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 A105861 #9 Oct 19 2019 04:11:29
%S A105861 1,3,10,23,76,102,442,695,1792,2828,11254,13334,53236,65418,155110,
%T A105861 347319,1114096,1259328,4980718,6223148,15033700,27548678,96468970,
%U A105861 108761942,352992576,529504212,1381165192,2314603370,7784628196
%N A105861 a(n) = (n/2) * Sum_{k=0..n} binomial(n,k)/gcd(n,k).
%C A105861 If instead the limits of the summation run from 1 to n-1, then the sum is A105861(n)-1.
%F A105861 a(n) = (n/2) * Sum_{k=0..n} binomial(n, k) / gcd(n, k).
%t A105861 f[n_] := n*Sum[ Binomial[n, k] / GCD[n, k], {k, 0, n}]/2; Table[ f[n], {n, 30}]
%o A105861 (PARI) a(n) = sum(k=0, n, binomial(n, k)/gcd(n, k))*n/2; \\ _Michel Marcus_, Oct 19 2019
%Y A105861 Cf. A105862, A105863.
%K A105861 nonn
%O A105861 1,2
%A A105861 _Robert G. Wilson v_, Apr 23 2005