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A256761 Numbers k such that R_k + 70 is prime, where R_k = 11...1 is the repunit (A002275) of length k. 0
1, 3, 4, 7, 9, 10, 16, 124, 243, 303, 459, 471, 520, 591, 1726, 2971, 5560, 5757, 13222 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
Also, numbers k such that (10^k + 629)/9 is prime.
Terms from Kamada data. Note that Kamada does not recognize k=1 as 81 is a degenerate case of form AAA..ABA.
a(20) > 2.5*10^5.
LINKS
EXAMPLE
For k=3, R_3 + 70 = 111 + 70 = 181 which is prime.
MATHEMATICA
Select[Range[0, 250000], PrimeQ[(10^# + 629)/9] &]
PROG
(Magma) [n: n in [1..350] | IsPrime((10^n+629) div 9)]; // Vincenzo Librandi, Apr 10 2015
CROSSREFS
Cf. A002275.
Sequence in context: A024799 A240531 A212013 * A039579 A274794 A115104
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Apr 09 2015
STATUS
approved

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Last modified July 14 12:35 EDT 2024. Contains 374318 sequences. (Running on oeis4.)