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A056659 Numbers n such that 10*R_n + 9 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 2
1, 4, 5, 7, 16, 49, 683, 719, 1451, 1678, 3145, 72820 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also numbers n such that (10^(n+1)+71)/9 is prime.

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) = A097685(n) - 1. - Robert Price, Nov 01 2014

MATHEMATICA

Do[ If[ PrimeQ[10*(10^n - 1)/9 + 9], Print[n]], {n, 5000}]

CROSSREFS

Cf. A002275, A097685.

Sequence in context: A216537 A085195 A107289 * A292321 A049896 A029520

Adjacent sequences:  A056656 A056657 A056658 * A056660 A056661 A056662

KEYWORD

hard,nonn

AUTHOR

Robert G. Wilson v, Aug 09 2000

EXTENSIONS

a(12) derived from A097685 by Robert Price, Nov 01 2014

STATUS

approved

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Last modified October 1 00:54 EDT 2020. Contains 337440 sequences. (Running on oeis4.)