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 A063880 #39 Aug 31 2024 04:33:09
%S A063880 108,540,756,1188,1404,1836,2052,2484,3132,3348,3780,3996,4428,4644,
%T A063880 5076,5724,5940,6372,6588,7020,7236,7668,7884,8316,8532,8964,9180,
%U A063880 9612,9828,10260,10476,10908,11124,11556,11772,12204,12420,12852,13716,14148
%N A063880 Numbers k such that sigma(k) = 2*usigma(k).
%C A063880 Numbers so far are all == 108 (mod 216). - _Ralf Stephan_, Jul 07 2003 [Confirmed up to 10^7 by _Robert G. Wilson v_.]
%C A063880 Also numbers whose unitary and nonunitary divisors have equal sum. - _Amiram Eldar_, Sep 30 2019
%C A063880 From _Amiram Eldar_, Aug 31 2024: (Start)
%C A063880 The primitive terms of this sequence (terms whose proper divisors are not in this sequence) are all powerful numbers (A001694).
%C A063880 All the other terms are of the form m*s, where m is primitive (powerful) and s is a squarefree number coprime to m.
%C A063880 The only primitive term below 10^18 is 108.
%C A063880 If there are no other primitive terms, then a(n) = 108 * A276378(n). (End)
%H A063880 Amiram Eldar, <a href="/A063880/b063880.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..1000 from Harry J. Smith)
%t A063880 usigma[n_] := Block[{d = Divisors[n]}, Plus @@ Select[d, GCD[ #, n/# ] == 1 &]]; Select[ Range[14363], DivisorSigma[1, # ] == 2 usigma[ # ] &] (* _Robert G. Wilson v_, Aug 28 2004 *)
%o A063880 (PARI) u(n) = sumdiv(n,d, if(gcd(d,n/d)==1,d));
%o A063880 j=[]; for(n=1,30000, if(sigma(n) == 2*u(n),j=concat(j,n))); j
%Y A063880 Cf. A000203, A001694, A034448, A048146, A097702, A276378.
%K A063880 easy,nonn
%O A063880 1,1
%A A063880 _Jason Earls_, Aug 27 2001