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%I #24 Jul 08 2021 00:48:10
%S 0,5,12,15,84,144,150,1235,1727,1812,8687,12390,28608,42959,51111,
%T 96798,99143
%N Numbers k such that 5*10^k + R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also numbers k such that (46*10^k - 1)/9 is prime.
%C a(18) > 10^5. - _Robert Price_, Dec 08 2014
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/5/51111.htm#prime">Prime numbers of the form 511...11</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t Do[ If[ PrimeQ[ 5*10^n + (10^n-1)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A068816.
%K hard,nonn
%O 1,2
%A _Robert G. Wilson v_, Aug 11 2000
%E a(11) from _Hugo Pfoertner_, Oct 19 2004
%E a(12)-a(17) from _Robert Price_, Dec 08 2014
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