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A256772 Numbers k such that 3*R_k - 20 is prime, where R_k = 11...1 is the repunit (A002275) of length k. 0
2, 3, 4, 7, 9, 37, 39, 67, 73, 183, 216, 1161, 2622, 2775, 6240, 22078, 30384, 189385 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Also, numbers k such that (10^k - 61)/3 is prime.
Terms from Kamada data.
a(19) > 2.5*10^5.
LINKS
EXAMPLE
For k=3, 3*R_3 - 20 = 333 - 20 = 313 which is prime.
MATHEMATICA
Select[Range[0, 250000], PrimeQ[(10^# - 61)/3] &]
PROG
(Magma) [n: n in [2..300] | IsPrime((10^n-61) div 3)]; // Vincenzo Librandi, Apr 10 2015
CROSSREFS
Cf. A002275.
Sequence in context: A319645 A071114 A078696 * A274976 A269729 A098115
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Apr 09 2015
STATUS
approved

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Last modified June 15 16:27 EDT 2024. Contains 373407 sequences. (Running on oeis4.)