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A256773
Numbers k such that 3*R_k - 10 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
2, 4, 6, 10, 12, 14, 17, 41, 69, 141, 170, 188, 204, 233, 306, 555, 743, 1908, 2006, 2140, 2772, 8105, 15170, 40038, 41908, 48604, 51652, 54038, 56781, 129051, 283342
OFFSET
1,1
COMMENTS
Also, numbers k such that (10^k - 31)/3 is prime.
Terms from Kamada data.
a(32) > 3*10^5.
EXAMPLE
For k=4, 3*R_4 - 10 = 3333 - 10 = 3323 which is prime.
MATHEMATICA
Select[Range[0, 250000], PrimeQ[(10^# - 31)/3] &]
PROG
(Magma) [n: n in [2..310] | IsPrime((10^n-31) div 3)]; // Vincenzo Librandi, Apr 10 2015
CROSSREFS
Cf. A002275.
Sequence in context: A022305 A103799 A175817 * A141104 A047410 A339331
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Apr 09 2015
EXTENSIONS
a(31) from Robert Price, Jul 12 2023
STATUS
approved