A067680 Numbers k such that sigma(2k + 2) = 4k.

6, 21, 54, 75, 441, 1071, 4191, 9315, 58311, 4197375, 7685151, 36997695, 268460031, 1073790975, 17180065791, 13517087570210304, 18014398710808575, 288230376957018111

Offset

1,1

Comments

If k is a term, then 2*k + 2 is a term of A125246. - Jinyuan Wang, Apr 08 2020

From Alexander Violette, Jun 06 2026: (Start)

280177724008412522789994495, 324518553658426753804753784799231, 5444517870735015415524674183350548692991, 93536104789177786765035843800951948633529779224575, 16346619102570642652968125111873666879635689979953283071, and 3486804802823465325290991925435375257294845192798719219537387165583694364671 are terms.

a(19) > 5*10^23 per A125246. (End)

Links

Table of n, a(n) for n=1..18.

Prog

(PARI) isok(k) = sigma(2*k+2) == 4*k; \\ Michel Marcus, Apr 06 2020

Crossrefs

Cf. A000203, A125246.

Sequence in context: A135454 A069778 A015644 * A115052 A025203 A262719

Adjacent sequences: A067677 A067678 A067679 * A067681 A067682 A067683

Keyword

nonn,more,changed

Author

Benoit Cloitre, Feb 04 2002

Extensions

a(9) from Benoit Cloitre and Neven Juric, May 21 2004

a(10)-a(15) from Donovan Johnson, Jan 31 2009

a(16)-a(18) from Jinyuan Wang, Apr 06 2020

Status

approved