OFFSET
1,1
COMMENTS
This sequence contains the odd members of A125639, which (empirically) accounts for only about 0.08% of them.
LINKS
Robert Israel, Table of n, a(n) for n = 1..1000
EXAMPLE
The third odd number which is doubly abundant is 38745. Hence a(3)=38745.
MAPLE
filter:= proc(n) local s;
s:= numtheory:-sigma(n)-n;
s > n and numtheory:-sigma(s)>2*s
end proc:
select(filter, [seq(i, i=3..400000, 2)]); # Robert Israel, Jun 04 2018
MATHEMATICA
s[n_]:=DivisorSigma[1, n]-n; DoublyAbundantNumberQ[k_]:=If[s[k]>k && s[s[k]]>s[k], True, False]; Select[Range[500000], OddQ[ # ] && DoublyAbundantNumberQ[ # ] & ]
PROG
(PARI) isok(n) = (n%2) && ((s=sigma(n)-n) > n) && (sigma(s) > 2*s); \\ Michel Marcus, Jun 05 2018
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Ant King, Apr 02 2007
STATUS
approved