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 A291611 #17 May 26 2024 22:59:50 %S A291611 1,3,6,16,30,33,36,37,85,288,561,805,850,1057,1192,1312,2571,4579, %T A291611 5223,5940,10191,18756,24564,29595,43891,65905,89118,97963,112003, %U A291611 139945,174101,195221 %N A291611 Numbers k such that 5*10^k + 81 is prime. %C A291611 For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 0 followed by the digits 81 is prime (see Example section). %C A291611 a(33) > 2*10^5. %H A291611 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>. %H A291611 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 50w81</a>. %e A291611 3 is in this sequence because 5*10^3 + 81 = 5081 is prime. %e A291611 Initial terms and associated primes: %e A291611 a(1) = 1, 131; %e A291611 a(2) = 3, 5081; %e A291611 a(3) = 6, 5000081; %e A291611 a(4) = 16, 50000000000000081; %e A291611 a(5) = 30, 5000000000000000000000000000081; etc. %t A291611 Select[Range[0, 100000], PrimeQ[5*10^# + 81] &] %Y A291611 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. %K A291611 nonn,more,hard %O A291611 1,2 %A A291611 _Robert Price_, Aug 27 2017 %E A291611 a(29)-a(32) from _Robert Price_, Mar 04 2019 %E A291611 Terms reordered into ascending order by _Robert Price_, Apr 03 2022