A353684 Numbers k for which phi(sigma(k)) >= phi(k).

1, 2, 3, 4, 6, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 22, 24, 25, 26, 28, 30, 32, 34, 36, 39, 40, 42, 44, 45, 46, 48, 50, 52, 54, 56, 60, 62, 63, 64, 66, 68, 70, 72, 74, 75, 76, 78, 80, 81, 84, 88, 90, 92, 93, 94, 96, 98, 100, 102, 104, 105, 106, 108, 110, 111, 112, 114, 117, 120, 122, 124, 126, 128, 130, 132, 136, 138

Offset

1,2

Comments

Numbers k such that A062401(k) >= A000010(k).

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Index entries for sequences related to sigma(n)

Maple

filter:= proc(k) uses NumberTheory;
    phi(sigma(k)) >= phi(k)
end proc:
select(filter, [$1..200]); # Robert Israel, Feb 24 2026

Mathematica

Select[Range[140], EulerPhi[DivisorSigma[1, #]] >= EulerPhi[#] &] (* Amiram Eldar, May 06 2022 *)

Prog

(PARI)
A353682(n) = (eulerphi(sigma(n))>=eulerphi(n));
isA353684(n) = A353682(n);

Crossrefs

Positions of nonnegative terms in A353636.

Cf. A000010, A000203, A062401, A353682 (characteristic function), A353686 (complement).

Disjoint union of A006872 and A353683.

Sequence in context: A173905 A171581 A063450 * A377236 A377182 A389634

Adjacent sequences: A353681 A353682 A353683 * A353685 A353686 A353687

Keyword

nonn

Author

Antti Karttunen, May 06 2022

Status

approved