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A125868
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Odd numbers n such that cos(2pi/n) is an algebraic number of a 5-smooth degree, but not 3-smooth.
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0
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11, 25, 31, 33, 41, 55, 61, 75, 77, 93, 99, 101, 123, 125, 143, 151, 155, 165, 175, 181, 183, 187, 205, 209, 217, 225, 231, 241, 251, 271, 275, 279, 287, 297, 303, 305, 325, 341, 369, 375, 385, 401, 403, 407, 425, 427, 429, 451, 453, 465, 475, 495, 505, 525
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| A regular polygon of a(n) sides can be constructed if one also has an angle trisector and 5-sector.
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MATHEMATICA
| Do[If[Take[FactorInteger[EulerPhi[2n+1]][[ -1]], 1]=={5}, Print[2n+1]], {n, 1, 1000}]
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CROSSREFS
| Cf. A058383, A061599, A125866-A125878.
Sequence in context: A108302 A182689 A084547 * A031025 A140675 A161532
Adjacent sequences: A125865 A125866 A125867 * A125869 A125870 A125871
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KEYWORD
| nonn
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AUTHOR
| Artur Jasinski (grafix(AT)csl.pl), Dec 13 2006
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EXTENSIONS
| Edited by Don Reble (djr(AT)nk.ca), Apr 24 2007
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