A125871 - OEIS

A125871

Numbers n such that p=14n+1 is prime and cos(2pi/p) is an algebraic number of a 7-smooth degree, but not 5-smooth.

2, 3, 5, 8, 9, 14, 15, 20, 24, 27, 30, 32, 35, 45, 48, 50, 54, 63, 72, 75, 98, 105, 144, 162, 180, 189, 192, 200, 224, 240, 252, 300, 320, 420, 450, 500, 504, 525, 540, 560, 588, 630, 750, 768, 864, 875, 900, 960, 980, 1029, 1080, 1134, 1215, 1280, 1323

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OFFSET

1,1

COMMENTS

Numbers n such that p=14n+1 is prime and the greatest prime divisor of p-1 is 7.

LINKS

Table of n, a(n) for n=1..55.

MATHEMATICA

Do[If[Take[FactorInteger[EulerPhi[14n+1]][[ -1]], 1]=={7} && PrimeQ[14n+1], Print[n]], {n, 1, 10000}] (*Artur Jasinski*)

CROSSREFS

Cf. A125866-A125878.

Sequence in context: A270151 A009388 A190803 * A141399 A104737 A286495