A067192 Composite c such that sigma(c)==2 (mod phi(c)).

10, 20, 52, 232, 976, 1332, 65152, 261376, 4191232, 67096576, 274877120512, 4398043365376

Offset

1,1

Comments

a(12) > 10^12. 4398043365376, 70368731594752 and 72057593635274752 are also terms. - Donovan Johnson, Feb 29 2012

a(13) > 10^13. If 2^k-3 is prime (A050414), then 2^(k-2)*(2^k-3) is a term. Up to 10^13 the only term not of this form is 1332. - Giovanni Resta, Mar 29 2020

Links

Table of n, a(n) for n=1..12.

Prog

(PARI) isok(c) = !isprime(c) && ((sigma(c) % eulerphi(c)) == 2); \\ Michel Marcus, Feb 17 2021

Crossrefs

Cf. A050414, A067193.

Sequence in context: A205879 A254030 A188896 * A030004 A271512 A294473

Adjacent sequences: A067189 A067190 A067191 * A067193 A067194 A067195

Keyword

nonn,more

Author

Benoit Cloitre, Feb 19 2002

Extensions

a(8)-a(10) from Donovan Johnson, Dec 14 2009

a(11) from Donovan Johnson, Feb 29 2012

a(12) from Giovanni Resta, Mar 29 2020

Status

approved