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 A140811 a(n) = 6*n^2 - 1. 12
 -1, 5, 23, 53, 95, 149, 215, 293, 383, 485, 599, 725, 863, 1013, 1175, 1349, 1535, 1733, 1943, 2165, 2399, 2645, 2903, 3173, 3455, 3749, 4055, 4373, 4703, 5045, 5399, 5765, 6143, 6533, 6935, 7349, 7775, 8213, 8663, 9125, 9599, 10085, 10583, 11093, 11615 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Also: The numerators in the j=2 column of the array a(i,j) defined in A140825, where the columns j=0 and j=1 are represented by A000012 and A005408. This could be extended to column j=3: 1, -1, 9, 55, 161, ... The common feature of these sequences derived from a(i,j) is that their j-th differences are constant sequences defined by A091137(j). a(n) is the set of all k such that 6k+6 is a perfect square. - Gary Detlefs, Mar 04 2010 The identity (6*n^2 - 1)^2 - (9*n^2 - 3)*(2*n)^2 = 1 can be written as a(n+1)^2 - A157872(n)*A005843(n+1)^2 = 1. - Vincenzo Librandi, Feb 05 2012 Apart from first term, sequence found by reading the line from 5, in the direction 5, 23, ..., in the square spiral whose vertices are the generalized pentagonal numbers A001318. - Omar E. Pol, Jul 18 2012 From Paul Curtz, Sep 17 2018: (Start) Terms from center to right in the following spiral: .                     65--63--61--59                     /             \                   67  31--29--27  57                   /   /         \   \                 69  33   9---7  25  55                 /   /   /     \   \   \               71  35  11  -1===5==23==53==>                   /   /   /   /   /   /                 37  13   1---3  21  51                   \   \         /   /                   39  15--17--19  49                     \             /                     41--43--45--47 (End) REFERENCES P. Curtz, Intégration numérique des systèmes différentiels à conditions initiales, Note 12, Centre de Calcul Scientifique de l'Armement, Arcueil, 1969, 132 pages, pp. 28-36. CCSA, then CELAR. Now DGA Maitrise de l'Information 35131 Bruz. LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..1000 Leo Tavares, Illustration: Barred Stars Index entries for linear recurrences with constant coefficients, signature (3,-3,1). FORMULA a(n) = 2*a(n-1) - a(n-2) + 12. First differences: a(n+1) - a(n) = A017593(n). Second differences: A071593(n+1) - A071593(n) = 12. G.f.: (1-8*x-5*x^2)/(x-1)^3. - Jaume Oliver Lafont, Aug 30 2009 a(n) = a(n-1) + 12*n -6. - Vincenzo Librandi, Feb 05 2012 a(n) = 3*a(n-1) -3*a(n-2) + a(n-3). - Vincenzo Librandi, Feb 05 2012 a(n) = A033581(n) - 1. - Omar E. Pol, Jul 18 2012 a(n) = A032528(2n) - 1. - Adriano Caroli, Jul 21 2013 For n > 0, a(n) = floor(3/(cosh(1/n) - 1)) = floor(1/(n*sinh(1/n) - 1)); for similar formulas for cosine and sine, see A033581. - Clark Kimberling, Oct 19 2014, corrected by M. F. Hasler, Oct 21 2014 a(-n) = a(n). - Paul Curtz, Sep 17 2018 From Amiram Eldar, Feb 04 2021: (Start) Sum_{n>=1} 1/a(n) = (1 - (Pi/sqrt(6))*cot(Pi/sqrt(6)))/2. Sum_{n>=1} (-1)^(n+1)/a(n) = ((Pi/sqrt(6))*csc(Pi/sqrt(6)) - 1)/2. Product_{n>=1} (1 + 1/a(n)) = (Pi/sqrt(6))*csc(Pi/sqrt(6)). Product_{n>=1} (1 - 1/a(n)) = csc(Pi/sqrt(6))*sin(Pi/sqrt(3))/sqrt(2). (End) a(n) = A003154(n+1) - 2*A016777(n). - Leo Tavares, May 13 2022 MATHEMATICA LinearRecurrence[{3, -3, 1}, {-1, 5, 23}, 40] (* Vincenzo Librandi, Feb 05 2012 *) CoefficientList[Series[(1-8*x-5*x^2)/(x-1)^3 , {x, 0, 40}], x] (* Stefano Spezia, Sep 17 2018 *) PROG (PARI) a(n)=6*n^2-1 \\ Charles R Greathouse IV, Jun 01 2011 (Magma) [6*n^2 - 1: n in [0..50]]; // Vincenzo Librandi, Jun 02 2011 CROSSREFS Cf. A005843, A157872. A060747, A103115(n+1), A141417 (array). Cf. A003154, A016777. Sequence in context: A147113 A135771 A327409 * A247657 A241099 A338977 Adjacent sequences:  A140808 A140809 A140810 * A140812 A140813 A140814 KEYWORD sign,easy AUTHOR Paul Curtz, Jul 16 2008 EXTENSIONS Edited and extended by R. J. Mathar, Aug 06 2008 Better description Ray Chandler, Feb 03 2009 STATUS approved

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Last modified August 15 08:41 EDT 2022. Contains 356133 sequences. (Running on oeis4.)