OFFSET
1,2
COMMENTS
The coefficients of the minimal polynomials C(N,x) of the algebraic number 2*cos(Pi/N) are given in A187360, where N is n.
The degree of C(N,x) is delta(N) = 1 if N=1 and delta(N) = phi(2*N)/2 if N > 1, with Euler's totient function phi(n) = A000010(n).
The forbidden degree values are shown in the complement (relative to the positive integers) A079695.
The array of the values N (the indices) for which the degree delta(N) = a(n), n >= 1, is given in A207334.
FORMULA
a(n) gives the allowed degree values, called delta, of the minimal polynomials C ordered increasingly, For C and delta see the comment section.
EXAMPLE
a(8) = 9 because there is at least one polynomial C(N,x) with degree delta(N)=9. In fact the only N values are 19 and 27.
7 is no member of this sequence (it belongs to the complement A079695).
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Wolfdieter Lang, Feb 19 2012
STATUS
approved