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 A382213 #46 Apr 24 2025 13:20:47 %S A382213 1,3,11,5,137,7,33,761,7129,671,83711,6617,1145993,1171733,1195757, %T A382213 143327,42142223,751279,275295799,55835135,18858053,830139,444316699, %U A382213 1347822955,34052522467,34395742267,312536252003,10876020307,214582477009,300151059037,290774257297357 %N A382213 Largest squarefree number dividing the numerator of harmonic number H(n). %F A382213 a(n) = A007947(A001008(n)). %e A382213 A001008(4) = 25, and the largest squarefree divisor of 25 is 5, so a(4) = 5. %p A382213 a:= n-> mul(i[1], i=ifactors(numer(harmonic(n)))[2]): %p A382213 seq(a(n), n=1..35); # _Alois P. Heinz_, Apr 23 2025 %o A382213 (PARI) a(n) = factorback(factor(numerator(sum(k=1, n, 1/k)))[, 1]); \\ _Michel Marcus_, Apr 24 2025 %Y A382213 Cf. A382329, A001008, A002805, A007913. %Y A382213 Cf. A333196. %K A382213 nonn %O A382213 1,2 %A A382213 _Ali Sada_, Mar 22 2025