

A259132


Numbers k such that 5*R_k + 7*10^k + 2 is prime, where R_k = 11...11 is the repunit (A002275) of length k.


0



2, 4, 10, 20, 22, 58, 74, 82, 208, 350, 422, 3812, 3982, 20924, 23786, 38852, 56042, 68504, 74434
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OFFSET

1,1


COMMENTS

Also, numbers k such that (68*10^k + 13)/9 is prime.
Terms from Kamada data.
a(20) > 10^5.


LINKS

Table of n, a(n) for n=1..19.
Makoto Kamada, Nearrepdigit numbers of the form ABB...BBA.
Makoto Kamada, Prime numbers of the form 755...557.
Index entries for primes involving repunits.


EXAMPLE

For k=2, 5*R_2 + 7*10^k + 2 = 55 + 700 + 2 = 757 which is prime.


MATHEMATICA

Select[Range[0, 100000], PrimeQ[(68*10^#+13)/9] &]


CROSSREFS

Cf. A002275.
Sequence in context: A011963 A083844 A026554 * A099413 A328761 A271624
Adjacent sequences: A259129 A259130 A259131 * A259133 A259134 A259135


KEYWORD

nonn,more,hard


AUTHOR

Robert Price, Jun 18 2015


STATUS

approved



