OFFSET
1,2
COMMENTS
The asymptotic density of this sequence is 0 (Dressler, 1975). - Amiram Eldar, Jul 23 2020
LINKS
Ivan Neretin, Table of n, a(n) for n = 1..10000
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
KEYWORD
nonn
AUTHOR
Lekraj Beedassy, Sep 07 2004
EXTENSIONS
Corrected and extended by Vladeta Jovovic, Sep 08 2004
STATUS
approved