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 A007367 Numbers k such that phi(x) = k has exactly 3 solutions. (Formerly M2163) 14
 2, 44, 56, 92, 104, 116, 140, 164, 204, 212, 260, 296, 332, 344, 356, 380, 392, 444, 452, 476, 524, 536, 564, 584, 588, 620, 632, 684, 692, 716, 744, 764, 776, 836, 860, 884, 932, 956, 980, 1004, 1016, 1112, 1124, 1136, 1172, 1196, 1284, 1292, 1304 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS From Torlach Rush, Jul 23 2018: (Start) For known terms: - The greatest common divisor of the three solutions is the distance of the middle solution from the least solution and is half the distance of the middle solution to the largest solution. - If the number of distinct prime factors of k equals the number of solutions of k = phi(x), then the greatest common divisor of the solutions is the least solution divided by the number of solutions. - Except for a(1), if the largest prime factor is the same for all solutions and is equal to the greatest common divisor of all solutions then the distance from a(n) to the least solution is gcd({k: phi(k) = a(n)}) + 2.  (End) By Ford's theorem on Euler totient function, this sequence is infinite. - Jianing Song, Jul 18 2018 REFERENCES M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards Applied Math. Series 55, 1964 (and various reprintings), p. 840. J.-M. De Koninck, Ces nombres qui nous fascinent, Entry 44, p. 17, Ellipses, Paris 2008. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS T. D. Noe, Table of n, a(n) for n = 1..1000 M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards, Applied Math. Series 55, Tenth Printing, 1972 [alternative scanned copy]. Wikipedia, Ford's theorem R. G. Wilson, V, Letter to N. J. A. Sloane, Jul. 1992 EXAMPLE phi(69) = phi(92) = phi(138) = 44, so 44 is a term. MATHEMATICA a = Table[ 0, {1500} ]; Do[ p = EulerPhi[ n ]; If[ p < 1501, a[ [ p ] ]++ ], {n, 1, 1500} ]; Select[ Range[ 1500 ], a[ [ # ] ] == 3 & ] Take[Select[Tally[EulerPhi[Range]], #[]==3&][[All, 1]], 50]//Sort (* Harvey P. Dale, Apr 02 2018 *) PROG (Haskell) a007367 n = a007367_list !! (n-1) a007367_list = map fst \$                filter ((== 3) . snd) \$ zip a002202_list a058277_list -- Reinhard Zumkeller, Nov 25 2015 CROSSREFS Cf. A000010, A007366, A060667-A060671. Cf. A002202, A058277, A085713. Sequence in context: A156478 A156508 A239865 * A006313 A059737 A123829 Adjacent sequences:  A007364 A007365 A007366 * A007368 A007369 A007370 KEYWORD nonn AUTHOR STATUS approved

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Last modified September 26 12:07 EDT 2020. Contains 337371 sequences. (Running on oeis4.)