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A332172
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a(n) = 7*(10^(2n+1)-1)/9 - 5*10^n.
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1
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2, 727, 77277, 7772777, 777727777, 77777277777, 7777772777777, 777777727777777, 77777777277777777, 7777777772777777777, 777777777727777777777, 77777777777277777777777, 7777777777772777777777777, 777777777777727777777777777, 77777777777777277777777777777, 7777777777777772777777777777777
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OFFSET
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0,1
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COMMENTS
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Indices of prime terms: {0, 1, 3, 7, 10, 12, 480, 949, ...} = A183178.
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LINKS
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FORMULA
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G.f.: (2 + 505*x - 1200*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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A332172 := n -> 7*(10^(n*2+1)-1)/9 -5*10^n;
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MATHEMATICA
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Array[7 (10^(2 # +1)-1)/9 -5*10^# &, 15, 0]
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PROG
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(PARI) apply( {A332172(n)=10^(n*2+1)\9*7-5*10^n}, [0..25])
(Python) def A332172(n): return 10**(n*2+1)//9*7-5*10^n
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CROSSREFS
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Cf. A138148 (cyclops numbers with binary digits only).
Cf. A332171 (analog with middle digit 1).
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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