The OEIS mourns the passing of Jim Simons and is grateful to the Simons Foundation for its support of research in many branches of science, including the OEIS.
login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
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 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
Numbers k such that A010554(k) = A000005(k).
LINKS
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
Sequence in context: A079360 A314365 A314366 * A110994 A015833 A072022
KEYWORD
nonn,easy,fini,full
AUTHOR
Benoit Cloitre, Mar 26 2002
STATUS
approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified May 29 00:29 EDT 2024. Contains 372921 sequences. (Running on oeis4.)