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A097987
Numbers k such that 4 does not divide phi(k), where phi is Euler's totient function (A000010).
4
1, 2, 3, 4, 6, 7, 9, 11, 14, 18, 19, 22, 23, 27, 31, 38, 43, 46, 47, 49, 54, 59, 62, 67, 71, 79, 81, 83, 86, 94, 98, 103, 107, 118, 121, 127, 131, 134, 139, 142, 151, 158, 162, 163, 166, 167, 179, 191, 199, 206, 211, 214, 223, 227, 239, 242, 243, 251, 254, 262, 263, 271
OFFSET
1,2
COMMENTS
The asymptotic density of this sequence is 0 (Dressler, 1975). - Amiram Eldar, Jul 23 2020
LINKS
Robert E. Dressler, A property of the phi and sigma_j functions, Compositio Mathematica, Vol. 31, No. 2 (1975), pp. 115-118.
FORMULA
a(n)=1, 2, 4, p^k, 2*p^k, with prime p == 3 (mod 4).
MATHEMATICA
Select[Range@275, ! Divisible[EulerPhi[#], 4] &] (* Ivan Neretin, Aug 24 2016 *)
PROG
(PARI) is(n)=my(o=valuation(n, 2), p); (o<2 && isprimepower(n>>o, &p) && p%4>1) || n<5 \\ Charles R Greathouse IV, Feb 21 2013
CROSSREFS
Essentially the same as A066499.
Cf. A000010.
Complement of A172019.
Sequence in context: A015851 A225529 A065156 * A049149 A332555 A373097
KEYWORD
nonn
AUTHOR
Lekraj Beedassy, Sep 07 2004
EXTENSIONS
Corrected and extended by Vladeta Jovovic, Sep 08 2004
STATUS
approved