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A257033
Numbers k such that 7*R_(k+2) + 10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
1, 2, 4, 5, 7, 11, 25, 31, 109, 205, 518, 1288, 1697, 1711, 1775, 1997, 6784, 8741, 13171, 20275, 67517
OFFSET
1,2
COMMENTS
Also, numbers k such that (709*10^k - 7)/9 is prime.
Terms from Kamada.
a(22) > 2*10^5.
EXAMPLE
For k=4, 7*R_6 + 10^4 = 777777 + 10000 = 787777 which is prime.
MATHEMATICA
Select[Range[0, 30000], PrimeQ[(709*10^#-7)/9 ] &]
PROG
(Magma) [n: n in [0..400] | IsPrime((709*10^n-7) div 9)]; // Vincenzo Librandi, Apr 15 2015
CROSSREFS
Cf. A002275.
Sequence in context: A164316 A323146 A250006 * A039674 A090649 A154686
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 14 2015
EXTENSIONS
a(21) from Robert Price, Sep 28 2017
STATUS
approved