A331006 Numbers k for which there is no number m < k such that U(k) = U(m), where U(k) = phi(k)/k for k > 2 even, phi(k)/(k-1) for k > 1 odd, U(2) = 1.

2, 4, 6, 9, 10, 14, 15, 21, 22, 25, 26, 27, 30, 33, 34, 35, 38, 39, 42, 45, 46, 49, 51, 55, 57, 58, 62, 63, 66, 69, 70, 74, 75, 77, 78, 81, 82, 85, 86, 87, 91, 93, 94, 95, 99, 102, 106, 110, 111, 114, 115, 117, 118, 119, 121, 122, 123, 125, 129, 130, 133, 134, 135, 138

Offset

1,1

Links

Amiram Eldar, Table of n, a(n) for n = 1..10000

Example

a(1) = 2 and U(2) = 1 = U(3) so 3 is not in the sequence.
U(4) = 1/2 so a(2) = 4.

Mathematica

u = <|1 -> 1|>; L = {2}; Do[v = EulerPhi[n]/(2 Floor[n/2]); If[! KeyExistsQ[u, v], AppendTo[L, n]; u[v] = 1], {n, 3, 140}]; L (* Giovanni Resta, Jan 06 2020 *)

Crossrefs

Cf. A000010.

Sequence in context: A003661 A065387 A219787 * A065388 A157936 A338348

Adjacent sequences: A331003 A331004 A331005 * A331007 A331008 A331009

Keyword

nonn

Author

Nick Bryant, Jan 05 2020

Status

approved