OFFSET
1,1
COMMENTS
Solutions to sigma(x)=2(x+d) with d > sqrt(x) being a proper divisor of x. The subsequence A165772 contains most of the terms. - M. F. Hasler, Oct 11 2009
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..1200
EXAMPLE
a(2)=30 because 1+2+3+5+10+15-6 = 30 and 6 > sqrt(30) = 5.477...
MATHEMATICA
aQ[n_] := (d = DivisorSigma[1, n] - 2n) > 0 && EvenQ[d] && Mod[n, d/2] == 0 && d < 2n && d^2 > 4n; Select[Range[35150], aQ] (* Amiram Eldar, Sep 22 2019 *)
PROG
(PARI) is_A109321(n)= my(d=sigma(n)-2*n); d>0 && bittest(d, 0)==0 && d<2*n && d*d>4*n && 2*n%d==0 \\ M. F. Hasler, Oct 11 2009
CROSSREFS
KEYWORD
nonn
AUTHOR
Jason Earls, Aug 20 2005
STATUS
approved