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 A125875 Odd numbers n such that cos(2pi/n) is an algebraic number of a 13-smooth degree, but not 11-smooth. 2
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS A regular polygon of a(n) sides can be constructed if one also has an angle trisector, 5-, 7-, 11- and 13-sector. LINKS MATHEMATICA Do[If[Take[FactorInteger[EulerPhi[2n+1]][[ -1]], 1]=={13}, Print[2n+1]], {n, 1, 10000}] CROSSREFS Cf. A125866-A125878. Sequence in context: A043992 A032421 A129257 * A059245 A268753 A125876 Adjacent sequences:  A125872 A125873 A125874 * A125876 A125877 A125878 KEYWORD nonn AUTHOR Artur Jasinski, Dec 13 2006 EXTENSIONS Edited by Don Reble, Apr 24 2007 STATUS approved

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Last modified December 8 20:37 EST 2021. Contains 349596 sequences. (Running on oeis4.)