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 A215007 a(n) = 7*a(n-1) - 14*a(n-2) + 7*a(n-3), a(0)=1, a(1)=3, a(2)=9. 23
 1, 3, 9, 28, 91, 308, 1078, 3871, 14161, 52479, 196196, 737793, 2785160, 10540390, 39955041, 151615947, 575723785, 2187128524, 8311078307, 31587815308, 120069510526, 456434707519, 1735184512425, 6596692255391, 25079305566420 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS The sequence a(n) we shall call Berndt-type sequence of number 1 for the argument 2*Pi/7 - our motivation from Berndt's et al. and myself papers comes (see the first formula below, which is in agreement with the respective identities discussed in these papers). We note that a(n) = A105849(n) for n=0,1,...,5, and A105849(6) - a(6) = 1. Moreover we have a(n) = 2*A215008(n) - A215008(n+1). REFERENCES R. Witula, E. Hetmaniok, D. Slota, Sums of the powers of any order roots taken from the roots of a given polynomial, Proceedings of the Fifteenth International Conference on Fibonacci Numbers and Their Applications, Eger, Hungary, 2012. 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. R. Wituła, P. Lorenc, M. Różański, M. Szweda, Sums of the rational powers of roots of cubic polynomials, Zeszyty Naukowe Politechniki Slaskiej, Seria: Matematyka Stosowana z. 4, Nr. kol. 1920, 2014. FORMULA a(n) = (1/sqrt(7))*(cot(8*Pi/7)*(s(1))^2n + cot(4*Pi/7)*(s(4))^2n + cot(2*Pi/7)*(s(2))^2n), where s(j) := 2*sin(2Pi*j/7). G.f.: (1-4*x+2*x^2)/(1-7*x+14*x^2-7*x^3). MAPLE seq(coeff(series((1-4*x+2*x^2)/(1-7*x+14*x^2-7*x^3), x, n+1), x, n), n = 0..30); # G. C. Greubel, Oct 03 2019 MATHEMATICA LinearRecurrence[{7, -14, 7}, {1, 3, 9}, 30] (* G. C. Greubel, Feb 01 2018 *) PROG (PARI) Vec((1-4*x+2*x^2)/(1-7*x+14*x^2-7*x^3)+O(x^30)) \\ Charles R Greathouse IV, Sep 27 2012 (MAGMA) R:=PowerSeriesRing(Rationals(), 30); Coefficients(R!((1-4*x+2*x^2)/(1-7*x+14*x^2-7*x^3))) // G. C. Greubel, Feb 01 2018 (Sage) def A215007_list(prec):     P. = PowerSeriesRing(ZZ, prec)     return P((1-4*x+2*x^2)/(1-7*x+14*x^2-7*x^3)).list() A215007_list(30) # G. C. Greubel, Oct 03 2019 (GAP) a:=[1, 3, 9];; for n in [4..30] do a[n]:=7*(a[n-1]-2*a[n-2]+a[n-3]); od; a; # G. C. Greubel, Oct 03 2019 CROSSREFS Cf. A122068, A215008. Sequence in context: A071752 A071756 A176673 * A105849 A243156 A228449 Adjacent sequences:  A215004 A215005 A215006 * A215008 A215009 A215010 KEYWORD nonn,easy AUTHOR Roman Witula, Jul 31 2012 STATUS approved

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Last modified May 29 07:47 EDT 2020. Contains 334697 sequences. (Running on oeis4.)