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A066218 Numbers k such that sigma(k) = Sum_{j|k, j<k} sigma(j). 24
198, 608, 11322, 20826, 56608, 3055150, 565344850, 579667086, 907521650, 8582999958, 13876688358, 19244570848, 195485816050, 255701999358, 1038635009650, 1410759512050 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

I propose this generalization of perfect numbers: for an arithmetical function f, the "f-perfect numbers" are the n such that f(n) = sum of f(k) where k ranges over proper divisors of n. The usual perfect numbers are i-perfect numbers, where i is the identity function. This sequence lists the sigma-perfect numbers. It is not hard to see that the EulerPhi-perfect numbers are the powers of 2 and the d-perfect numbers are the squares of primes (d(n) denotes the number of divisors of n).

Problems: Find an expression generating sigma-perfect numbers. Are there infinitely many of these? Find other interesting sets generated by other f's. 3.

a(17) > 2*10^12. - Giovanni Resta, Jun 20 2013

LINKS

Table of n, a(n) for n=1..16.

J. Pe, On a Generalization of Perfect Numbers, J. Rec. Math., 31(3) (2002-2003), 168-172.

Giovanni Resta, 34 numbers > 3*10^12 which belong to the sequence

FORMULA

Integer n = p1^k1 * p2^k2 * ... * pm^km is in this sequence if and only if g(p1^k1)*g(p2^k2)*...*g(pm^km)=2, where g(p^k) = (p^(k+2)-(k+2)*p+k+1)/(p^(k+1)-1)/(p-1) for prime p and integer k>=1. - Max Alekseyev, Oct 23 2008

EXAMPLE

Proper divisors of 198 = {1, 2, 3, 6, 9, 11, 18, 22, 33, 66, 99}; sum of their sigma values = 1 + 3 + 4 + 12 + 13 + 12 + 39 + 36 + 48 + 144 + 156 = 468 = sigma(198).

MAPLE

with(numtheory): P:=proc(q) local a, k, n; for n from 1 to q do a:=sort([op(divisors(n))]);

if sigma(n)=add(sigma(a[k]), k=1..nops(a)-1); then print(n); fi; od; end: P(10^9); # Paolo P. Lava, Dec 04 2017

MATHEMATICA

f[ x_ ] := DivisorSigma[ 1, x ]; Select[ Range[ 1, 10^5 ], 2 * f[ # ] == Apply[ Plus, Map[ f, Divisors[ # ] ] ] & ]

PROG

(PARI) is(n)=sumdiv(n, d, sigma(d))==2*sigma(n) \\ Charles R Greathouse IV, Mar 09 2014

CROSSREFS

Cf. A211779, A000203, A066229, A066230.

Sequence in context: A202526 A221219 A238765 * A304614 A252952 A252945

Adjacent sequences:  A066215 A066216 A066217 * A066219 A066220 A066221

KEYWORD

nonn,more

AUTHOR

Joseph L. Pe, Dec 17 2001

EXTENSIONS

More terms from Naohiro Nomoto, May 07 2002

2 more terms from Farideh Firoozbakht, Sep 18 2006

a(9)-a(13) from Donovan Johnson, Jun 25 2012

a(14)-a(16) from Giovanni Resta, Jun 20 2013

STATUS

approved

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Last modified November 30 04:28 EST 2021. Contains 349418 sequences. (Running on oeis4.)