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A120353
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Sum of 5 consecutive powers of 3, starting with a power of 9.
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2
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121, 1089, 9801, 88209, 793881, 7144929, 64304361, 578739249, 5208653241, 46877879169, 421900912521, 3797108212689, 34173973914201, 307565765227809, 2768091887050281, 24912826983452529, 224215442851072761, 2017938985659654849, 18161450870936893641
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OFFSET
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1,1
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COMMENTS
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Always a square.
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LINKS
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FORMULA
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a(n) = 121*3^(2n-2).
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EXAMPLE
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a(2) = 3^2 + 3^3 + 3^4 + 3^5 + 3^6 = 121*(3^2) = 1089.
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MATHEMATICA
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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 *)
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PROG
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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