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 A269503 #57 Jan 20 2024 22:21:44 %S A269503 101,13,137,9091,9901,909091,5882353,52579,333667,9091,99990001, %T A269503 1058313049,265371653,909091,2906161,21993833369,999999000001, %U A269503 909090909090909091,1111111111111111111,909091,1056689261,549797184491917,11111111111111111111111 %N A269503 Largest prime factor of A138148(n). %C A269503 Largest prime factor of (10^(n+1)+1)*(10^n-1)/9. %H A269503 Max Alekseyev, <a href="/A269503/b269503.txt">Table of n, a(n) for n = 1..344</a> %F A269503 a(n) = max(A003020(n),A003021(n+1)) for n >= 2. - _Robert Israel_, May 11 2016 %e A269503 a(4) = 9091 because largest prime factor of 111101111 is 9091. %p A269503 seq(max(max(numtheory:-factorset((10^n-1)/9)), %p A269503 max(numtheory:-factorset(10^(n+1)+1))), n=1..30); # _Robert Israel_, May 11 2016 %o A269503 (PARI) a006530(n) = if(n>1, vecmax(factor(n)[, 1]), 1); %o A269503 for(n=1, 25, print1(a006530((10^(2*n+1)-1)/9-10^n), ", ")); %Y A269503 Cf. A003020, A003021, A006530, A138148. %K A269503 nonn %O A269503 1,1 %A A269503 _Altug Alkan_, May 11 2016