OFFSET
1,2
MATHEMATICA
Select[Range[210], Nest[EulerPhi, #, 2] === DivisorSigma[0, #] &] (* Amiram Eldar, Jun 12 2022 *)
PROG
(PARI) is(k) = numdiv(k) == eulerphi(eulerphi(k)); \\ Jinyuan Wang, Apr 05 2020
CROSSREFS
KEYWORD
nonn,easy,fini,full
AUTHOR
Benoit Cloitre, Mar 26 2002
STATUS
approved