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A064012 usigma(usigma(n))=3*n where usigma(n) is the sum of unitary divisors of n (A034448). 3

%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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Last modified March 28 22:04 EDT 2024. Contains 371254 sequences. (Running on oeis4.)