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 A295325 #13 May 05 2024 00:20:56 %S A295325 1,4,7,8,18,19,73,143,192,408,533,792,3179,7709,9554,35598,41587, %T A295325 52919,56021,61604,78672,81624 %N A295325 Numbers k such that 15*10^k + 1 is prime. %C A295325 For k > 1, numbers k such that the digits 15 followed by k-1 occurrences of the digit 0 followed by the digit 1 is prime (see Example section). %C A295325 a(23) > 10^5. %H A295325 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>. %H A295325 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 150w1</a>. %e A295325 4 is in this sequence because 15*10^4 + 1 = 150001 is prime. %e A295325 Initial terms and associated primes: %e A295325 a(1) = 1, 151; %e A295325 a(2) = 4, 150001; %e A295325 a(3) = 7, 150000001; %e A295325 a(4) = 8, 1500000001; %e A295325 a(5) = 18, 15000000000000000001; etc. %t A295325 Select[Range[0, 100000], PrimeQ[15*10^# + 1] &] %o A295325 (PARI) isok(k) = isprime(15*10^k + 1); \\ _Michel Marcus_, Nov 20 2017 %Y A295325 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. %K A295325 nonn,more,hard %O A295325 1,2 %A A295325 _Robert Price_, Nov 19 2017