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A088826 Solutions to sigma(n)-2*n = phi(n): abundance of n equals Euler-phi of n. 1
12, 42, 1242, 6137440, 1385119360, 1588268480 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

a(7) > 10^12. - Donovan Johnson, Feb 29 2012

10^13 < a(7) <= 479535318548480. Other terms of the form 2^k*5*p*q are 983990190817280, 528322308638228480, 1374972658786140160, 9222951307429806080 and 13732480001814200320. - Giovanni Resta, Jul 12 2013

248248256622696037089280 and 29053620223944172891013120 are the next two terms of the form 2^k*5*p*q where p&q are distinct primes. 12, 42 and 1242 are the only terms of one of the three forms 4*p, 2*p*q and 2*p^3*q where p and q are two distinct primes. Farideh Firoozbakht, Aug 19 2013

LINKS

Table of n, a(n) for n=1..6.

EXAMPLE

n=1242, sigma(1242)=2880, 2880-2484=396=phi(1242).

MATHEMATICA

Do[If[Equal[DivisorSigma[1, n]-2*n, EulerPhi[n]], Print[n]], {n, 1, 10000000}]

CROSSREFS

Cf. A000203, A000010, A077374, A033880.

Sequence in context: A074356 A172294 A238225 * A125221 A253911 A082829

Adjacent sequences:  A088823 A088824 A088825 * A088827 A088828 A088829

KEYWORD

more,nonn

AUTHOR

Labos Elemer, Oct 27 2003

EXTENSIONS

a(5)-a(6) from Donovan Johnson, Sep 30 2009

STATUS

approved

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Last modified April 18 05:44 EDT 2021. Contains 343072 sequences. (Running on oeis4.)