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 A051913 Numbers n such that phi(n)/phi(phi(n)) = 3. 4
 7, 9, 13, 14, 18, 19, 21, 26, 27, 28, 35, 36, 37, 38, 39, 42, 45, 52, 54, 56, 57, 63, 65, 70, 72, 73, 74, 76, 78, 81, 84, 90, 91, 95, 97, 104, 105, 108, 109, 111, 112, 114, 117, 119, 126, 130, 133, 135, 140, 144, 146, 148, 152, 153, 156, 162, 163, 168, 171, 180, 182 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Also numbers n such that phi(n) = 2^a*3^b with a, b > 0. Also numbers n such that a regular n-gon can be constructed using conics but not with merely a compass and straightedge. "Constructed using conics" means that one can draw any conic, once its focus, its vertex and a point on its directrix are constructed. Points at intersections are thereby constructed. (Videla's definition is slightly more complicated, but equivalent.) One can use parabolas to take cube roots; hyperbolas yield trisected angles. - Don Reble, Apr 23 2007 REFERENCES George E. Martin, Geometric Constructions, Springer, 1997, p. 140. LINKS T. D. Noe, Table of n, a(n) for n=1..1000 C. R. Videla, On points constructible from conics, Mathematical Intelligencer, 19, No. 2, pp. 53-57 (1997). FORMULA Numbers n of the form 2^a*3^b*p*q*r*..., where p, q, r, ... are distinct primes of the form 2^x*3^y + 1 (i.e., belong to A005109) and phi(n) is not a power of 2 [Videla]. - Robert G. Wilson v, Apr 05 2005 EXAMPLE Phi(999) = Phi(3*3*3*37) = 648 = 8*81. MATHEMATICA lf[x_] := Length[FactorInteger[x]] eu[x_] := EulerPhi[x] Do[s=lf[eu[n]]; If[Equal[s, 2]&&Equal[Mod[eu[n], 6], 0], Print[n]], {n, 1, 1000}] (* Labos Elemer, Dec 28 2001 *) f[n_] := Block[{m = n}, If[m > 0, While[ IntegerQ[m/2], m /= 2]; While[ IntegerQ[m/3], m /= 3]]; m == 1]; fQ[n_] := Block[{pff = Select[ FactorInteger[n], #[[1]] > 3 &]}, pf = Flatten[{2, Table[ #[[1]], {1}] & /@ pff}]; pfe = Union[ Flatten[{1, Table[ #[[2]], {1}] & /@ pff}]]; If[ Union[f /@ (pf - 1)] == {True} && pfe == {1} && !IntegerQ[ Log[2, EulerPhi[ n]]], True, False]]; Select[ Range[184], fQ[ # ] &] (* Robert G. Wilson v, Apr 05 2005 *) PROG (Magma) [n: n in [1..200] | EulerPhi(n)/EulerPhi(EulerPhi(n)) eq 3]; // Vincenzo Librandi, Apr 17 2015 CROSSREFS Cf. A000010, A003401, A003586, A058383. Sequence in context: A102306 A066962 A067020 * A180645 A196091 A329166 Adjacent sequences: A051910 A051911 A051912 * A051914 A051915 A051916 KEYWORD nonn,easy AUTHOR J. H. Conway and Antreas P. Hatzipolakis (xpolakis(AT)otenet.gr), Dec 17 1999 EXTENSIONS Additional comments from Labos Elemer, Dec 28 2001 Additional comments from Benoit Cloitre, Jan 26 2002 Edited by N. J. A. Sloane, Apr 21 2007 Entries checked by Don Reble, Apr 23 2007 STATUS approved

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Last modified July 25 12:21 EDT 2024. Contains 374588 sequences. (Running on oeis4.)