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 A102964 #27 Jan 17 2019 13:44:07
%S A102964 1,2,5,10,29,39,114,484,865,4180,5385,13122,13832,22676,25020,30057,
%T A102964 35910,37935,42295,50194,110076,184124,191152
%N A102964 Numbers n such that 3*10^n-7 is prime.
%C A102964 Also numbers n such that 2*10^n + 9*R_n - 6 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C A102964 a(21) > 10^5. - _Robert Price_, Jan 25 2015
%C A102964 a(24) > 2*10^5. - _Robert Price_, Aug 09 2015
%H A102964 Makoto Kamada, <a href="https://stdkmd.net/nrr/2/29993.htm#prime">Prime numbers of the form 299...993</a>.
%H A102964 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F A102964 a(n) = A101973(n) + 1.
%e A102964 For n=1,2,5 the corresponding primes are 23,293,29993.
%t A102964 Do[ If[ PrimeQ[3*10^n-7], Print[n]], {n, 0, 10000}]
%o A102964 (PARI) is_ok(n)=isprime(3*10^n-7); /* _Anders Hellström_, Aug 09 2015 */
%Y A102964 Cf. A002275, A101973.
%K A102964 more,nonn
%O A102964 1,2
%A A102964 _Robert G. Wilson v_, Dec 17 2004
%E A102964 Addition of a(12)-a(15) from Kamada data by _Robert Price_, Dec 12 2010
%E A102964 a(16)-a(20) from Kamada data by _Robert Price_, Jan 25 2015
%E A102964 a(21)-a(23) from _Robert Price_, Aug 09 2015