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%I #17 Sep 28 2018 05:48:39
%S 2520,31320,1163160,2208384,3053232,6535296,13472928,123165600,
%T 365959296,4401782352,5517818880,612014161920
%N Nonunitary doubly perfect numbers: the sum of the nonunitary divisors of n is 2n; i.e., sigma(n) - usigma(n) = 2n.
%C There are no others up to 7.9*10^12, but here's a larger one: 2846972800010800140288.
%C Ligh & Wall found the first 11 terms of this sequence. - _Amiram Eldar_, Sep 27 2018
%H Steve Ligh and Charles R. Wall, <a href="https://www.fq.math.ca/Scanned/25-4/ligh.pdf">Functions of Nonunitary Divisors</a>, Fibonacci Quarterly, Vol. 25 (1987), pp. 333-338.
%t nusigma[ n_ ] := DivisorSigma[ 1, n ]-Times@@(1+Power@@#&/@FactorInteger[ n ]); For[ n=1, True, n++, If[ nusigma[ n ]==2n, Print[ n ] ] ]
%o (PARI) isok(n) = sumdiv(n, d, if (gcd(d, n/d)!=1, d)) == 2*n; \\ _Michel Marcus_, Jul 31 2017
%Y Cf. A048146, A064591, A064593, A064594, A064595, A064596.
%K nonn,hard
%O 1,1
%A _Dean Hickerson_, Sep 25 2001
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