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A256829
Numbers k such that 7*R_k - 30 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
2, 5, 8, 13, 17, 50, 169, 193, 407, 1496, 1718, 3610, 6527, 6889, 12665, 15949, 23498, 32969, 44773, 110027
OFFSET
1,1
COMMENTS
Also, numbers k such that (7*10^k - 277)/9 is prime.
Terms from Kamada.
a(21) > 200000. - Tyler Busby, Apr 20 2024
EXAMPLE
For k=5, 7*R_5 - 30 = 77777 - 30 = 77747 which is prime.
MATHEMATICA
Select[Range[0, 250000], PrimeQ[(7*10^# - 277)/9] &]
PROG
(Magma) [n: n in [2..500] | IsPrime((7*10^n-277) div 9)]; // Vincenzo Librandi, Apr 11 2015
CROSSREFS
Cf. A002275.
Sequence in context: A225255 A076145 A332774 * A342431 A049617 A375996
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 10 2015
EXTENSIONS
a(19)-a(20) from Kamada data by Tyler Busby, Apr 20 2024
STATUS
approved