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 A273907 #16 May 26 2024 15:23:24 %S A273907 2,3,4,9,10,13,15,24,26,38,39,42,433,489,495,513,597,829,2019,2738, %T A273907 3096,3691,5437,7537,8536,34125,40105,41790,52713,104811,173809,175860 %N A273907 Numbers k such that 2*10^k - 87 is prime. %C A273907 For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 9 followed by the digits 13 is prime (see Example section). %C A273907 a(33) > 2*10^5. %H A273907 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>. %H A273907 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 19w13</a>. %e A273907 3 is in this sequence because 2*10^3 - 87 = 1913 is prime. %e A273907 Initial terms and associated primes: %e A273907 a(1) = 2, 113; %e A273907 a(2) = 3, 1913; %e A273907 a(3) = 4, 19913; %e A273907 a(4) = 9, 1999999913; %e A273907 a(5) = 10, 19999999913, etc. %t A273907 Select[Range[0, 100000], PrimeQ[2*10^# - 87] &] %o A273907 (PARI) is(n)=ispseudoprime(2*10^n - 87) \\ _Charles R Greathouse IV_, Jun 08 2016 %Y A273907 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. %K A273907 nonn,more %O A273907 1,1 %A A273907 _Robert Price_, Jun 03 2016 %E A273907 a(30)-a(32) from _Robert Price_, Apr 13 2018