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%I #35 Apr 13 2024 14:56:41
%S 108,540,756,1188,1404,1836,2052,2484,3132,3348,3780,3996,4428,4644,
%T 5076,5724,5940,6372,6588,7020,7236,7668,7884,8316,8532,8964,9180,
%U 9612,9828,10260,10476,10908,11124,11556,11772,12204,12420,12852,13716,14148
%N Numbers k such that sigma(k) = 2*usigma(k).
%C Numbers so far are all == 108 (mod 216). - _Ralf Stephan_, Jul 07 2003 [Confirmed up to 10^7 by _Robert G. Wilson v_.]
%C Also numbers whose unitary and nonunitary divisors have equal sum. - _Amiram Eldar_, Sep 30 2019
%H 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 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 (PARI) u(n) = sumdiv(n,d, if(gcd(d,n/d)==1,d));
%o j=[]; for(n=1,30000, if(sigma(n) == 2*u(n),j=concat(j,n))); j
%Y Cf. A000203, A048146, A034448, A097702.
%K easy,nonn,changed
%O 1,1
%A _Jason Earls_, Aug 27 2001
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