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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.
0
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
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
KEYWORD
nonn
AUTHOR
Nick Bryant, Jan 05 2020
STATUS
approved