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 A120353 Sum of 5 consecutive powers of 3, starting with a power of 9. 2
 121, 1089, 9801, 88209, 793881, 7144929, 64304361, 578739249, 5208653241, 46877879169, 421900912521, 3797108212689, 34173973914201, 307565765227809, 2768091887050281, 24912826983452529, 224215442851072761, 2017938985659654849, 18161450870936893641 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Always a square. LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..300 Tanya Khovanova, Recursive Sequences Index entries for linear recurrences with constant coefficients, signature (9). FORMULA a(n) = 121*3^(2n-2). EXAMPLE a(2) = 3^2 + 3^3 + 3^4 + 3^5 + 3^6 = 121*(3^2) = 1089. MATHEMATICA Total/@Select[Partition[3^Range[0, 60], 5, 1], IntegerQ[Log[9, First[#]]]&] (* or *) Table[121 3^(2n-2), {n, 30}] (* Harvey P. Dale, May 03 2011 *) PROG (MAGMA) [121*3^(2*n-2): n in [1..30]]; // Vincenzo Librandi, Jun 10 2011 (PARI) a(n)=121*9^(n-1) \\ Charles R Greathouse IV, Jul 11 2016 CROSSREFS Sequence in context: A223389 A243810 A302561 * A175983 A307858 A208261 Adjacent sequences:  A120350 A120351 A120352 * A120354 A120355 A120356 KEYWORD nonn,easy AUTHOR Anton Joha, Jun 25 2006 EXTENSIONS Corrected and extended by Harvey P. Dale, May 03 2011 STATUS approved

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Last modified August 22 17:52 EDT 2019. Contains 326182 sequences. (Running on oeis4.)