A125868 Odd numbers k such that cos(2*Pi/k) is an algebraic number of a 5-smooth degree, but not 3-smooth.

11, 25, 31, 33, 41, 55, 61, 75, 77, 93, 99, 101, 123, 125, 143, 151, 155, 165, 175, 181, 183, 187, 205, 209, 217, 225, 231, 241, 251, 271, 275, 279, 287, 297, 303, 305, 325, 341, 369, 375, 385, 401, 403, 407, 425, 427, 429, 451, 453, 465, 475, 495, 505, 525

Offset: 1

Author

Artur Jasinski, Dec 13 2006

Keywords

• nonn

Comments

A regular polygon of a(n) sides can be constructed if one also has an angle trisector and 5-sector.

Crossrefs

Cf. A058383, A061599, A125866-A125878.

Programs

Mathematica

Do[If[Take[FactorInteger[EulerPhi[2n+1]][[ -1]],1]=={5},Print[2n+1]],{n,1,1000}]

Extensions

Edited by Don Reble, Apr 24 2007