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A332117
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a(n) = (10^(2n+1)-1)/9 + 6*10^n.
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7
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7, 171, 11711, 1117111, 111171111, 11111711111, 1111117111111, 111111171111111, 11111111711111111, 1111111117111111111, 111111111171111111111, 11111111111711111111111, 1111111111117111111111111, 111111111111171111111111111, 11111111111111711111111111111, 1111111111111117111111111111111
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OFFSET
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0,1
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COMMENTS
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See A107127 = {0, 3, 33, 311, 2933, ...} for the indices of primes.
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LINKS
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FORMULA
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G.f.: (7 - 606*x + 500*x^2)/((1 - x)(1 - 10*x)(1 - 100*x)).
a(n) = 111*a(n-1) - 1110*a(n-2) + 1000*a(n-3) for n > 2.
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MAPLE
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A332117 := n -> (10^(2*n+1)-1)/9+6*10^n;
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MATHEMATICA
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Array[(10^(2 # + 1)-1)/9 + 6*10^# &, 15, 0]
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PROG
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(PARI) apply( {A332117(n)=10^(n*2+1)\9+6*10^n}, [0..15])
(Python) def A332117(n): return 10**(n*2+1)//9+6*10**n
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CROSSREFS
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Cf. A138148 (cyclops numbers with binary digits), A002113 (palindromes).
Cf. A332127 .. A332197 (variants with different repeated digit 2, ..., 9).
Cf. A332112 .. A332119 (variants with different middle digit 2, ..., 9).
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KEYWORD
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nonn,base,easy
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AUTHOR
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STATUS
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approved
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