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A223607 Numbers n whose deficiency is 20: sigma(n) - 2*n = -20. 7
46, 154, 190, 2656, 6490, 44650, 318250, 1360810, 1503370, 1788490, 3214090, 103712410, 3915380170, 6077111050, 9796360330, 10828121356, 33086522327050, 35966517350410, 11577093570201610, 16726040141635450, 576460762503970816 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Suggested by N. J. A. Sloane and Robert G. Wilson v.

a(17) > 10^12.

a(17) > 10^13. - Giovanni Resta, Mar 29 2013

a(22) > 10^18. - Hiroaki Yamanouchi, Aug 21 2018

Any term x of this sequence can be combined with any term y of A223611 to satisfy the property (sigma(x)+sigma(y))/(x+y) = 2, which is a necessary (but not sufficient) condition for two numbers to be amicable. - Timothy L. Tiffin, Sep 13 2016

LINKS

Table of n, a(n) for n=1..21.

EXAMPLE

n = 1360810. sigma(n)-2*n = -20.

MATHEMATICA

Select[Range[1, 10^8], DivisorSigma[1, #] - 2 # == - 20 &] (* Vincenzo Librandi, Sep 14 2016 *)

PROG

(PARI) for(n=1, 10^8, if(sigma(n)-2*n==-20, print1(n ", ")))

(MAGMA) [n: n in [1..9*10^6] | (SumOfDivisors(n)-2*n) eq -20]; // Vincenzo Librandi, Sep 14 2016

CROSSREFS

Cf. A000203, A033879, A223611 (abundance 20).

Sequence in context: A053019 A044378 A044759 * A189809 A251467 A251460

Adjacent sequences:  A223604 A223605 A223606 * A223608 A223609 A223610

KEYWORD

nonn,more

AUTHOR

Donovan Johnson, Mar 23 2013

EXTENSIONS

a(17)-a(21) from Hiroaki Yamanouchi, Aug 21 2018

STATUS

approved

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Last modified September 17 14:07 EDT 2021. Contains 347478 sequences. (Running on oeis4.)