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A256426
Numbers k such that 7*R_k + 20 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
2, 3, 5, 17, 44, 47, 78, 504, 533, 768, 915, 1155, 1313, 7128, 8088, 15717, 33074, 43568, 58689, 82517, 85647, 154211
OFFSET
1,1
COMMENTS
Also, numbers k such that (7*10^k + 173)/9 is prime.
Terms from Kamada.
a(23) > 200000. - Tyler Busby, Apr 20 2024
EXAMPLE
For k=3, 7*R_11 + 20 = 777 + 20 = 797 which is prime.
MATHEMATICA
Select[Range[0, 30000], PrimeQ[(7*10^# + 173)/9] &]
PROG
(Magma) [n: n in [0..400] | IsPrime((7*10^n + 173) div 9)]; // Vincenzo Librandi, Apr 13 2015
CROSSREFS
Cf. A002275.
Sequence in context: A339855 A076706 A243441 * A019350 A235630 A235636
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 12 2015
EXTENSIONS
a(18)-a(22) from Kamada data by Tyler Busby, Apr 20 2024
STATUS
approved