OFFSET
1,1
COMMENTS
It is conjectured that there are no odd terms and that the sequence is infinite.
a(25) > 10^12. - Donovan Johnson, Nov 15 2013
LINKS
Yong-Gao Chen and Jin-Hui Fang, On n phi(n)d(n) + 2 and n phi(n) sigma(n)+1, INTEGERS: Electronic J. Combinatorial Number Theory: 8 (2008), #A7.
Donovan Johnson, 10 terms > 10^12
MATHEMATICA
Select[Range[172*10^4], CompositeQ[#]&&Divisible[2*DivisorSigma[1, #]-DivisorSigma[ 0, #], #]&] (* The program generates the first 9 terms of the sequence. To generate more, increase the Range constant. *) (* Harvey P. Dale, May 11 2020 *)
PROG
(PARI) isok(n) = (! isprime(n) && (2*sigma(n) - numdiv(n)) % n == 0) \\ Michel Marcus, Jun 09 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
R. K. Guy, Feb 08 2008
EXTENSIONS
Corrected by Emeric Deutsch, Feb 29 2008
a(15)-a(24) from Donovan Johnson, Oct 13 2008
STATUS
approved