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A172019
Numbers k such that 4 divides phi(k) (i.e., A000010(k)).
5
5, 8, 10, 12, 13, 15, 16, 17, 20, 21, 24, 25, 26, 28, 29, 30, 32, 33, 34, 35, 36, 37, 39, 40, 41, 42, 44, 45, 48, 50, 51, 52, 53, 55, 56, 57, 58, 60, 61, 63, 64, 65, 66, 68, 69, 70, 72, 73, 74, 75, 76, 77, 78, 80, 82, 84, 85, 87, 88, 89, 90, 91, 92, 93, 95, 96, 97, 99, 100, 101
OFFSET
1,1
COMMENTS
Complement of A097987.
The asymptotic density of this sequence is 1 (Dressler, 1975). - Amiram Eldar, Feb 12 2021
LINKS
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