A007367 - OEIS

%I M2163 #78 Nov 17 2024 07:18:28

%S 2,44,56,92,104,116,140,164,204,212,260,296,332,344,356,380,392,444,

%T 452,476,524,536,564,584,588,620,632,684,692,716,744,764,776,836,860,

%U 884,932,956,980,1004,1016,1112,1124,1136,1172,1196,1284,1292,1304

%N Numbers k such that phi(x) = k has exactly 3 solutions.

%C From _Torlach Rush_, Jul 23 2018: (Start)

%C For known terms:

%C - 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.

%C - 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.

%C - 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)

%C By Ford's theorem on Euler totient function, this sequence is infinite. - _Jianing Song_, Jul 18 2018

%D 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.

%D Jean-Marie De Koninck, Ces nombres qui nous fascinent, Entry 44, p. 17, Ellipses, Paris, 2008.

%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

%H Amiram Eldar, <a href="/A007367/b007367.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..1000 from T. D. Noe)

%H M. Abramowitz and I. A. Stegun, eds., <a href="http://www.convertit.com/Go/ConvertIt/Reference/AMS55.ASP">Handbook of Mathematical Functions</a>, National Bureau of Standards, Applied Math. Series 55, Tenth Printing, 1972 [alternative scanned copy].

%H Max Alekseyev, <a href="https://oeis.org/wiki/User:Max_Alekseyev/gpscripts">PARI/GP Scripts for Miscellaneous Math Problems</a> (invphi.gp).

%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Euler%27s_totient_function#Ford&#39;s_theorem">Ford's theorem</a>.

%H Robert G. Wilson v, <a href="/A007015/a007015.pdf">Letter to N. J. A. Sloane, Jul. 1992</a>.

%e phi(69) = phi(92) = phi(138) = 44, so 44 is a term.

%t a = Table[ 0, {1500} ]; Do[ p = EulerPhi[ n ]; If[ p < 1501, a[ [ p ] ]++ ], {n, 1, 1500} ]; Select[ Range[ 1500 ], a[ [ # ] ] == 3 & ]

%t Take[Select[Tally[EulerPhi[Range[50000]]],#[[2]]==3&][[All,1]],50]//Sort (* _Harvey P. Dale_, Apr 02 2018 *)

%o (Haskell)

%o a007367 n = a007367_list !! (n-1)

%o a007367_list = map fst $

%o                filter ((== 3) . snd) $ zip a002202_list a058277_list

%o -- _Reinhard Zumkeller_, Nov 25 2015

%o (PARI) is(k) = invphiNum(k) == 3 \\ _Amiram Eldar_, Nov 17 2024, using _Max Alekseyev_'s invphi.gp

%Y Cf. A000010, A002202, A058277, A085713.

%Y Number of solutions: A007617 (0), A007366 (2), this sequence (3), A060667 (4), A060668 (5), A060669 (6), A060670 (7), A060671 (8), A060672 (9), A060673 (10), A060674 (11), A060675 (12).

%K nonn

%O 1,1

%A _N. J. A. Sloane_, _Mira Bernstein_, _Robert G. Wilson v_