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 A127725 #46 Feb 16 2020 07:55:29
%S A127725 2,12,40,252,880,10880,75852,715816960,62549517598720
%N A127725 Numbers that are 2-imperfect.
%C A127725 This sequence also contains n = 3074457344902430720 = 2^31*5*17*257*65537, which has the product of four Fermat primes (A019434). For this n, 3*n is a 3-imperfect number (A127726). - _T. D. Noe_, Apr 03 2009
%C A127725 a(9) > 2*10^11. - _Donovan Johnson_, Feb 07 2013
%C A127725 62549517598720 is also a term (see the "43 terms > 2*10^11" link by Donovan Johnson in A127724). - _Michel Marcus_, Nov 05 2017
%H A127725 Laszlo Toth, <a href="http://macs.elte.hu/downloads/abstracts/MaCS_abs_Toth.pdf">The alternating sum-of-divisors function</a>, 9th Joint Conf. on Math. and Comp. Sci., February 9-12, 2012, Siofok, Hungary.
%H A127725 Laszlo Toth, <a href="http://arxiv.org/abs/1111.4842">A survey of the alternating sum-of-divisors function</a>, arXiv:1111.4842 [math.NT], 2011-2014.
%e A127725 40 = 2^3 * 5, (8 - 4 + 2 - 1)(5 - 1) = 20 = 40 / 2, so 40 is in the sequence. - _Jud McCranie_, Aug 17 2019
%t A127725 okQ[n_] := 2 Sum[d*(-1)^PrimeOmega[n/d], {d, Divisors[n]}] == n;
%t A127725 For[k = 2, k <= 10^9, k = k+2, If[okQ[k], Print[k]]] (* _Jean-François Alcover_, Jan 27 2019 *)
%o A127725 (PARI) isok(n) = 2*sumdiv(n, d, d*(-1)^bigomega(n/d)) == n; \\ _Michel Marcus_, Oct 28 2017
%Y A127725 Cf. A127726 (3-imperfect numbers), A127724 (k-imperfect numbers).
%K A127725 nonn,more,hard
%O A127725 1,1
%A A127725 _T. D. Noe_, Jan 25 2007
%E A127725 a(9) by _Jud McCranie_, Aug 17 2019