

A259134


Numbers k such that 9*R_k + 7*10^k  2 is prime, where R_k = 11...11 is the repunit (A002275) of length k.


0



0, 2, 4, 28, 156, 322, 352, 1212, 1284, 7984, 15192, 84772, 119930, 148860
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OFFSET

1,2


COMMENTS

Also, numbers k such that 8*10^k  3 is prime.
Terms from Kamada data.
Note that Kamada does not recognize k=0 as 5 is a degenerate case of form ABB..BBA.
a(15) > 2*10^5.


LINKS



EXAMPLE

For k=2, 9*R_2 + 7*10^k  2 = 99 + 700  2 = 797 which is prime.


MATHEMATICA

Select[Range[0, 200000], PrimeQ[8*10^#3] &]


CROSSREFS



KEYWORD

nonn,more,hard


AUTHOR



STATUS

approved



