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A123368
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Numbers k such that 10*(10^(k+1) + 10^k - 1) + 7 is prime.
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0
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0, 1, 4, 5, 7, 22, 23, 40, 65, 73, 169, 271, 802, 1451, 2623, 4107, 7315, 7583, 7747, 8407, 9841, 19647, 23923, 24825
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OFFSET
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1,3
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LINKS
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EXAMPLE
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107, 1097, 1099997, 10999997 are prime so terms are 0, 1, 4, 5.
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MATHEMATICA
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Join[{0}, Select[Range[3000], PrimeQ[10*(10^(# + 1) + 10^# - 1) + 7] &]] (* G. C. Greubel, Oct 13 2017 *)
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PROG
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(PARI) g(n) = for(x=0, n, y=10*(10^(x+1)+10^x-1)+7; if(ispseudoprime(y), print1(x", ")))
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CROSSREFS
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KEYWORD
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more,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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