OFFSET
1,3
COMMENTS
See a comment on A216325 on the degree delta(n) = A055034(n) of the polynomial C(n,x) of 2*cos(Pi/n) (coefficients in A187360), Here n is prime.
For p prime, delta(p) = (p - 1)/2 if p > 2 and 1 if p = 2. a(n) is the number of divisors of delta(prime(n)), with prime(n) = A000040(n).
a(n) is also the number of distinct Modd p orders, p = prime, in row prime(n) of the table A216320. (For Modd n see a comment on A203571).
See also A008328 for the mod p analog of this sequence.
FORMULA
EXAMPLE
a(6) = 4 because prime(6) = 13, and row n=13 of A216320 is [1 3 2 6 3 6] with 4 distinct numbers (Modd 13 orders).
PROG
(PARI) delta(n) = if (n==1, 1, eulerphi(2*n)/2); \\ A055034
a(n) = numdiv(delta(prime(n))); \\ Michel Marcus, Sep 12 2023
CROSSREFS
KEYWORD
nonn
AUTHOR
Wolfdieter Lang, Sep 27 2012
STATUS
approved