OFFSET
1,1
COMMENTS
The first four numbers in this sequence {6, 12, 28, 42} are featured in the cult film "Donnie Darko", in which the title character is warned that the world will end in 28 days, 6 hours, 42 minutes and 12 seconds. - Don Rogers (donrogers42(AT)aol.com), Sep 21 2006
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..104
MAPLE
select(t -> not isprime(t) and (numtheory:-sigma(t) - 2*t) mod numtheory:-phi(t) = 0, [$4..2*10^6]); # Robert Israel, Jan 15 2017
MATHEMATICA
Select[Range[15*10^5], CompositeQ[#]&&Divisible[DivisorSigma[1, #]-2#, EulerPhi[ #]]&] (* Harvey P. Dale, Mar 08 2023 *)
PROG
(PARI) for(n=2, 300000, if((sigma(n)-2*n)%(eulerphi(n))==isprime(n), print1(n, ", ")))
(PARI) is(n) = {my(f = factor(n)); !((sigma(f) - 2*n) % (eulerphi(f))); } \\ Amiram Eldar, Feb 01 2024
CROSSREFS
KEYWORD
nonn
AUTHOR
Benoit Cloitre, Mar 03 2002
EXTENSIONS
More terms from Rick L. Shepherd, Apr 08 2002
STATUS
approved