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A064012 usigma(usigma(n))=3*n where usigma(n) is the sum of unitary divisors of n (A034448). 2
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

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), 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,changed

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 20 03:36 EDT 2019. Contains 322294 sequences. (Running on oeis4.)