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A185123
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a(n) = n 9's sandwiched between two 1's.
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0
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11, 191, 1991, 19991, 199991, 1999991, 19999991, 199999991, 1999999991, 19999999991, 199999999991, 1999999999991, 19999999999991, 199999999999991, 1999999999999991, 19999999999999991, 199999999999999991, 1999999999999999991, 19999999999999999991
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OFFSET
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0,1
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LINKS
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FORMULA
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a(0)=11, a(n) = 10*a(n-1) + 81.
G.f.: (11 + 70*x)/( (1-x)*(1-10*x) ).
E.g.f.: 20*exp(10*x) - 9*exp(x). (End)
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MATHEMATICA
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H[n_]:=10^n+1+Sum[10^i*9, {i, 1, n-1}]; Array[H, 100]
CoefficientList[Series[(11 + 70*x)/( (1-x)*(1-10*x) ), {x, 0, 50}], x] (* G. C. Greubel, Jun 23 2017 *)
Table[10FromDigits[PadRight[{1}, n, 9]]+1, {n, 20}] (* or *) LinearRecurrence[ {11, -10}, {11, 191}, 20] (* Harvey P. Dale, May 18 2021 *)
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PROG
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CROSSREFS
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KEYWORD
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nonn,easy,less,base
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AUTHOR
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STATUS
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approved
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