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 A256375 #30 Jul 08 2021 01:19:28 %S A256375 4,5,6,8,13,14,25,30,44,58,383,455,463,504,511,1358,2293,4096,4374, %T A256375 8664,13676,23993,84134,90155,115794,127858 %N A256375 Numbers k such that 3*R_(k+2) - 10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k. %C A256375 Also, numbers k such that (97*10^k - 1)/3 is prime. %C A256375 Terms from Kamada. %C A256375 a(27) > 250000. %H A256375 Makoto Kamada, <a href="https://stdkmd.net/nrr/abaaa.htm">Near-repdigit numbers of the form ABAA...AA</a>. %H A256375 Makoto Kamada, <a href="https://stdkmd.net/nrr/3/32333.htm#prime">Prime numbers of the form 3233...33</a>. %H A256375 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>. %e A256375 For k=4, 3*R_6 - 10^4 = 333333 - 10000 = 323333 which is prime. %t A256375 Select[Range[0, 250000], PrimeQ[(97*10^#-1)/3 ] &] %Y A256375 Cf. A002275. %K A256375 more,hard,nonn %O A256375 1,1 %A A256375 _Robert Price_, Apr 14 2015