A332173 revision #3
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A332173
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a(n) = 7*(10^(2n+1)-1)/9 - 4*10^n.
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1
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3, 737, 77377, 7773777, 777737777, 77777377777, 7777773777777, 777777737777777, 77777777377777777, 7777777773777777777, 777777777737777777777, 77777777777377777777777, 7777777777773777777777777, 777777777777737777777777777, 77777777777777377777777777777, 7777777777777773777777777777777
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OFFSET
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0,1
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COMMENTS
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According to M. Kamada, n = 0 and n = 2 are the only indices of a prime up to n = 2*10^4.
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LINKS
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FORMULA
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G.f.: (1 + 404*x - 1100*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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A332173 := n -> 7*(10^(n*2+1)-1)/9 - 4*10^n;
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MATHEMATICA
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Array[7 (10^(2 # + 1) - 1)/9 - 4*10^# &, 15]
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PROG
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(PARI) apply( {A332173(n)=10^(n*2+1)\9*7-4*10^n}, [0..15])
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CROSSREFS
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Cf. A138148 (cyclops numbers with binary digits only).
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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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editing
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