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 A102931 #28 Feb 01 2023 23:06:59
%S A102931 1,3,6,51,57,63,150,393,420,547,42024,43063,101613,107385
%N A102931 Numbers k such that 10^k + 2*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A102931 Also numbers k such that (11*10^k + 61)/9 is prime.
%C A102931 a(13) > 10^5. - _Robert Price_, Jan 17 2015
%C A102931 a(15) > 2*10^5. - _Tyler Busby_, Feb 01 2023
%H A102931 Makoto Kamada, <a href="https://stdkmd.net/nrr/1/12229.htm#prime">Prime numbers of the form 122...229</a>.
%H A102931 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F A102931 a(n) = A102011(n) + 1.
%t A102931 Do[ If[ PrimeQ[(11*10^n + 61)/9], Print[n]], {n, 0, 10000}]
%Y A102931 Cf. A002275, A102011.
%K A102931 more,nonn
%O A102931 1,2
%A A102931 _Robert G. Wilson v_, Dec 16 2004
%E A102931 a(11)-a(12) from Erik Branger May 01 2013 by _Ray Chandler_, Aug 16 2013
%E A102931 a(13)-a(14) from _Tyler Busby_, Jan 31 2023