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A256722
Numbers k such that R_k + 20 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
2, 3, 5, 8, 9, 11, 18, 20, 29, 39, 123, 153, 267, 327, 627, 1020, 1686, 1764, 2960, 18236, 34613, 53991, 67404, 119880, 221654, 230706
OFFSET
1,1
COMMENTS
Also, numbers k such that (10^k + 179)/9 is prime.
Terms from Kamada data.
a(27) > 2.5*10^5.
EXAMPLE
For k=3, R_3 + 20 = 111 + 20 = 131 which is prime.
MATHEMATICA
Select[Range[0, 250000], PrimeQ[(10^# + 179)/9] &]
PROG
(Magma) [n: n in [1..400] | IsPrime((10^n+179) div 9)]; // Vincenzo Librandi, Apr 10 2015
CROSSREFS
Cf. A002275.
Sequence in context: A226815 A284890 A051214 * A364926 A363097 A192391
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 09 2015
STATUS
approved