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 A365928 #59 Oct 22 2023 06:45:10 %S A365928 101,3,7,7,3,317,40637,3,7,7,3,1487,101,3,7,7,3,39855301,641,3,7,7,3, %T A365928 162340676822011484150719,101,3,7,7,3,121068683,47,3,7,7,3,107,71,3,7, %U A365928 7,3,67,695841737,3,7,7,3,47,101,3,7,7,3,8933,677,3,7,7,3,10305833206337 %N A365928 Smallest prime factor of f(n) = 10^(2*n) + (10^n - 1)/9. %F A365928 a(3k + 2) = 3, a(6k + 3) = 7, a(6k + 4) = 7. %e A365928 a(1) = 101, because f(1) = 101 is prime. %e A365928 a(2) = 3, because the smallest prime factor of f(2) = 10011 = 3 * 337 is 3. %t A365928 a[n_]:=Min[First/@FactorInteger[10^(2*n)+(10^n-1)/9]]; Array[a,59] (* _Stefano Spezia_, Sep 24 2023 *) %o A365928 (PARI) a(n)=my(x=10^(2*n)+(10^n-1)/9);m=factor(x);return(m[1,1]) %o A365928 (PARI) a(n) = my(x=10^(2*n)+(10^n-1)/9, k=10); if (ispseudoprime(x), return(x)); while (1, m=factor(x, k); if (m[1,1]<x, return(m[1,1])); k*=10); \\ _Michel Marcus_, Sep 24 2023 %Y A365928 Cf. A020639, A067063, A365966, A102380. %K A365928 nonn %O A365928 1,1 %A A365928 _Jean-Marc Rebert_, Sep 23 2023 %E A365928 a(60) from _Jinyuan Wang_, Sep 24 2023