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 A276118 #17 May 26 2024 14:57:53 %S A276118 0,1,2,4,13,19,39,62,76,79,109,184,222,265,370,626,670,679,763,1950, %T A276118 2174,3379,7369,9087,34990,47535,97970 %N A276118 Numbers k such that 42 * 10^k + 1 is prime. %C A276118 For k > 0, numbers k such that the digits 42 followed by k - 1 occurrences of the digit 0 followed by the digit 1 is prime (see the Example section). %C A276118 a(28) > 10^5. %H A276118 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>. %H A276118 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 420w1</a>. %e A276118 4 is in this sequence because 42*10^4+1 = 420001 is prime. %e A276118 Initial terms and associated primes: %e A276118 a(1) = 0, 43; %e A276118 a(2) = 1, 421; %e A276118 a(3) = 2, 4201; %e A276118 a(4) = 4, 420001; %e A276118 a(5) = 13, 420000000000001, etc. %t A276118 Select[Range[0, 100000], PrimeQ[42 * 10^# + 1] &] %o A276118 (PARI) is(n)=ispseudoprime(42*10^n+1) \\ _Charles R Greathouse IV_, Jun 13 2017 %Y A276118 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. %K A276118 nonn,more %O A276118 1,3 %A A276118 _Robert Price_, Aug 20 2016