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 A136024 #14 Aug 31 2025 12:55:45 %S A136024 3,31,331,3331,33331,333331,3333331,33333331,333333313,3333333323, %T A136024 33333333329,333333333323,3333333333301,33333333333323, %U A136024 333333333333307,3333333333333301,33333333333333323 %N A136024 Largest prime factor of odd composites less than 10^n. %C A136024 Last instance of the largest prime factor of odd N <= 10^n-1 associated with A136021. %C A136024 This sequence is not the same as A051200. E.g., A051200(9)=333333331 is not prime and is different from a(9)=333333313. However, if A051200(n) is prime, then a(n)=A051200(n). %F A136024 a(n) = precprime(10^n/3) = A007917((10^n-1)/3). - _Max Alekseyev_, Sep 29 2015 %e A136024 a(1)=31 because it is the largest factor of odd N <= 10^2-1. The value of odd N where this factor first occurs is 3*31 = 93. %t A136024 a[n_]:=NextPrime[10^n/3,-1]; Array[a,17] (* _Stefano Spezia_, Aug 31 2025 *) %o A136024 (PARI) a(n)=precprime(10^n\3) %Y A136024 Cf. A051200, A136021. %K A136024 easy,nonn,changed %O A136024 1,1 %A A136024 _Enoch Haga_, Dec 12 2007 %E A136024 Clarified and extended by _Charles R Greathouse IV_, Oct 11 2009