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A256724
Numbers k such that R_k + 30 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
1, 2, 8, 56, 675470, 718580
OFFSET
1,2
COMMENTS
Also, numbers k such that (10^k + 269)/9 is prime.
Terms from Kamada data. Note that Kamada does not recognize k=1 as 31 is a degenerate case of form AAA..ABA.
a(7) > 10^6.
EXAMPLE
For k=8, R_8 + 30 = 11111111 + 30 = 11111141 which is prime.
MATHEMATICA
Select[Range[0, 250000], PrimeQ[(10^# + 269)/9] &]
PROG
(Magma) [n: n in [1..400] | IsPrime((10^n+269) div 9)]; // Vincenzo Librandi, Apr 10 2015
CROSSREFS
Cf. A002275.
Sequence in context: A191508 A113725 A345677 * A153538 A153566 A153529
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 09 2015
STATUS
approved