A125875 Odd numbers k such that cos(2*Pi/k) is an algebraic number of a 13-smooth degree, but not 11-smooth.

53, 79, 131, 157, 159, 169, 237, 265, 313, 371, 393, 395, 471, 477, 507, 521, 547, 553, 583, 655, 677, 689, 711, 785, 795, 845, 859, 869, 901, 911, 917, 937, 939, 1007, 1027, 1093, 1099, 1113, 1171, 1179, 1183, 1185, 1219, 1249, 1301, 1325, 1343

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-, 7-, 11- and 13-sector.

Crossrefs

Cf. A125866-A125878.

Programs

Mathematica

Do[If[Take[FactorInteger[EulerPhi[2n+1]][[ -1]],1]=={13},Print[2n+1]],{n,1,10000}]

Extensions

Edited by Don Reble, Apr 24 2007