A172019 Numbers k such that 4 divides phi(k) (i.e., A000010(k)).

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

Author

Giovanni Teofilatto, Jan 22 2010

Keywords

• easy
• nonn

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.

Crossrefs

Cf. A000010, A097987, A066499, A066498, A066500, A066502.

Programs

Mathematica

Select[Range[200], Mod[EulerPhi[#], 4] == 0 &] (* Geoffrey Critzer, Nov 30 2014 *)

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