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 A056700 Numbers n such that 2*10^n + R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n. 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 n such that (19*10^n-1)/9 is prime. LINKS Makoto Kamada, Prime numbers of the form 211...11. MATHEMATICA Do[ If[ PrimeQ[ 2*10^n + (10^n-1)/9], Print[n]], {n, 0, 10000}] CROSSREFS Cf. A002275, A068814. Sequence in context: A073452 A112976 A100570 * A299547 A140989 A281086 Adjacent sequences:  A056697 A056698 A056699 * A056701 A056702 A056703 KEYWORD nonn 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 March 25 12:11 EDT 2019. Contains 321470 sequences. (Running on oeis4.)