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URL: https://oeis.org/A256830

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A256830
Numbers k such that 7*R_k - 20 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
3, 4, 24, 106, 124, 162, 243, 258, 1344, 1386, 2494, 4200, 5859, 8844, 13122, 19908, 86844, 106066, 180732
OFFSET
1,1
COMMENTS
Also, numbers k such that (7*10^k - 187)/9 is prime.
Terms from Kamada.
a(17) > 30000.
a(20) > 2*10^5. - Tyler Busby, Feb 01 2023
EXAMPLE
For k=4, 7*R_4 - 20 = 7777 - 20 = 7757 which is prime.
MATHEMATICA
Select[Range[0, 250000], PrimeQ[(7*10^# - 187)/9] &]
PROG
(Magma) [n: n in [2..400] | IsPrime((7*10^n-187) div 9)]; // Vincenzo Librandi, Apr 11 2015
CROSSREFS
Cf. A002275.
Sequence in context: A124632 A048091 A189738 * A065900 A065809 A093600
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 10 2015
EXTENSIONS
a(17)-a(19) from Tyler Busby, Feb 01 2023
STATUS
approved