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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

%I

%S 0,2,3,12,18,23,57,128,543,584,833,2450,2810,2873,3671,6384,10296,

%T 16704,53049,56544,74253

%N Numbers n such that 2*10^n + R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n.

%C Also numbers n such that (19*10^n-1)/9 is prime.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/2/21111.htm#prime">Prime numbers of the form 211...11</a>.

%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>

%t Do[ If[ PrimeQ[ 2*10^n + (10^n-1)/9], Print[n]], {n, 0, 10000}]

%Y Cf. A002275, A068814.

%K nonn

%O 1,2

%A _Robert G. Wilson v_, Aug 10 2000

%E 6384 from _Hugo Pfoertner_, Oct 16 2004

%E 10296 and 16704 from Ray Chandler, Sep 2010

%E 0 added by _Arkadiusz Wesolowski_, Mar 10 2011

%E 53049 and 56544 from Serge Batalov, Mar 2009 confirmed as next terms by _Ray Chandler_, Feb 03 2012

%E 74253 from _Ray Chandler_, Feb 03 2012

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Last modified April 25 21:06 EDT 2019. Contains 322461 sequences. (Running on oeis4.)