A353679 Odd numbers k for which phi(k) = phi(sigma(k)).

1, 3, 15, 39, 45, 111, 117, 175, 183, 195, 219, 333, 357, 471, 549, 555, 579, 585, 657, 831, 915, 939, 1071, 1095, 1191, 1263, 1371, 1413, 1443, 1623, 1665, 1737, 1839, 1983, 2019, 2199, 2271, 2275, 2295, 2355, 2379, 2493, 2631, 2745, 2817, 2847, 2895, 2991, 3279, 3285, 3459, 3573, 3603, 3639, 3711, 3789, 3963

Offset: 1

Author

Antti Karttunen, May 06 2022

Keywords

• nonn

Links

• Index entries for sequences where odd perfect numbers must occur, if they exist at all
• Index entries for sequences related to sigma(n)

Crossrefs

Odd terms in A006872.

Cf. A000010, A000035, A000203, A062401, A353637, A353680 (characteristic function).

Programs

Mathematica

Select[Range[1, 4000, 2], EulerPhi[#] == EulerPhi[DivisorSigma[1, #]] &] (* Amiram Eldar, May 06 2022 *)

PARI

A353680(n) = ((n%2) && (eulerphi(sigma(n))==eulerphi(n)));
isA353679(n) = A353680(n);