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A340762
Numbers k such that iterations of phi(k), phi(phi(k)), ... end in ... 4, 2, 1.
2
4, 5, 8, 10, 11, 12, 13, 15, 16, 17, 20, 21, 22, 23, 24, 25, 26, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72
OFFSET
1,1
COMMENTS
Infinite set (see reference).
LINKS
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.
EXAMPLE
11 is in the list because phi(phi(11)))) = phi(10) = 4.
MAPLE
filter:= proc(n) option remember;
if n < 4 then false
elif n = 4 then true
else procname(numtheory:-phi(n))
fi
end proc:
select(filter, [$4..100]); # Robert Israel, Dec 23 2021
MATHEMATICA
Select[Range[4, 72], FixedPointList[EulerPhi, #][[-4]] == 4 &] (* Amiram Eldar, Jan 27 2021 *)
PROG
(PARI) isok(k) = if (k>=4, while((k!=6) && (k!=4), k=eulerphi(k))); k == 4; \\ Michel Marcus, Feb 01 2021
CROSSREFS
Cf. A000010, A340765 (complement relative to {n>=4}).
Sequence in context: A064394 A346457 A141220 * A092022 A346302 A377897
KEYWORD
nonn
AUTHOR
Franz Vrabec, Jan 20 2021
STATUS
approved