login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 60th year, we have over 367,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).

Other ways to Give
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
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
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
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 A350556 A251467
KEYWORD
nonn,more
AUTHOR
Donovan Johnson, Mar 23 2013
EXTENSIONS
a(17)-a(21) from Hiroaki Yamanouchi, Aug 21 2018
STATUS
approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified December 6 07:56 EST 2023. Contains 367597 sequences. (Running on oeis4.)