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 A139271 a(n) = 2*n*(4*n-3). 17
 0, 2, 20, 54, 104, 170, 252, 350, 464, 594, 740, 902, 1080, 1274, 1484, 1710, 1952, 2210, 2484, 2774, 3080, 3402, 3740, 4094, 4464, 4850, 5252, 5670, 6104, 6554, 7020, 7502, 8000, 8514, 9044, 9590, 10152, 10730, 11324, 11934, 12560 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Sequence found by reading the line from 0, in the direction 0, 2, ..., in the square spiral whose vertices are the triangular numbers A000217. Opposite numbers to the members of A033585 in the same spiral. Twice decagonal numbers (or twice 10-gonal numbers). - Omar E. Pol, May 15 2008 a(n) is the number of walks in a cubic lattice of n dimensions that reach the point of origin for the first time after 4 steps. - Shel Kaphan, Mar 20 2023 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 - 6*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) = A001107(n)*2. - Omar E. Pol, May 15 2008 a(n) = 16*n + a(n-1) - 14 (with a(0)=0). - Vincenzo Librandi, Aug 03 2010 From G. C. Greubel, Jul 18 2017: (start) G.f.: (2*x)*(7*x+1)/(1-x)^3. E.g.f.: (8*x^2 + 2*x)*exp(x). (End) Sum_{n>=1} 1/a(n) = Pi/12 + log(2)/2. - Amiram Eldar, Mar 28 2023 MATHEMATICA Table[8n^2-6n, {n, 0, 40}] (* or *) LinearRecurrence[{3, -3, 1}, {0, 2, 20}, 50] (* Harvey P. Dale, Sep 26 2016 *) PROG (PARI) a(n)=2*n*(4*n-3) \\ Charles R Greathouse IV, Jun 17 2017 CROSSREFS Cf. A000217, A014634, A014635, A033585, A033586, A033587, A035008, A051870, A069129, A085250, A139272, A139273, A139274, A139275, A139276, A139278, A139279, A139280, A139281, A139282. Cf. A001107. Cf. numbers of the form n*(n*k-k+4))/2 listed in A226488 (this sequence is the case k=16). - Bruno Berselli, Jun 10 2013 Row n=2 of A361397. Sequence in context: A225065 A059211 A331090 * A133217 A001504 A192351 Adjacent sequences: A139268 A139269 A139270 * A139272 A139273 A139274 KEYWORD easy,nonn,walk AUTHOR Omar E. Pol, Apr 26 2008 EXTENSIONS Corrected by Harvey P. Dale, Sep 26 2016 STATUS approved

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Last modified May 18 15:24 EDT 2024. Contains 372664 sequences. (Running on oeis4.)