A353686 Numbers k for which phi(sigma(k)) < phi(k).

5, 7, 11, 13, 17, 19, 23, 27, 29, 31, 33, 35, 37, 38, 41, 43, 47, 49, 51, 53, 55, 57, 58, 59, 61, 65, 67, 69, 71, 73, 77, 79, 82, 83, 85, 86, 87, 89, 91, 95, 97, 99, 101, 103, 107, 109, 113, 115, 116, 118, 119, 121, 123, 125, 127, 129, 131, 133, 134, 135, 137, 139, 141, 143, 145, 147, 149, 151, 153, 155, 157, 158, 159

Offset: 1

Author

Antti Karttunen, May 06 2022

Keywords

• nonn

Comments

Numbers k such that A062401(k) < A000010(k).

Links

• Index entries for sequences related to sigma(n)

Crossrefs

Cf. A000010, A000203, A062401, A353684 (complement).

Positions of terms < 0 in A353636. Positions of zeros in A353682.

Setwise difference of A353685 and A006872.

Programs

Mathematica

Select[Range[160], EulerPhi[DivisorSigma[1, #]] < EulerPhi[#] &] (* Amiram Eldar, May 06 2022 *)

PARI

isA353686(n) = (eulerphi(sigma(n))