A125870 Odd numbers k such that cos(2*Pi/k) is an algebraic number of a 7-smooth degree, but not 5-smooth.

29, 43, 49, 71, 87, 113, 127, 129, 145, 147, 197, 203, 211, 213, 215, 245, 261, 281, 301, 319, 337, 339, 343, 355, 377, 379, 381, 387, 421, 435, 441, 449, 473, 491, 493, 497, 539, 551, 559, 565, 591, 609, 631, 633, 635, 637, 639, 645, 673, 701, 725, 731

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, 5-sector and 7-sector.

Crossrefs

Cf. A125866-A125878.

Programs

Mathematica

Do[If[Take[FactorInteger[EulerPhi[2n+1]][[ -1]],1]=={7},Print[2n+1]],{n,1,1000}]

Extensions

Edited by Don Reble, Apr 24 2007