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A056726 Numbers k such that 9*10^k + R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k. 1
2, 5, 20, 41, 47, 92, 161, 401, 455, 8570, 18592, 73714, 78635, 100780 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Also numbers k such that (82*10^k - 1)/9 is prime.

LINKS

Table of n, a(n) for n=1..14.

Makoto Kamada, Prime numbers of the form 911...11.

Index entries for primes involving repunits

MATHEMATICA

Do[ If[ PrimeQ[ 9*10^n + (10^n-1)/9], Print[n]], {n, 0, 5000}]

CROSSREFS

Cf. A002275, A093634.

Sequence in context: A247131 A192164 A244087 * A176042 A185593 A136899

Adjacent sequences:  A056723 A056724 A056725 * A056727 A056728 A056729

KEYWORD

hard,nonn

AUTHOR

Robert G. Wilson v, Aug 11 2000

EXTENSIONS

8570 from Hugo Pfoertner, Oct 22 2004

a(11)-a(14) from Robert Price, Dec 05 2014

STATUS

approved

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Last modified July 4 08:01 EDT 2022. Contains 355070 sequences. (Running on oeis4.)