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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
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)
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
KEYWORD
nonn,base,hard,more
AUTHOR
Mikk Heidemaa, Nov 17 2015
STATUS
approved