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 A272193 #16 May 02 2024 22:55:07 %S A272193 1,2,5,7,13,16,17,25,44,52,197,233,241,389,838,856,2252,2945,5207, %T A272193 8020,10708,14663,16885,20366,20450,24121,24437,29348,134939 %N A272193 Numbers k such that (73*10^k + 143)/9 is prime. %C A272193 For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 1 followed by the digits 27 is prime (see Example section). %C A272193 a(29) > 2*10^5. %H A272193 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>. %H A272193 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 81w27</a>. %e A272193 5 is in this sequence because (73*10^5 + 143)/9 = 811127 is prime. %e A272193 Initial terms and associated primes: %e A272193 a(1) = 1, 97; %e A272193 a(2) = 2, 827;; %e A272193 a(3) = 5, 811127; %e A272193 a(4) = 7, 81111127; %e A272193 a(5) = 13, 81111111111127, etc. %t A272193 Select[Range[0, 100000], PrimeQ[(73*10^# + 143)/9] &] %o A272193 (PARI) lista(nn) = for(n=1, nn, if(ispseudoprime((73*10^n + 143)/9), print1(n, ", "))); \\ _Altug Alkan_, Apr 22 2016 %Y A272193 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. %K A272193 nonn,more %O A272193 1,2 %A A272193 _Robert Price_, Apr 22 2016 %E A272193 a(29) from _Robert Price_, Jul 31 2019