

A256711


Numbers k such that R_k  10 is prime, where R_k = 11...1 is the repunit (A002275) of length k.


0



3, 8, 12, 17, 86, 146, 1428, 1949, 4809, 16922, 33102, 125792, 211610
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OFFSET

1,1


COMMENTS

Also, numbers k such that (10^k  91)/9 is prime.
Terms from Kamada data.
a(14) > 2.5*10^5.


LINKS



EXAMPLE

For k=3, R_3  10 = 111  10 = 101 which is prime.


MATHEMATICA

Select[Range[0, 250000], PrimeQ[(10^#  91)/9] &]


PROG

(Magma) [n: n in [0..1400]  IsPrime((10^n91) div 9)]; // Vincenzo Librandi, Apr 09 2015


CROSSREFS



KEYWORD

more,hard,nonn


AUTHOR



STATUS

approved



