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 A084217 #20 Apr 27 2025 03:20:46 %S A084217 1,1,1,1,2,1,2,1,1,1,2,1,2,2,2,1,2,1,2,1,1,2,2,1,3,2,1,2,2,1,2,1,2,2, %T A084217 4,1,2,2,2,1,2,1,2,2,2,2,2,1,3,1,2,3,2,1,2,2,2,2,2,1,2,2,1,1,4,2,2,2, %U A084217 2,3,2,1,2,2,3,2,4,1,2,2,1,2,2,1,4,2,2,3,2,1,4,2,2,2,5,1,2,3,2,1,2,2,2,2,2 %N A084217 Number of k's 1 <= k <= n dividing the numerator of 2n-th Bernoulli number. %H A084217 Antti Karttunen, <a href="/A084217/b084217.txt">Table of n, a(n) for n = 1..5530</a> %H A084217 Wikipedia, <a href="https://en.wikipedia.org/wiki/Bernoulli_number">Bernoulli numbers</a>. %t A084217 a[n_] := Count[Range[n], _?(Divisible[Numerator[BernoulliB[2*n]], #] &)]; Array[a, 100] (* _Amiram Eldar_, Apr 27 2025 *) %o A084217 (PARI) a(n)=sum(k=1,n,if(numerator(bernfrac(2*n))%k,0,1)) %Y A084217 Cf. A000367. %K A084217 nonn %O A084217 1,5 %A A084217 _Benoit Cloitre_, Jun 21 2003 %E A084217 Missing a(1)=1 inserted and data section extended to a(105) by _Antti Karttunen_, Jan 19 2025