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 A064012 usigma(usigma(n))=3*n where usigma(n) is the sum of unitary divisors of n (A034448). 3
 10, 30, 288, 660, 720, 2146560 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Is this sequence finite? 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 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 LINKS V. Sitaramaiah and M. V. Subbarao, On the equation usigma(usigma(n)) = 2n, Util. Math. 53 (1998), 101--124. EXAMPLE 10 belongs to the sequence because usigma(usigma(10)) = 30 = 3*10. CROSSREFS Cf. A034448, A038843. Sequence in context: A098684 A268117 A256879 * A276180 A247207 A187521 Adjacent sequences:  A064009 A064010 A064011 * A064013 A064014 A064015 KEYWORD nonn AUTHOR Felice Russo, Sep 07 2001 EXTENSIONS One more term from Naohiro Nomoto, Oct 21 2001 No other terms < 1070000000, Jud McCranie, Oct 28 2001 STATUS approved

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Last modified April 1 14:29 EDT 2020. Contains 333163 sequences. (Running on oeis4.)