A129087 Odd doubly abundant numbers (A125639).

11025, 34155, 38745, 39585, 41895, 75735, 85995, 99225, 118755, 131355, 135135, 193725, 208845, 218025, 237195, 241395, 241605, 245385, 255645, 271215, 272745, 275625, 276885, 279279, 306495, 307125, 323505, 342225, 347985, 364455, 377685

Offset: 1

Author

Ant King, Apr 02 2007

Keywords

• easy
• nonn

Comments

This sequence contains the odd members of A125639, which (empirically) accounts for only about 0.08% of them.

Examples

			The third odd number which is doubly abundant is 38745. Hence a(3)=38745.
		

Links

• Robert Israel, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A125639, A001065, A005101.

Programs

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[ # ] & ]

PARI

isok(n) = (n%2) && ((s=sigma(n)-n) > n) && (sigma(s) > 2*s); \\ Michel Marcus, Jun 05 2018

Formula

Odd numbers k, such that both k and s(k) (A001065) are abundant (A005101).