

A064012


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


3




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

Table of n, a(n) for n=1..6.
V. Sitaramaiah and M. V. Subbarao, On the equation usigma(usigma(n)) = 2n, Util. Math. 53 (1998), 101124.


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



