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A056694
Numbers k such that 80*R_k + 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
2
0, 1, 2, 4, 7, 8, 14, 50, 70, 76, 223, 295, 314, 2089, 2905, 3394, 3881, 5113, 6055, 7253, 7994, 18172, 18970, 35005, 69673
OFFSET
1,3
COMMENTS
Also numbers k such that (8*10^(k+1)-53)/9 is prime.
a(26) > 10^5. - Robert Price, Oct 31 2014
FORMULA
a(n) = A099422(n) - 1. [adapted by Georg Fischer, Jan 04 2021]
MATHEMATICA
Do[ If[ PrimeQ[80*(10^n - 1)/9 + 3], Print[n]], {n, 0, 5000}]
PROG
(Magma) [n: n in [0..500] | IsPrime((8*10^(n+1)-53) div 9)]; // Vincenzo Librandi, Nov 01 2014
CROSSREFS
KEYWORD
hard,nonn
AUTHOR
Robert G. Wilson v, Aug 10 2000
EXTENSIONS
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(22)-a(25) from Robert Price, Oct 31 2014
STATUS
approved