A125872 Odd numbers k such that cos(2*Pi/k) is an algebraic number of an 11-smooth degree, but not 7-smooth.

23, 67, 69, 89, 115, 121, 161, 199, 201, 207, 253, 267, 299, 331, 335, 345, 353, 363, 391, 397, 437, 445, 463, 469, 483, 575, 597, 603, 605, 617, 621, 623, 661, 667, 713, 727, 737, 759, 801, 805, 847, 851, 871, 881, 897, 943, 979, 989, 991, 993, 995, 1005

Offset

1,1

Comments

A regular polygon of a(n) sides can be constructed if one also has an angle trisector, 5-sector, 7-sector and 11-sector.

Links

Table of n, a(n) for n=1..52.

Mathematica

Do[If[Take[FactorInteger[EulerPhi[2n+1]][[ -1]], 1]=={11}, Print[2n+1]], {n, 1, 10000}]]

Crossrefs

Cf. A125866-A125878.

Sequence in context: A323220 A001346 A051875 * A228611 A104945 A141849

Adjacent sequences: A125869 A125870 A125871 * A125873 A125874 A125875

Keyword

nonn

Author

Artur Jasinski, Dec 13 2006

Extensions

Edited by Don Reble, Apr 24 2007

Status

approved