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 A305740 #14 Jun 26 2018 04:56:03 %S A305740 1,1,4,7,7,170716,170926,26373004,247201983,10562770680,118345066231, %T A305740 54717848613610 %N A305740 a(n) is the smallest k such that 10^m*k + 1 is prime for all m in 1..n. %C A305740 a(12) > 3*10^11. %e A305740 10^1*1 + 1 = 11 (prime), so a(1) = 1. %e A305740 10^2*1 + 1 = 101 (also prime), so a(2) = 1 as well. %e A305740 10^3*1 + 1 = 1001 = 7*143, so a(3) > 1; %e A305740 10^1*2 + 1 = 21 = 3*7, so a(3) > 2; %e A305740 10^2*3 + 1 = 301 = 7*43, so a(3) > 3; %e A305740 however, for m = 1..3, 10^m*4 + 1 yields 41, 401, and 4001, each of which is prime, so a(3) = 4. %Y A305740 Cf. A000040 (primes), A124417, A124516. %K A305740 nonn,hard,more %O A305740 1,3 %A A305740 _Jon E. Schoenfield_, Jun 23 2018 %E A305740 a(12) from _Giovanni Resta_, Jun 25 2018