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 A272271 #18 May 26 2024 14:40:21 %S A272271 1,2,3,23,29,34,35,38,52,57,61,82,186,209,251,366,394,426,786,979, %T A272271 1382,2037,4557,8995,12774,19170,21828,23259,32003,41831,44999,56785, %U A272271 76483,97987,110468 %N A272271 Numbers k such that 7*10^k - 23 is prime. %C A272271 For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 9 followed by the digits 77 is prime (see Example section). %C A272271 a(36) > 3*10^5. %H A272271 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>. %H A272271 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 69w77</a>. %e A272271 3 is in this sequence because 7*10^3 - 23 = 6977 is prime. %e A272271 Initial terms and associated primes: %e A272271 a(1) = 1, 47; %e A272271 a(2) = 2, 677; %e A272271 a(3) = 3, 6977; %e A272271 a(4) = 23, 699999999999999999999977; %e A272271 a(5) = 29, 699999999999999999999999999977, etc. %t A272271 Select[Range[0, 100000], PrimeQ[7*10^# - 23] &] %o A272271 (PARI) is(n)=ispseudoprime(7*10^n - 23) \\ _Charles R Greathouse IV_, Jun 13 2017 %Y A272271 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. %K A272271 nonn,more %O A272271 1,2 %A A272271 _Robert Price_, Apr 24 2016 %E A272271 a(35) from _Robert Price_, Jul 27 2019