OFFSET
1,2
COMMENTS
Old name was: Numbers n such that sum of divisors of n, sigma (n), minus n is a square.
All primes are terms, since for p prime, A001065(p)=1 and 1 is a square. - Michel Marcus, Apr 22 2018
LINKS
Ivan Neretin, Table of n, a(n) for n = 1..10000
FORMULA
EXAMPLE
a(6) = 9 because the divisors of 9 are 1, 3, 9, and (1+3+9)-9 = 4 = 2^2.
The number 10 is not in the sequence because (1+2+5+10)-10 = 8, which is not a square.
a(7) = 11 because (1+11)-11 = 1, a square.
MAPLE
with(numtheory); a := []; for n from 1 to 2000 do if issqr(sigma(n)-n) then a := [op(a), n]; fi; od: a;
MATHEMATICA
Select[Range[200], IntegerQ[Sqrt[-# + Plus@@Divisors[#]]] &] (* Alonso del Arte, Dec 08 2010 *)
PROG
(PARI) isok(n) = issquare(sigma(n) - n); \\ Michel Marcus, Apr 22 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Aug 24 2002
EXTENSIONS
Name edited by Altug Alkan, Apr 22 2018
STATUS
approved