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 A102953 #20 Jul 08 2021 01:35:19
%S A102953 1,4,7,12,30,94,178,196,564,1801,3520,3538,8233,35161,37405,42330,
%T A102953 70051,90792,124096,152670
%N A102953 Numbers k such that 2*10^k + 4*R_k - 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A102953 Also numbers k such that (22*10^k - 13)/9 is prime.
%C A102953 a(21) > 2*10^5. - _Robert Price_, Jun 11 2018
%H A102953 Makoto Kamada, <a href="https://stdkmd.net/nrr/2/24443.htm#prime">Prime numbers of the form 244...443</a>.
%H A102953 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F A102953 a(n) = A101958(n) + 1.
%t A102953 Do[ If[ PrimeQ[(22*10^n - 13)/9], Print[n]], {n, 0, 10000}]
%Y A102953 Cf. A002275, A101958.
%K A102953 more,nonn
%O A102953 1,2
%A A102953 _Robert G. Wilson v_, Dec 17 2004
%E A102953 a(14)-a(16) from Erik Branger May 01 2013 by _Ray Chandler_, Aug 16 2013
%E A102953 a(17)-a(18) from _Robert Price_, Mar 16 2015
%E A102953 a(19)-a(20) from _Robert Price_, Jun 11 2018