OFFSET
1,1
COMMENTS
Sequence is infinite, since 2n is in the sequence if and only if n is in the sequence. What is its density? - Charles R Greathouse IV, Feb 21 2013
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
EXAMPLE
7 is here because phi(7)=6 divided by 2 is prime; 21 is here because phi(21)=12 divided by 4 is prime; 15 is not here because phi(15)=8 divided by 8 is not prime.
MATHEMATICA
Select[Range@ 138, PrimeQ@ Last@ Most@ NestWhileList[#/2 &, EulerPhi@ #, IntegerQ@ # &] &] (* Michael De Vlieger, Mar 18 2017 *)
PROG
(PARI) is(n)=n=eulerphi(n); isprime(n>>valuation(n, 2)) \\ Charles R Greathouse IV, Feb 21 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
Labos Elemer, Dec 18 2001
STATUS
approved