OFFSET
1,1
COMMENTS
Infinite set (see reference).
Contains 3^k for k >= 2 and 2*3^k for k >= 1, and all members of A111974 except 3. - Robert Israel, Dec 23 2021
LINKS
Robert Israel, Table of n, a(n) for n = 1..1000
Eliot T. Jacobson and Alan E. Parks, Infinite branches of the phi-tree, Amer. Math. Monthly, Vol. 93, No. 7 (August-September 1986), pp. 552-554.
Keith Matthews, Solving phi(x)=n, where phi(x) is Euler's totient function.
EXAMPLE
19 is in the list because phi(phi(19)) = phi(18) = 6.
MAPLE
R:= {6}: Agenda:= {6}: count:= 1:
while count - nops(Agenda) < 99 do
v:= min(Agenda);
W:= convert(numtheory:-invphi(v), set);
count:= count + nops(W);
Agenda:= Agenda minus {v} union W;
R:= R union W;
od:
sort(select(`<=`, convert(R, list), min(Agenda))); # Robert Israel, Dec 23 2021
MATHEMATICA
Select[Range[4, 10000], FixedPointList[EulerPhi, #][[-4]] == 6 &] (* Amiram Eldar, Jan 27 2021 *)
PROG
(PARI) isok(k) = if (k>=6, while((k!=6) && (k!=4), k=eulerphi(k))); k == 6; \\ Michel Marcus, Feb 01 2021
CROSSREFS
KEYWORD
nonn
AUTHOR
Franz Vrabec, Jan 20 2021
STATUS
approved