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 A125877 Numbers n such that p=26n+1 is prime and cos(2pi/p) is an algebraic number of a 13-smooth degree, but not 11-smooth. 2
 2, 3, 5, 6, 12, 20, 21, 26, 33, 35, 36, 42, 45, 48, 50, 72, 75, 77, 78, 80, 90, 98, 105, 110, 120, 125, 128, 132, 135, 143, 147, 156, 182, 192, 225, 231, 252, 260, 275, 288, 297, 308, 315, 330, 336, 351, 363, 378, 390, 392, 405, 441, 450, 455, 486, 500, 507, 512 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Numbers n such that p=26n+1 is prime and the greatest prime divisor of p-1 is 13. LINKS Harvey P. Dale, Table of n, a(n) for n = 1..1000 MATHEMATICA Do[If[Take[FactorInteger[EulerPhi[26n+1]][[ -1]], 1]=={13} && PrimeQ[26n+1], Print[n]], {n, 1, 10000}] (*Artur Jasinski*) Select[Range[600], PrimeQ[26#+1]&&FactorInteger[26#][[-1, 1]]==13&] (* Harvey P. Dale, Jun 01 2019 *) CROSSREFS Cf. A125866-A125878. Sequence in context: A179333 A128958 A007435 * A118787 A191783 A098930 Adjacent sequences:  A125874 A125875 A125876 * A125878 A125879 A125880 KEYWORD nonn AUTHOR Artur Jasinski, Dec 13 2006 EXTENSIONS Edited by Don Reble, Apr 24 2007 STATUS approved

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Last modified April 13 16:27 EDT 2021. Contains 342936 sequences. (Running on oeis4.)