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 A099418 #26 May 04 2024 02:52:49
%S A099418 2,8,12,18,26,32,138,188,222,338,1002,2744,6530,38690,39464,335480,
%T A099418 343734
%N A099418 Numbers k such that 5*R_k + 4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A099418 Also numbers k such that (5*10^k + 31)/9 is prime.
%H A099418 Makoto Kamada, <a href="https://stdkmd.net/nrr/5/55559.htm#prime">Prime numbers of the form 55...559</a>.
%H A099418 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A099418 a(n) = A056687(n) + 1.
%t A099418 Do[ If[ PrimeQ[ 5(10^n - 1)/9 + 4], Print[n]], {n, 0, 5000}]
%Y A099418 Cf. A002275, A093403, A056687.
%K A099418 nonn,more
%O A099418 1,1
%A A099418 _Robert G. Wilson v_, Oct 14 2004
%E A099418 a(13)-a(15) from Kamada link by _Ray Chandler_, Feb 27 2012
%E A099418 a(16)-a(17) from Kamada data by _Tyler Busby_, May 03 2024