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A257031
Numbers k such that 7*R_(k+2) - 2*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
1, 2, 5, 10, 11, 16, 23, 247, 1700, 2891, 3019, 5549, 5837, 9326, 14417, 23312, 24155, 30740, 61907, 64421, 69997, 78656, 163106, 177266
OFFSET
1,2
COMMENTS
Also, numbers k such that (682*10^k - 7)/9 is prime.
Terms from Kamada.
EXAMPLE
For k=5, 7*R_7 - 2*10^5 = 7777777 - 200000 = 7577777 which is prime.
MATHEMATICA
Select[Range[0, 30000], PrimeQ[(682*10^#-7)/9 ] &]
PROG
(Magma) [n: n in [0..400] | IsPrime((682*10^n-7) div 9)]; // Vincenzo Librandi, Apr 15 2015
CROSSREFS
Cf. A002275.
Sequence in context: A163624 A353356 A363890 * A167799 A179871 A258116
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 14 2015
EXTENSIONS
a(18)-a(24) from Kamada data by Tyler Busby, Apr 16 2024
STATUS
approved