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