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 A215143 a(n) = 7*a(n-1) -14*a(n-2) +7*a(n-3), with a(0)=1, a(1)=2, a(2)=7. 17
 1, 2, 7, 28, 112, 441, 1715, 6615, 25382, 97069, 370440, 1411788, 5375839, 20458921, 77833217, 296038498, 1125816895, 4281011812, 16277915640, 61891962377, 235320000363, 894697938743, 3401649302758, 12933013979445, 49170893188704, 186945601728004, 710757805310287 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS The Berndt-type sequence number 3 for the argument 2Pi/7 (see A215007 and A215008 for the respective sequences numbers 1 and 2) is defined by the following relations: sqrt(7) *a(n) = s(1)*s(2)^(2n) + s(2)*s(4)^(2n) + s(4)*s(1)^(2n) = s(4)*s(1)^(2n) + s(1)*s(2)^(2n) + s(2)*s(4)^(2n), where s(j) := 2*sin(2*Pi*j/7). REFERENCES R. Witula, Complex numbers, Polynomials and Fractial Partial Decompositions, T.3, Silesian Technical University Press, Gliwice 2010 (in Polish). LINKS G. C. Greubel, Table of n, a(n) for n = 0..1000 B. C. Berndt, A. Zaharescu, Finite trigonometric sums and class numbers, Math. Ann. 330 (2004), 551-575. B. C. Berndt, L.-C. Zhang, Ramanujan's identities for eta-functions, Math. Ann. 292 (1992), 561-573. Z.-G. Liu, Some Eisenstein series identities related to modular equations of the seventh order, Pacific J. Math. 209 (2003), 103-130. Roman Witula, Ramanujan Type Trigonometric Formulae, Demonstratio Math. 45 (2012) 779-796. Roman Witula and Damian Slota, New Ramanujan-Type Formulas and Quasi-Fibonacci Numbers of Order 7, Journal of Integer Sequences, Vol. 10 (2007), Article 07.5.6 Index entries for linear recurrences with constant coefficients, signature (7,-14,7). FORMULA G.f.: (1-5*x+7*x^2)/(1-7*x+14*x^2-7*x^3). MATHEMATICA LinearRecurrence[{7, -14, 7}, {1, 2, 7}, 40] PROG (PARI) Vec((1-5*x+7*x^2)/(1-7*x+14*x^2-7*x^3) + O(x^30)) \\ Michel Marcus, Apr 19 2016 (MAGMA) I:=[1, 2, 7]; [n le 3 select I[n] else 7*Self(n-1) - 14*Self(n-2) + 7*Self(n-3): n in [1..30]]; // G. C. Greubel, Apr 19 2018 CROSSREFS Cf. A215007, A215008. Sequence in context: A099488 A289607 A068944 * A289158 A012855 A224066 Adjacent sequences:  A215140 A215141 A215142 * A215144 A215145 A215146 KEYWORD nonn,easy AUTHOR Roman Witula, Aug 04 2012 STATUS approved

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Last modified February 28 08:13 EST 2020. Contains 332321 sequences. (Running on oeis4.)