A125869 Numbers k such that p=10*k+1 is prime and cos(2*Pi/p) is an algebraic number of a 5-smooth degree, but not 3-smooth.

1, 3, 4, 6, 10, 15, 18, 24, 25, 27, 40, 54, 60, 64, 75, 81, 120, 160, 162, 180, 216, 225, 300, 400, 405, 480, 486, 648, 768, 810, 864, 900, 960, 972, 1125, 1440, 1536, 1600, 1944, 2160, 2187, 2250, 2304, 2400, 2560, 3240, 3375, 3645, 3750, 4096, 4320

Offset: 1

Author

Artur Jasinski, Dec 13 2006

Keywords

• nonn

Comments

Numbers k such that p=10*k+1 is prime and the greatest prime divisor of p-1 is 5.

Crossrefs

Cf. A024912, A125866-A125878.

Programs

Mathematica

Do[If[Take[FactorInteger[EulerPhi[10n+1]][[ -1]],1]=={5} && PrimeQ[10n+1],Print[n]],{n,1,10000}]

Extensions

Edited by Don Reble, Apr 24 2007