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A332178
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a(n) = 7*(10^(2n+1)-1)/9 + 10^n.
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8
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8, 787, 77877, 7778777, 777787777, 77777877777, 7777778777777, 777777787777777, 77777777877777777, 7777777778777777777, 777777777787777777777, 77777777777877777777777, 7777777777778777777777777, 777777777777787777777777777, 77777777777777877777777777777, 7777777777777778777777777777777
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OFFSET
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0,1
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COMMENTS
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See A183182 = {1, 3, 39, 54, 168, 240, ...} for the indices of primes.
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LINKS
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FORMULA
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G.f.: (8 - 101*x - 600*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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A332178 := n -> 7*(10^(n*2+1)-1)/9 + 10^n;
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MATHEMATICA
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Array[7 (10^(2 # + 1) - 1)/9 + 10^# &, 15, 0]
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PROG
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(PARI) apply( {A332178(n)=10^(n*2+1)\9*7+10^n}, [0..15])
(Python) def A332178(n): return 10**(n*2+1)//9*7+10^n
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CROSSREFS
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Cf. A138148 (cyclops numbers with binary digits only).
Cf. A332171 .. A332179 (variants with different middle digit 1, ..., 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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