OFFSET
1,1
COMMENTS
Although the definition here is similar to the one in A019278, it appears that this sequence does not have the same nice features as A019278.
Otherwise said: sigma(sigma(k))/k is half-integer, or: sigma(sigma(k)) is an odd multiple of k/2. This also implies that all terms are even. - M. F. Hasler, Jan 06 2020
LINKS
Giovanni Resta, Table of n, a(n) for n = 1..38 (terms < 10^13)
Michel Marcus, Unexhaustive list of terms
EXAMPLE
sigma(sigma(30))/30 = sigma(72)/30 = 195/30 = 13/2 so 30 is a term.
PROG
(PARI) isok(n) = denominator(sigma(sigma(n))/n) == 2;
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Marcus, Dec 19 2019
EXTENSIONS
a(22)-a(26) from Giovanni Resta, Dec 20 2019
STATUS
approved