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 A239205 #17 Mar 03 2020 09:00:43 %S A239205 28,140,190,248,420,477,496,672,840,910,1428,1488,1547,1638,1848,1892, %T A239205 2295,2398,2480,2660,3417,3472,3515,3640,3720,4064,4095,4590,4641, %U A239205 4655,4845,5456,5655,5995,6200,6678,6888,6944,7030,7440,8008,8128,8190,9214,9282 %N A239205 Numbers k such that sigma(k) divides the sum of the numbers x coprime to k, with x <= k. %C A239205 Numbers k such that sigma(k) | k*phi(k)/2. %H A239205 Amiram Eldar, <a href="/A239205/b239205.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..500 from Paolo P. Lava) %e A239205 28 is a term since 28*phi(28)/2 = 168, sigma(28) = 56 and 168 / 56 = 3. %p A239205 with(numtheory); P:=proc(q) local a,n; %p A239205 for n from 1 to q do a:=n*phi(n)/2; if type(a/sigma(n),integer) then print(n); %p A239205 fi; od; end: P(10^6); %t A239205 Select[Range[10^4], Divisible[# * EulerPhi[#]/2, DivisorSigma[1, #]] &] (* _Amiram Eldar_, Mar 03 2020 *) %Y A239205 Cf. A000010, A000203, A238232, A239208. %K A239205 nonn,easy %O A239205 1,1 %A A239205 _Paolo P. Lava_, Mar 12 2014