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A271983 The smaller of a pair n, m such that phi(n) = phi(m) and there is no other k such that phi(n) = phi(k). 0
1, 11, 23, 29, 31, 47, 53, 81, 59, 67, 71, 79, 83, 103, 107, 121, 127, 131, 137, 139, 149, 151, 167, 173, 179, 191, 197, 199, 211, 223, 227, 229, 239, 251, 263, 269, 271, 283, 293, 343, 307, 311, 317, 331, 361, 347, 359, 367, 373, 379, 383, 389, 419, 431, 439, 443, 463, 467, 479, 491, 499 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

If phi(x) = N has exactly two solutions, x = n and x = m, say (see A007366), it is conjectured that one of n and m is odd and the other even.

This sequence differs from A058340 in that it contains nonprime integers.  The first few are 81, 121, 343, 361, 529, 649, 841, 961, 1219, 1331, 1537, 1633, ...

LINKS

Table of n, a(n) for n=1..61.

EXAMPLE

81 is a term because phi(81) = phi(162) = 54 (= A007366(8)).

MATHEMATICA

(* takes about 2 minutes, can return the sequence up to terms less than 5760=Euler phi(13 primorial) *)

Prepend[Select[

   Table[Flatten[Position[Table[EulerPhi[n], {n, 1, 30030}], m]], {m,

     2, 500, 2}], Length[#] == 2 &][[All, 1]], 1]

CROSSREFS

Cf. A007366, A058340.

Sequence in context: A086102 A058340 A138537 * A136000 A054723 A109981

Adjacent sequences:  A271980 A271981 A271982 * A271984 A271985 A271986

KEYWORD

nonn

AUTHOR

Geoffrey Critzer, Apr 17 2016

EXTENSIONS

Edited by N. J. A. Sloane, Apr 22 2016 at the suggestion of Franklin T. Adams-Watters.

STATUS

approved

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Last modified December 5 12:24 EST 2021. Contains 349557 sequences. (Running on oeis4.)