OFFSET
1,1
LINKS
Daniel Suteu, Table of n, a(n) for n = 1..58
EXAMPLE
The divisors of 5 are 1 and 5, which add up to 6. 6 is a perfect number because its proper divisors are 1, 2 and 3, which also add up to 6.
MAPLE
with(numtheory); P:=proc(q) local n; for n from 1 to q do
if sigma(sigma(n))=2*sigma(n) then print(n);
fi; od; end: P(10^9); # Paolo P. Lava, Oct 22 2013
PROG
(PARI) isok(n) = sigma(sigma(n)) == 2*sigma(n); \\ Michel Marcus, Oct 22 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
Howard Berman (howard_berman(AT)hotmail.com), Oct 31 2008
EXTENSIONS
Two missing terms added and a(10)-a(19) from Donovan Johnson, Jan 20 2012
a(20) from Daniel Suteu, May 23 2022
STATUS
approved