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A332174
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a(n) = 7*(10^(2n+1)-1)/9 - 3*10^n.
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1
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4, 747, 77477, 7774777, 777747777, 77777477777, 7777774777777, 777777747777777, 77777777477777777, 7777777774777777777, 777777777747777777777, 77777777777477777777777, 7777777777774777777777777, 777777777777747777777777777, 77777777777777477777777777777, 7777777777777774777777777777777
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OFFSET
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0,1
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COMMENTS
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See A183179 = {2, 3, 6, 23, 36, 69, 561, ...} for the indices of primes.
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LINKS
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FORMULA
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G.f.: (4 + 303*x - 1000*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.
E.g.f.: (1/9)*exp(x)*(70*exp(99*x) - 27*exp(9*x) - 7). - Stefano Spezia, Feb 08 2020
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MAPLE
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A332174 := n -> 7*(10^(n*2+1)-1)/9 - 3*10^n;
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MATHEMATICA
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Array[7 (10^(2 # + 1) - 1)/9 - 3*10^# &, 15, 0]
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PROG
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(PARI) apply( {A332174(n)=10^(n*2+1)\9*7-3*10^n}, [0..15])
(Python) def A332174(n): return 10**(n*2+1)//9*7-3*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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