login
A068580
Let phi_m(x) = phi(phi(...(phi(x))...)) m times; sequence gives values of k such that phi_2(k) = tau(k).
3
1, 5, 7, 15, 21, 22, 26, 40, 56, 66, 70, 78, 108, 120, 126, 168, 210
OFFSET
1,2
COMMENTS
Numbers k such that A010554(k) = A000005(k).
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