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 A239208 #7 Mar 12 2014 11:21:56 %S A239208 18,135,891,4095,10560,13120,14144,21600,23199,74655,144495,192311, %T A239208 404415,4197375,4612608,5675775,6664680,9180800,10953215,11110400, %U A239208 14381055,18162144,18420480,18920000,20765024,25159680,32058351,41055200,55889920,65327104,65982464 %N A239208 Numbers n such that sigma(n) divides the sum of the numbers x not coprime to n, with x<=n. %C A239208 Numbers n such that sigma(n) | n/2(n+1-phi(n)). %H A239208 Giovanni Resta, <a href="/A239208/b239208.txt">Table of n, a(n) for n = 1..100</a> %e A239208 18/2*(19-phi(18)) = 117, sigma(18) = 39 and 117 / 39 = 3. %p A239208 with(numtheory); P:=proc(q) local a,n; %p A239208 for n from 1 to q do a:=n/2*(n+1-phi(n)); if type(a/sigma(n),integer) then print(n); %p A239208 fi; od; end: P(10^6); %Y A239208 Cf. A000010, A000230, A238232, A239205. %K A239208 nonn,easy %O A239208 1,1 %A A239208 _Paolo P. Lava_, Mar 12 2014 %E A239208 a(14)-a(31) from _Giovanni Resta_, Mar 12 2014