

A099416


Numbers k such that 5*R_k  2 is prime, where R_k = 11...1 is the repunit (A002275) of length k.


2



1, 2, 8, 26, 66, 74, 233, 473, 540, 2774, 64715
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OFFSET

1,2


COMMENTS

Also numbers k such that (5*10^k23)/9 is prime.
a(12) > 10^5.  Robert Price, Nov 13 2014


LINKS

Table of n, a(n) for n=1..11.
Makoto Kamada, Prime numbers of the form 55...553.
Index entries for primes involving repunits


FORMULA

a(n) = A056685(n) + 1.


MATHEMATICA

Do[ If[ PrimeQ[ 5(10^k  1)/9  2], Print[k]], {k, 1, 5000}]


CROSSREFS

Cf. A002275, A056685.
Sequence in context: A216929 A100504 A327600 * A211885 A101696 A155861
Adjacent sequences: A099413 A099414 A099415 * A099417 A099418 A099419


KEYWORD

nonn


AUTHOR

Robert G. Wilson v, Oct 14 2004


EXTENSIONS

a(11) from Robert Price, Nov 13 2014
0 removed by Georg Fischer, Jan 03 2021


STATUS

approved



