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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
OFFSET
1,1
COMMENTS
Always a square.
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 A329100
KEYWORD
nonn,easy
AUTHOR
Anton Joha, Jun 25 2006
EXTENSIONS
Corrected and extended by Harvey P. Dale, May 03 2011
STATUS
approved