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A102937 Numbers n such that 10^n + 5*R_n + 2 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 3

%I

%S 0,1,2,5,7,19,20,28,58,250,397,7591,32716,70978,83587

%N Numbers n such that 10^n + 5*R_n + 2 is prime, where R_n = 11...1 is the repunit (A002275) of length n.

%C Also numbers n such that (14*10^n+13)/9 is prime.

%C a(16) > 10^5. - _Robert Price_, Jan 17 2015

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/1/15557.htm#prime">Prime numbers of the form 155...557</a>.

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

%F a(n) = A102020(n-1) + 1.

%t Do[ If[ PrimeQ[(14*10^n + 13)/9], Print[n]], {n, 0, 10000}]

%Y Cf. A002275, A102020.

%K more,nonn

%O 1,3

%A _Robert G. Wilson v_, Dec 16 2004

%E 0 added by _Arkadiusz Wesolowski_, Mar 10 2011

%E a(13) from Erik Branger May 01 2013 by _Ray Chandler_, Aug 16 2013

%E a(14)-a(15) from _Robert Price_, Jan 17 2015

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Last modified June 16 04:54 EDT 2019. Contains 324145 sequences. (Running on oeis4.)