login
A099422
Numbers k such that 8*R_k - 5 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
3
1, 2, 3, 5, 8, 9, 15, 51, 71, 77, 224, 296, 315, 2090, 2906, 3395, 3882, 5114, 6056, 7254, 7995, 18173, 18971, 35006, 69674, 175428, 253313
OFFSET
1,2
COMMENTS
Also numbers k >= 1 such that (8*10^k - 53)/9 is prime.
a(26) > 10^5. - Robert Price, Oct 31 2014
FORMULA
a(n) = A056694(n) + 1.
MATHEMATICA
Do[ If[ PrimeQ[ 8(10^n - 1)/9 - 5], Print[n]], {n, 1, 5000}]
PROG
(Magma) [n: n in [1..500] | IsPrime((8*10^n-53) div 9)]; // Vincenzo Librandi, Nov 01 2014
CROSSREFS
KEYWORD
nonn
AUTHOR
Robert G. Wilson v, Oct 14 2004
EXTENSIONS
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(22)-a(25) from Robert Price, Oct 31 2014
a(1)=0 removed by Georg Fischer, Jan 03 2021
a(26)-a(27) from Kamada data by Tyler Busby, Apr 16 2024
STATUS
approved