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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
MATHEMATICA
Do[ If[ PrimeQ[ 9*10^n + (10^n-1)/9], Print[n]], {n, 0, 5000}]
CROSSREFS
Sequence in context: A192164 A244087 A361134 * A176042 A185593 A136899
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 June 17 12:36 EDT 2024. Contains 373445 sequences. (Running on oeis4.)