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A259140
Numbers k such that 2*R_k + 9*10^k + 7 is prime, where R_k = 11...11 is the repunit (A002275) of length k.
0
2, 6, 12, 110, 3608, 37784
OFFSET
1,1
COMMENTS
Also, numbers k such that (83*10^k + 61)/9 is prime.
Terms from Kamada data.
a(7) > 10^5.
EXAMPLE
For k=2, 2*R_2 + 9*10^k + 7 = 22 + 900 + 7 = 929 which is prime.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(83*10^#+61)/9] &]
PROG
(Magma) [n: n in [0..300] | IsPrime((83*10^n+61) div 9)]; // _Vincenzo Librandi_, Jun 19 2015
CROSSREFS
Cf. A002275.
Sequence in context: A141288 A038787 A144098 * A075071 A250269 A193987
KEYWORD
nonn,more,hard
AUTHOR
_Robert Price_, Jun 18 2015
STATUS
approved