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

1,1

Comments

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

Links

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

Mathematica

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

Crossrefs

Cf. A125866-A125878.

Sequence in context: A086149 A344515 A066502 * A076439 A341658 A168474

Adjacent sequences: A125867 A125868 A125869 * A125871 A125872 A125873

Keyword

nonn

Author

Artur Jasinski, Dec 13 2006

Extensions

Edited by Don Reble, Apr 24 2007

Status

approved