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A056700
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Numbers n such that 2*10^n + R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n.
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2
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0, 2, 3, 12, 18, 23, 57, 128, 543, 584, 833, 2450, 2810, 2873, 3671, 6384, 10296, 16704, 53049, 56544, 74253
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OFFSET
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1,2
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COMMENTS
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Also numbers n such that (19*10^n-1)/9 is prime.
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LINKS
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Table of n, a(n) for n=1..21.
Makoto Kamada, Factorizations of 211...11
Index entries for primes involving repunits
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MATHEMATICA
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Do[ If[ PrimeQ[ 2*10^n + (10^n-1)/9], Print[n]], {n, 0, 10000}]
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CROSSREFS
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Cf. A068814.
Sequence in context: A073452 A112976 A100570 * A140989 A130089 A126292
Adjacent sequences: A056697 A056698 A056699 * A056701 A056702 A056703
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KEYWORD
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nonn
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AUTHOR
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Robert G. Wilson v, Aug 10 2000
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EXTENSIONS
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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
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STATUS
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approved
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