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A257027
Numbers k such that 7*R_(k+2) - 6*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
0, 2, 3, 9, 11, 18, 74, 131, 144, 161, 224, 282, 390, 398, 614, 791, 1313, 1866, 9708, 10544, 13292, 13394, 29703, 30779, 72446
OFFSET
1,2
COMMENTS
Also, numbers k such that (646*10^k - 7)/9 is prime.
Terms from Kamada.
a(26) > 10^5. Robert Price, Jul 31 2016
EXAMPLE
For k=2, 7*R_4 - 6*10^2 = 7777 - 600 = 7177 which is prime.
a(1)=0 associated with 71, a(2)=2 associated with 7177, a(3)=3 associated with 71777, a(4)=9 associated with 71777777777, etc. - Robert Price, Jul 31 2016
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(646*10^#-7)/9 ] &]
PROG
(PARI) for(n=0, 200, if(isprime((646*10^n-7)/9), print1(n, ", "))) \\ Derek Orr, Apr 14 2015
(Magma) [n: n in [0..300] | IsPrime((646*10^n-7) div 9)]; // Vincenzo Librandi, Apr 15 2015
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 14 2015
EXTENSIONS
a(25) from Robert Price, Jul 31 2016
STATUS
approved