OFFSET
1,1
COMMENTS
Totients of prime powers are prime powers only for powers of 2.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
FORMULA
Numbers of the form phi(p^k) = (p-1)*p^(k-1), where p is prime and k > 1.
Sum_{n>=1} 1/a(n) = Sum_{p prime} 1/(p-1)^2 = A086242 = 1.3750649947... - Amiram Eldar, Dec 21 2020
EXAMPLE
The 10th pure power of prime (but not a prime) is 81, so a(10) = EulerPhi(81) = 54.
MATHEMATICA
EulerPhi[Select[Range[2^13], CompositeQ[#] && PrimePowerQ[#] &]] (* Amiram Eldar, Dec 21 2020 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Labos Elemer, Mar 03 2000
STATUS
approved