OFFSET
1,1
COMMENTS
Suggested by N. J. A. Sloane and Robert G. Wilson v.
a(22) > 10^12.
a(23) > 10^13. - Giovanni Resta, Mar 29 2013
a(29) > 10^18. - Hiroaki Yamanouchi, Aug 23 2018
Any term x of this sequence can be combined with any term y of A223607 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
Every number of the form 2^(j-1)*(2^j - 21), where 2^j - 21 is prime, is a term. - Jon E. Schoenfield, Jun 02 2019
LINKS
Hiroaki Yamanouchi, Table of n, a(n) for n = 1..28
EXAMPLE
For k = 544310, sigma(k) - 2*k = 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
KEYWORD
nonn
AUTHOR
Donovan Johnson, Mar 23 2013
EXTENSIONS
a(22) from Giovanni Resta, Mar 29 2013
a(23)-a(25) from Hiroaki Yamanouchi, Aug 23 2018
STATUS
approved