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 A366921 #11 Oct 29 2023 09:47:10 %S A366921 11,37,11,41,7,239,11,37,11,21649,7,53,11,31,11,2071723,7, %T A366921 1111111111111111111,11,37,11,11111111111111111111111,7,41,11,37,11, %U A366921 3191,7,2791,11,37,11,41,7,2028119,11,37,11,83,7,173,11,31,11,35121409,7,239,11,37,11,107 %N A366921 a(n) is the least prime factor > 3 of 10^n - 1. %e A366921 a(2) = 11 because 99 = 3^2 * 11 = 3^A366922(2) * A003020(2). %e A366921 a(9) = 37 because 10^9 - 1 = 3^4 * 37 * 333667 = 3^A366922(9) * 37 * A003020(9). %o A366921 (PARI) a366921(n) = factor(10^n-1)[2,1] %Y A366921 Cf. A002283, A003020, A102347, A366922. %K A366921 nonn %O A366921 2,1 %A A366921 _Hugo Pfoertner_, Oct 28 2023