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
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