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A129087 Odd doubly abundant numbers (A125639). 1
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 (list; graph; refs; listen; history; text; internal format)
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

FORMULA

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

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

Cf. A125639, A001065, A005101.

Sequence in context: A233720 A285845 A160711 * A239828 A156942 A164518

Adjacent sequences:  A129084 A129085 A129086 * A129088 A129089 A129090

KEYWORD

easy,nonn

AUTHOR

Ant King, Apr 02 2007

STATUS

approved

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Last modified January 19 20:26 EST 2019. Contains 319310 sequences. (Running on oeis4.)