A015126 Least k such that phi(k) = phi(n).

1, 1, 3, 3, 5, 3, 7, 5, 7, 5, 11, 5, 13, 7, 15, 15, 17, 7, 19, 15, 13, 11, 23, 15, 25, 13, 19, 13, 29, 15, 31, 17, 25, 17, 35, 13, 37, 19, 35, 17, 41, 13, 43, 25, 35, 23, 47, 17, 43, 25, 51, 35, 53, 19, 41, 35, 37, 29, 59, 17, 61, 31, 37, 51, 65, 25, 67, 51, 69, 35, 71, 35, 73

Offset: 1

Author

Vladeta Jovovic, Jan 12 2002

Keywords

• nonn
• look

Comments

From Jianing Song, Nov 11 2022: (Start)

The first even term is a(33817088) = 16842752 (see A002181 and A143510).

Conjecture: a(n) is always odd for odd n. (End)

Links

• Antti Karttunen, Table of n, a(n) for n = 1..10000
• Max Alekseyev, PARI/GP Scripts for Miscellaneous Math Problems (invphi.gp).

Crossrefs

Cf. A000010, A028476, A066412, A066659.

Cf. A002181, A143510.

Programs

PARI

a(n) = {my(en = eulerphi(n)); k = 1; while (eulerphi(k) != en, k++); return (k);} \\ Michel Marcus, Jun 17 2013

PARI

a(n) = vecmin(select(x -> x<=n, invphi(eulerphi(n)))); \\ Amiram Eldar, Nov 14 2024, using Max Alekseyev's invphi.gp