OFFSET
1,4
COMMENTS
a(n) is always a power of 2.
It would appear that a(n) <= a(n+1) and that for a(n)=2^k, the count for k beginning with 0 is 3, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, ...; or that the count for k is k+2 for k > 0. - Robert G. Wilson v, Jul 31 2014
Apparently v_2(a(n)) = A052146(n-1) for n >= 2 where v_2 is the 2-adic valuation. - Joerg Arndt, Jul 29 2014 [incorrect for n >= 561, Joerg Arndt, Mar 03 2019]
LINKS
Robert G. Wilson v, Table of n, a(n) for n = 1..1632
Eric Weisstein's World of Mathematics, Trigonometry Angles
MATHEMATICA
f[n_] := Exponent[MinimalPolynomial[Cos[Pi/n]][x], x]; Table[ f@ n, {n, Select[ Range@ 1300, IntegerQ[ Log[2, EulerPhi[#]]] &]}] (* Robert G. Wilson v, Jul 28 2014 *)
A092506 = {2, 3, 5, 17, 257, 65537}; s = Sort[Times @@@ Subsets@ A092506]; mx = 2500; t = Union@ Flatten@ Table[(2^n)*s[[i]], {i, 64}, {n, 0, Log2[mx/s[[i]]]}]; f[n_] := EulerPhi[ 2n]/2; f[1] = 1; f@# & /@ t (* Robert G. Wilson v, Jul 28 2014 *)
CROSSREFS
KEYWORD
nonn,easy,nice
AUTHOR
Eric W. Weisstein, Oct 28 2005
STATUS
approved