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A256933
Numbers k such that R_(k+2) + 7*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
1, 2, 11, 62, 157, 170, 251, 500, 2275, 7525, 12230, 13658, 54727, 183227, 212779
OFFSET
1,2
COMMENTS
Also, numbers k such that (163*10^k - 1)/9 is prime.
Terms from Kamada.
a(16) > 250000.
EXAMPLE
For k=2, R_4 + 7*10^2 = 1111 + 700 = 1811 which is prime.
MATHEMATICA
Select[Range[0, 250000], PrimeQ[(163*10^#-1)/9 ] &]
PROG
(Magma) [n: n in [0..300] | IsPrime((163*10^n-1) div 9)]; // Vincenzo Librandi, Apr 14 2015
(PARI) for(n=0, 300, if(isprime((163*10^n-1)/9), print1(n, ", "))) \\ Derek Orr, Apr 14 2015
CROSSREFS
Cf. A002275.
Sequence in context: A126034 A308729 A034726 * A162274 A183160 A020078
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 13 2015
STATUS
approved