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