login
This site is supported by donations to The OEIS Foundation.

 

Logo

Annual Appeal: Please make a donation (tax deductible in USA) to keep the OEIS running. Over 5000 articles have referenced us, often saying "we discovered this result with the help of the OEIS".

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A125866 Odd numbers n such that cos(2pi/n) is an algebraic number of a 3-smooth degree, but not 2-smooth. 14
7, 9, 13, 19, 21, 27, 35, 37, 39, 45, 57, 63, 65, 73, 81, 91, 95, 97, 105, 109, 111, 117, 119, 133, 135, 153, 163, 171, 185, 189, 193, 195, 219, 221, 243, 247, 259, 273, 285, 291, 315, 323, 327, 333, 351, 357, 365, 399, 405, 433, 455, 459, 481, 485, 487, 489 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Odd terms of A051913.

This sequence is infinite (unlike A004729), because it contains any A058383(n) times any power of 3.

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

LINKS

Table of n, a(n) for n=1..56.

MATHEMATICA

Do[If[Take[FactorInteger[EulerPhi[2n+1]][[ -1]], 1]=={3}, Print[2n+1]], {n, 1, 10000}]

CROSSREFS

Cf. A004729, A051913, A058383, A125866-A125878.

Sequence in context: A222945 A129069 A258616 * A027692 A185720 A032487

Adjacent sequences:  A125863 A125864 A125865 * A125867 A125868 A125869

KEYWORD

nonn

AUTHOR

Artur Jasinski, Dec 13 2006

EXTENSIONS

Edited by Don Reble (djr(AT)nk.ca), Apr 24 2007

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy .

Last modified December 8 19:05 EST 2016. Contains 278948 sequences.