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 A215917 a(n) = -3*a(n-1) + a(n-3), with a(0)=0, a(1)=6, and a(2)=-15. 7
 0, 6, -15, 45, -129, 372, -1071, 3084, -8880, 25569, -73623, 211989, -610398, 1757571, -5060724, 14571774, -41957751, 120812529, -347865813, 1001639688, -2884106535, 8304453792, -23911721688, 68851058529, -198248721795, 570834443697, -1643652272562 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS The Berndt-type sequence number 9 for the argument 2Pi/9 defined by the first relation from the section "Formula" below. We have a(n) = 3*(-1)^(n+1)*A215448(n+1). From the recurrence formula for a(n) it follows that all a(3*n) are divisible by 9, a(3*n+1)/3 are congruent to 2 modulo 3, and a(3*n+2)/3 are congruent to 1 modulo 3. In the consequence also all sums a(n)+a(n+1)+a(n+2) are divisible by 9. From general recurrence X(n) = -3*X(n-1) + X(n-3) the following formula can be deduced: 3*sum{k=2,..,n-1} X(k) = -X(n)-X(n-1)-X(n-2)+X(2)+X(1)+X(0). Hence, in the case of a(n) we obtain  3*sum{k=2,..,n-1} a(k) = -a(n)-a(n-1)-a(n-2)-9. If we set X(n) = -3*X(n-1) + X(n-3), n in Z, with a(n) = X(n) for n=0,1,... then X(-n) = abs(A215666(n)) = (-1)^n*A215666(n), for every n=0,1,... The following decomposition holds true (X - c(1)*(-c(4))^(-n))*(X - c(2)*(-c(1))^(-n))*(X - c(4)*(-c(2))^(-n)) = X^3 - a(n)*X^2 + (-1)^n*(A215665(n) - A215664(n))*X + 1. REFERENCES D. Chmiela and R. Witula, Two parametric quasi-Fibonacci numbers of the ninth order, (submitted, 2012). R. Witula, Ramanujan type formulas for arguments 2Pi/7 and 2Pi/9, Demonstratio Math. (in press, 2012). LINKS Index entries for linear recurrences with constant coefficients, signature (-3, 0, 1). FORMULA a(n) = c(1)*(-c(4))^(-n) + c(2)*(-c(1))^(-n) + c(4)*(-c(2))^(-n), where c(j) := 2*cos(2*Pi*j/9). a(n) = (-1)^n*(A215885(n+1) - A215885(n)). G.f.: 3*x(x+2)/(1+3*x-x^3). MAPLE We have a(3) + 3*a(2) = 0, a(8) + 24*a(5) = 48 = a(3) + a(1)/2. MATHEMATICA LinearRecurrence[{-3, 0, 1}, {0, 6, -15}, 50]. PROG (PARI) concat(0, Vec(3*(x+2)/(1+3*x-x^3)+O(x^99))) \\ Charles R Greathouse IV, Oct 01 2012 CROSSREFS Cf. A215448, A215885, A215664, A215665, A215666. Sequence in context: A327769 A318482 A095122 * A082637 A244023 A271131 Adjacent sequences:  A215914 A215915 A215916 * A215918 A215919 A215920 KEYWORD sign,easy AUTHOR Roman Witula, Aug 27 2012 STATUS approved

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Last modified February 28 09:52 EST 2020. Contains 332323 sequences. (Running on oeis4.)