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 A272059 #26 May 02 2024 22:55:03 %S A272059 1,2,4,7,10,13,15,20,22,33,34,108,117,130,193,273,280,654,775,1144, %T A272059 4014,4015,7701,10356,11478,12427,15075,44107,102597,118635 %N A272059 Numbers k such that (17*10^k + 13)/3 is prime. %C A272059 For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 6 followed by the digits 71 is prime (see Example section). %C A272059 a(31) > 2*10^5. %H A272059 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>. %H A272059 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 56w71</a>. %e A272059 4 is in this sequence because (17*10^4 + 13)/3 = 56671 is prime. %e A272059 Initial terms and associated primes: %e A272059 a(1) = 1, 61; %e A272059 a(2) = 2, 571: %e A272059 a(3) = 4, 56671; %e A272059 a(4) = 7, 56666671; %e A272059 a(5) = 10, 56666666671, etc. %t A272059 Select[Range[0, 100000], PrimeQ[(17*10^# + 13)/3] &] %o A272059 (PARI) is(n)=ispseudoprime((17*10^n + 13)/3) \\ _Charles R Greathouse IV_, Jun 13 2017 %Y A272059 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. %K A272059 nonn,more %O A272059 1,2 %A A272059 _Robert Price_, May 19 2016 %E A272059 a(29)-a(30) from _Robert Price_, Jan 22 2019