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 A260899 Positive numbers k such that 1...123 = (10^(k+2) + 107) / 9 is prime. 1
 2, 12, 14, 38, 158, 170, 548 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS From Robert Israel, Nov 30 2015: (Start) All terms == 0 or 2 (mod 6), because otherwise (10^(n+2)+107)/9 is divisible by 3, 7, or 13. a(8) > 42000 (if it exists). (End) LINKS EXAMPLE 2 appears because 1123 is prime. 12 appears because 11111111111123 is prime. MATHEMATICA Select[Select[Range[10^3], EvenQ], PrimeQ[(10^(# + 2) + 107)/9] &] Select[Range[3, 560], PrimeQ[FromDigits[PadLeft[{2, 3}, #, 1]]]&]-2 (* Harvey P. Dale, May 16 2021 *) PROG (Magma) [n: n in [1..300] | IsPrime((10^(n+2)+107) div 9)]; // Vincenzo Librandi, Nov 18 2015 (PARI) is(n)=ispseudoprime((10^(n+2)+107)/9) \\ Charles R Greathouse IV, Jun 13 2017 CROSSREFS See A260898 for the actual primes. Sequence in context: A280942 A270119 A340016 * A108969 A269130 A265485 Adjacent sequences:  A260896 A260897 A260898 * A260900 A260901 A260902 KEYWORD nonn,base,hard,more AUTHOR Mikk Heidemaa, Nov 17 2015 STATUS approved

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Last modified May 25 12:00 EDT 2022. Contains 354071 sequences. (Running on oeis4.)