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

1,1

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

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

Sequence in context: A344443 A332556 A049195 * A064362 A173298 A248356

Adjacent sequences: A172016 A172017 A172018 * A172020 A172021 A172022

Keyword

easy,nonn

Author

Giovanni Teofilatto, Jan 22 2010

Status

approved