OFFSET
1,2
COMMENTS
n | sigma(n) gives the multi-perfect numbers A007691, n | sigma(n)+1 if n is a power of 2 (A000079).
This contains A191363 as subsequence, so for any Fermat prime F(k) = 2^2^k+1, the triangular number A000217(2^2^k)=(F(k)-1)*F(k)/2 is in this sequence. See also A055708 which is identical up to the first term. - M. F. Hasler, Oct 02 2014
a(7) > 10^13. - Giovanni Resta, Jul 13 2015
MATHEMATICA
Do[If[Mod[DivisorSigma[1, n]+2, n]==0, Print[n]], {n, 1, 7*10^8}]
PROG
(PARI) for(n=1, 5e9, if((sigma(n)+2)%n==0, print1(n", "))) \\ Charles R Greathouse IV, Jun 01 2011
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Robert G. Wilson v, Jul 24 2000
EXTENSIONS
a(6) from Charles R Greathouse IV, Jun 01 2011
Edited by M. F. Hasler, Oct 02 2014
STATUS
approved