OFFSET
1,1
COMMENTS
Complement of A097987.
The asymptotic density of this sequence is 1 (Dressler, 1975). - Amiram Eldar, Feb 12 2021
LINKS
Rémy Sigrist, 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.
MATHEMATICA
Select[Range[200], Mod[EulerPhi[#], 4] == 0 &] (* Geoffrey Critzer, Nov 30 2014 *)
PROG
(PARI) is(n)=my(o=valuation(n, 2), p); (o>1 || !isprimepower(n>>o, &p) || p%4<2) && n>4 \\ Charles R Greathouse IV, Mar 05 2013
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Giovanni Teofilatto, Jan 22 2010
STATUS
approved