%I #17 Apr 10 2019 02:45:58
%S 10,30,288,660,720,2146560
%N usigma(usigma(n))=3*n where usigma(n) is the sum of unitary divisors of n (A034448).
%C Is this sequence finite?
%C 18 is the only known integer usigma(usigma(n)) = kn for some integer k >= 4 (no other one <= 2^30). - _Tomohiro Yamada_, Apr 22 2017
%C a(7) > 2.85*10^11, if it exists. The same bound holds also for any n > 18 such usigma(usigma(n)) = k*n for some integer k >= 4. - _Giovanni Resta_, Apr 10 2019
%H V. Sitaramaiah and M. V. Subbarao, <a href="http://www.math.ualberta.ca/~subbarao/documents/Sitaramaiah_Subbarao1998.pdf">On the equation usigma(usigma(n)) = 2n</a>, Util. Math. 53 (1998), 101--124.
%e 10 belongs to the sequence because usigma(usigma(10)) = 30 = 3*10.
%Y Cf. A034448, A038843.
%K nonn
%O 1,1
%A _Felice Russo_, Sep 07 2001
%E One more term from _Naohiro Nomoto_, Oct 21 2001
%E No other terms < 1070000000, _Jud McCranie_, Oct 28 2001
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