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A056700 Numbers k such that 2*10^k + R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k. 2
0, 2, 3, 12, 18, 23, 57, 128, 543, 584, 833, 2450, 2810, 2873, 3671, 6384, 10296, 16704, 53049, 56544, 74253 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
Also numbers k such that (19*10^k-1)/9 is prime.
LINKS
MATHEMATICA
Do[ If[ PrimeQ[ 2*10^n + (10^n-1)/9], Print[n]], {n, 0, 10000}]
CROSSREFS
Sequence in context: A073452 A112976 A100570 * A299547 A140989 A281086
KEYWORD
nonn,hard,more
AUTHOR
Robert G. Wilson v, Aug 10 2000
EXTENSIONS
6384 from Hugo Pfoertner, Oct 16 2004
10296 and 16704 from Ray Chandler, Sep 2010
0 added by Arkadiusz Wesolowski, Mar 10 2011
53049 and 56544 from Serge Batalov, Mar 2009 confirmed as next terms by Ray Chandler, Feb 03 2012
74253 from Ray Chandler, Feb 03 2012
STATUS
approved

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Last modified May 18 09:54 EDT 2024. Contains 372620 sequences. (Running on oeis4.)