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A256721
Numbers k such that R_k + 10 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
1, 6, 10, 34, 36, 370, 8256, 13290, 14724, 427912, 685224
OFFSET
1,2
COMMENTS
Also, numbers k such that (10^k + 89)/9 is prime.
Terms from Kamada data. Note Kamada does not recognize k=1 as 11 is a degenerate case of form AAA..ABA.
a(12) > 10^6.
EXAMPLE
For k=6, R_6 + 10 = 111111 + 10 = 111121 which is prime.
MATHEMATICA
Select[Range[0, 250000], PrimeQ[(10^# + 89)/9] &]
PROG
(Magma) [n: n in [1..400] | IsPrime((10^n+89) div 9)]; // Vincenzo Librandi, Apr 10 2015
CROSSREFS
Cf. A002275.
Sequence in context: A266957 A137272 A121801 * A192774 A218860 A242866
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 09 2015
STATUS
approved