

A097685


Numbers n such that R_n + 8 is prime, where R_n = 11...1 is the repunit (A002275) of length n.


3



2, 5, 6, 8, 17, 50, 684, 720, 1452, 1679, 3146, 72821
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OFFSET

1,1


COMMENTS

Also numbers n such that (10^n+71)/9 is prime.
Values indicate primes of the form "(n1) ones followed by a nine". Related to the base 10 repunit primes.
a(13) > 10^5.  Robert Price, Nov 01 2014


LINKS

Table of n, a(n) for n=1..12.
Makoto Kamada, Prime numbers of the form 11...119.
Index entries for primes involving repunits


FORMULA

a(n) = A056659(n) + 1.


MATHEMATICA

Do[ If[ PrimeQ[(10^n  1)/9 + 8], Print[n]], {n, 0, 5000}] (* Robert G. Wilson v, Oct 14 2004 *)


CROSSREFS

Cf. A004023, A056659, A097683, A097684.
Sequence in context: A191140 A271430 A326854 * A136369 A305511 A269472
Adjacent sequences: A097682 A097683 A097684 * A097686 A097687 A097688


KEYWORD

nonn,more,hard


AUTHOR

Carl R. White, Aug 19 2004


EXTENSIONS

a(12) from Robert Price, Nov 01 2014


STATUS

approved



