OFFSET
1,1
COMMENTS
Contains 2^k(2^(k+2)-5) where 2^(k+2)-5 is prime; contains 2^k*3*(2^(k+2)-3) where 2^(k+2)-3 is prime; complete up to 5000000000. - David W. Wilson, May 16 2001
a(23) > 10^12. - Donovan Johnson, Feb 29 2012
MATHEMATICA
Do[If[Mod[DivisorSigma[1, n], EulerPhi[n]+1]==0, Print[n]], {n, 1, 10^7}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Robert G. Wilson v, Jul 27 2000
EXTENSIONS
a(20)-a(22) from Donovan Johnson, Feb 29 2012
STATUS
approved