OFFSET
1,2
EXAMPLE
24 is in the sequence because gcd(24; sigma(24)=60) = (sigma(24)=60) mod 24 = 12.
MATHEMATICA
Select[Range[10000], ! PrimeQ[#] && Mod[GCD[#, DivisorSigma[1, #]] - DivisorSigma[1, #], #] == 0 &] (* T. D. Noe, Feb 03 2012 *)
PROG
(PARI) isok(k) = if (!isprime(k), my(s=sigma(k)); Mod(gcd(k, s), k) == Mod(s, k)); \\ Michel Marcus, Feb 09 2021
CROSSREFS
KEYWORD
nonn
AUTHOR
Jaroslav Krizek, Jan 28 2012
EXTENSIONS
Corrected by T. D. Noe, Feb 03 2012
STATUS
approved