A067702 Numbers k such that sigma(k) == 0 (mod k+2).

12, 70, 88, 180, 1888, 4030, 5830, 32128, 521728, 1848964, 8378368, 34359083008, 66072609790, 549753192448, 259708613909470

Offset

1,1

Comments

a(12) > 10^8. - Michel Marcus, Nov 22 2013

If 2^i-5 is prime for i > 2 then let x = (2^i-5)*2^(i-1). Then sigma(x)=2*(x+2), so x is in the sequence. There are other terms that are not of this form. - Jud McCranie, Jan 12 2019

Links

Table of n, a(n) for n=1..15.

Example

sigma(180) = 546 = 3(180+2), so 180 is in the sequence.

Mathematica

Select[Range[84*10^5], Divisible[DivisorSigma[1, #], #+2]&] (* Harvey P. Dale, May 11 2018 *)

Prog

(PARI) isok(n) = sigma(n) % (n+2) == 0; \\ Michel Marcus, Nov 22 2013

Crossrefs

Cf. A045768, A156560, A059608.

Sequence in context: A212753 A210427 A101097 * A163193 A088832 A366748

Adjacent sequences: A067699 A067700 A067701 * A067703 A067704 A067705

Keyword

nonn,more

Author

Benoit Cloitre, Feb 05 2002

Extensions

a(9)-a(11) from Michel Marcus, Nov 22 2013

a(12)-a(13) from Jud McCranie, Jan 12 2019

a(14) from Jud McCranie, Jan 13 2019

a(15) from Jud McCranie, Dec 02 2019

Status

approved