A097987 - OEIS

A097987

Numbers k such that 4 does not divide phi(k), where phi is Euler's totient function (A000010).

1, 2, 3, 4, 6, 7, 9, 11, 14, 18, 19, 22, 23, 27, 31, 38, 43, 46, 47, 49, 54, 59, 62, 67, 71, 79, 81, 83, 86, 94, 98, 103, 107, 118, 121, 127, 131, 134, 139, 142, 151, 158, 162, 163, 166, 167, 179, 191, 199, 206, 211, 214, 223, 227, 239, 242, 243, 251, 254, 262, 263, 271

(list; graph; refs; listen; history; text; internal format)

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 *)