

A256772


Numbers k such that 3*R_k  20 is prime, where R_k = 11...1 is the repunit (A002275) of length k.


0



2, 3, 4, 7, 9, 37, 39, 67, 73, 183, 216, 1161, 2622, 2775, 6240, 22078, 30384, 189385
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OFFSET

1,1


COMMENTS

Also, numbers k such that (10^k  61)/3 is prime.
Terms from Kamada data.
a(19) > 2.5*10^5.


LINKS



EXAMPLE

For k=3, 3*R_3  20 = 333  20 = 313 which is prime.


MATHEMATICA

Select[Range[0, 250000], PrimeQ[(10^#  61)/3] &]


PROG

(Magma) [n: n in [2..300]  IsPrime((10^n61) div 3)]; // Vincenzo Librandi, Apr 10 2015


CROSSREFS



KEYWORD

nonn,more,hard


AUTHOR



STATUS

approved



