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 A276545 #19 May 25 2024 15:38:53 %S A276545 2,5,7,8,11,13,25,26,61,82,131,289,377,547,845,929,1786,5887,6562, %T A276545 10546,28033,33493,150515,205183 %N A276545 Numbers k such that (43*10^k - 421)/9 is prime. %C A276545 For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 7 followed by the digits 31 is prime (see Example section). %C A276545 a(25) > 3*10^5. %H A276545 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>. %H A276545 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 47w31</a>. %e A276545 4 is in this sequence because (43*10^4 - 421)/9 = 477731 is prime. %e A276545 Initial terms and associated primes: %e A276545 a(1) = 2, 431; %e A276545 a(2) = 5, 477731; %e A276545 a(3) = 7, 47777731; %e A276545 a(4) = 8, 477777731; %e A276545 a(5) = 11, 477777777731; etc. %t A276545 Select[Range[1, 100000], PrimeQ[(43*10^# - 421)/9] &] %Y A276545 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. %K A276545 nonn,more,hard %O A276545 1,1 %A A276545 _Robert Price_, Apr 09 2017 %E A276545 a(23) from _Robert Price_, Jan 21 2019 %E A276545 a(24) from _Robert Price_, Oct 25 2023