|
%I #19 Jan 17 2019 13:44:07
%S 0,2,3,5,17,47,177,8462,36615
%N Numbers n such that 8*10^n + 2*R_n - 1 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C Also numbers n such that (74*10^n-11)/9 is prime.
%C a(10) > 10^5. - _Robert Price_, Oct 13 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/8/82221.htm#prime">Prime numbers of the form 822...221</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101061(n-1) + 1 for n>1.
%t Do[ If[ PrimeQ[(74*10^n - 11)/9], Print[n]], {n, 0, 10000}]
%o (PARI) for(n=0, 50000, if(isprime((74*10^n-11)/9), print1(n", "))) \\ _Altug Alkan_, Oct 13 2015
%Y Cf. A002275, A101061.
%K more,nonn
%O 1,2
%A _Robert G. Wilson v_, Jan 19 2005
%E a(9) from Erik Branger May 01 2013 by _Ray Chandler_, Aug 16 2013
%E Inserted a(1)=0 by _Robert Price_, Oct 13 2015
| 