A331006 - OEIS

The OEIS is supported by the many generous donors to the OEIS Foundation.

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).

Other ways to Give

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

(list; graph; refs; listen; history; text; internal format)

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,changed

AUTHOR

Nick Bryant, Jan 05 2020

STATUS

approved