This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A056704 #25 Jan 17 2019 13:44:05
%S A056704 0,1,2,5,10,11,13,34,47,52,77,88,554,580,1310,1505,8537,15892,24022,
%T A056704 27041,37922,40033,134122,165358,183760
%N A056704 Numbers k such that 3*10^k + 1*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056704 Also numbers k such that (28*10^k - 1)/9 is prime.
%C A056704 Although perhaps a degenerate case, A002275 defines R(0)=0. Thus zero belongs in this sequence since 3*10^0 + 0 = 3*1 + 0 = 3 is prime. - _Robert Price_, Oct 28 2014
%C A056704 a(26) > 2*10^5. - _Robert Price_, Dec 19 2014
%H A056704 Makoto Kamada, <a href="https://stdkmd.net/nrr/3/31111.htm#prime">Prime numbers of the form 311...11</a>.
%H A056704 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t A056704 Do[ If[ PrimeQ[ 3*10^n + (10^n-1)/9], Print[n]], {n, 0, 10000}]
%Y A056704 Cf. A002275, A068813.
%K A056704 hard,nonn
%O A056704 1,3
%A A056704 _Robert G. Wilson v_, Aug 10 2000
%E A056704 Added zero by _Robert Price_, Oct 28 2014
%E A056704 a(18)-a(25) from Kamada data by _Robert Price_, Dec 19 2014