A378641 Largest m <= n such that phi(m) does not divide n, or -1 if no such m exists, where phi is the Euler totient function (A000010).

-1, -1, 3, -1, 5, 5, 7, 7, 9, 10, 11, 11, 13, 14, 15, 14, 17, 17, 19, 20, 21, 22, 23, 23, 25, 26, 27, 28, 29, 30, 31, 31, 33, 34, 35, 35, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 47, 49, 50, 51, 52, 53, 53, 55, 56, 57, 58, 59, 60, 61, 62, 63, 63, 65, 66, 67, 68, 69

Offset: 1

Author

Paolo Xausa, Dec 05 2024

Keywords

• sign
• easy

Links

• Paolo Xausa, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000010, A378636, A378637, A378640, A378642.

Programs

Mathematica

A378641[n_] := Module[{m = n}, While[m > 0 && Divisible[n, EulerPhi[m]], m--]; If[m == 0, -1, m]];
Array[A378641, 100]

Formula

a(n) = n if n is an odd number >= 3.