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A256463 Numbers k such that 3*R_k + 20 is prime, where R_k = 11...1 is the repunit (A002275) of length k. 0
1, 2, 3, 5, 24, 37, 53, 63, 84, 299, 506, 1687, 6660, 7473, 8754, 24048, 68078, 71223 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
Also, numbers k such that (10^k + 59)/3 is prime.
Terms from Kamada data. Note that Kamada does not recognize k=1 as 23 is a degenerate case of form AAA..ABA.
a(19) > 2.5*10^5.
LINKS
EXAMPLE
For k=3, 3*R_3 + 20 = 333 + 20 = 353 which is prime.
MATHEMATICA
Select[Range[0, 250000], PrimeQ[(10^# + 59)/3] &]
PROG
(PARI) is(n)=ispseudoprime(10^n\3+20) \\ Charles R Greathouse IV, Apr 10 2015
(Magma) [n: n in [0..300] | IsPrime((10^n+59) div 3)]; // Vincenzo Librandi, Apr 11 2015
CROSSREFS
Cf. A002275.
Sequence in context: A137070 A084730 A117696 * A279864 A324289 A280258
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 10 2015
STATUS
approved

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Last modified April 22 06:39 EDT 2024. Contains 371888 sequences. (Running on oeis4.)