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A064591 Nonunitary perfect numbers: n is the sum of its nonunitary divisors; i.e., sigma(n) - usigma(n) = n. 25
24, 112, 1984, 32512, 134201344, 34359476224, 549754765312 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

There are no other terms up to 1.2*10^14.

If P (A000396) is an even perfect number, then 4*P is in the sequence. Are there any others?

If there are no terms of another form, the sequence goes on with 9223372032559808512 = 2^32 * A000668(8), 10633823966279326978618770463815368704 = 2^62 * A000668(9), 766247770432944429179173512337214552523989286192676864 = 2^90 * A000668(10), ... - Michel Lagneau, Jan 21 2015

Conjecture: let s0 be the sum of the inverses of the even divisors of a number n and s1 the sum of the inverses of the odd divisors of n; then n is in the sequence iff 2*s0-s1 = 1. - Michel Lagneau, Jan 21 2015

Ligh & Wall proved that 2^(p+1)*(2^p-1) is a term if p and 2^p-1 are primes, and that all the nonunitary perfect numbers below 10^6 are of this form. - Amiram Eldar, Sep 27 2018

LINKS

Table of n, a(n) for n=1..7.

Steve Ligh and Charles R. Wall, Functions of Nonunitary Divisors, Fibonacci Quarterly, Vol. 25 (1987), pp. 333-338.

EXAMPLE

The sum of the nonunitary divisors of 24 is 2 + 4 + 6 + 12 = 24.

MATHEMATICA

nusigma[ n_ ] := DivisorSigma[ 1, n ]-Times@@(1+Power@@#&/@FactorInteger[ n ]); For[ n=1, True, n++, If[ nusigma[ n ]==n, Print[ n ] ] ]

Do[s0=0; s1=0; Do[d=Divisors[n][[i]]; If[Mod[d, 2]==0, s0=s0+1/d, s1=s1+1/d], {i, 1, Length[Divisors[n]]}]; If[2*s0-s1==1, Print[n]], {n, 2, 10^9, 2}] (* Michel Lagneau, Jan 21 2015 *)

CROSSREFS

Cf. A048146, A063870.

Cf. A064592, A064593, A064594, A064595, A064596, A064597, A064598, A064599.

Sequence in context: A263542 A281133 A064595 * A228433 A124952 A126411

Adjacent sequences:  A064588 A064589 A064590 * A064592 A064593 A064594

KEYWORD

nonn,hard,more

AUTHOR

Dean Hickerson, Sep 25 2001

STATUS

approved

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Last modified September 18 16:05 EDT 2020. Contains 337169 sequences. (Running on oeis4.)