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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 Table of n, a(n) for n=1..21. Makoto Kamada, Prime numbers of the form 211...11. Index entries for primes involving repunits 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,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.)