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A247842
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Sum(9^k, k=2..n).
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2
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0, 81, 810, 7371, 66420, 597861, 5380830, 48427551, 435848040, 3922632441, 35303692050, 317733228531, 2859599056860, 25736391511821, 231627523606470, 2084647712458311, 18761829412124880, 168856464709124001, 1519708182382116090, 13677373641439044891
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OFFSET
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1,2
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LINKS
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FORMULA
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G.f.: 81*x^2/((1-x)*(1-9*x)).
a(n) = a(n-1) + 9^n = (9^(n+1) - 81)/8 = 10*a(n-1) - 9*a(n-2).
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MAPLE
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MATHEMATICA
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RecurrenceTable[{a[1] == 0, a[n] == a[n-1] + 9^n}, a, {n, 30}] (* or *) CoefficientList[Series[81 x / ((1 - x) (1 - 9 x)), {x, 0, 30}], x]
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PROG
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(Magma) [0] cat [&+[9^k: k in [2..n]]: n in [2..30]]; /* or */ [(9^(n+1)-81)/8: n in [1..30]];
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CROSSREFS
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Cf. similar sequences listed in A247817.
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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