to A139271-A139278, positive or negative c. - R. J. Mathar, May 12 2008 a(n) = 16*n+a(n-1)-1 (with a(0)=0). - Vincenzo Librandi, Aug 03 2010 From G. C. Greubel, Jul 18 2017: (Start) G.f.: x*(x+15)/(1-x)^3. E.g.f.: (8*x^2 + 15*x)*exp(x). (End) Sum_{n>=1} 1/a(n) = 8/49 + (sqrt(2)+1)*Pi/14 - 4*log(2)/7 - sqrt(2)*log(sqrt(2)+1)/7. - Amiram Eldar, Mar 17 2022 MATHEMATICA Table[n (8 n + 7), {n, 0, 40}] (* or *) LinearRecurrence[{3, -3, 1}, {0, 15, 46}, 50] (* Harvey P. Dale, Oct 07 2015 *) PROG (PARI) a(n)=n*(8*n+7) \\ Charles R Greathouse IV, Jun 17 2017 CROSSREFS Cf. A000217, A014634, A014635, A033585, A033586, A033587, A035008, A051870, A069129, A085250, A072279, A139272, A139273, A139274, A139275, A139276, A139277, A139279, A139280, A139281, A139282. Sequence in context: A033480 A041434 A136430 * A041436 A219885 A042007 Adjacent sequences: A139275 A139276 A139277 * A139279 A139280 A139281 KEYWORD nonn,easy AUTHOR Omar E. Pol, Apr 26 2008 STATUS approved

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Last modified June 13 03:07 EDT 2024. Contains 373366 sequences. (Running on oeis4.)