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A216325 Number of divisors of the degree of the minimal polynomial for 2*cos(Pi/n), n >= 1. 1
1, 1, 1, 2, 2, 2, 2, 3, 2, 3, 2, 3, 4, 4, 3, 4, 4, 4, 3, 4, 4, 4, 2, 4, 4, 6, 3, 6, 4, 4, 4, 5, 4, 5, 6, 6, 6, 6, 6, 5, 6, 6, 4, 6, 6, 4, 2, 5, 4, 6, 5, 8, 4, 6, 6, 8, 6, 6, 2, 5, 8, 8, 6, 6, 8, 6, 4, 6, 4, 8, 4, 8, 9, 9, 6, 9, 8, 8, 4, 6, 4, 8, 2, 8, 6, 8, 6, 8, 6, 8, 9, 6, 8, 4, 9, 6, 10, 8 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,4

COMMENTS

For the minimal polynomials C(n,x) of the algebraic number rho = 2*cos(Pi/n), n >= 1, see their coefficient table A187360. Their degree is delta(n)= phi(2*n)/2, if n >= 2, and delta(1) = 1, with Euler's totient A000010. The delta sequence is given in A055034. a(n) is the number of divisors of delta(n).

a(n) is also the number of distinct Modd n orders given in the table A216320 in row n. (For Modd n see a comment on A203571).

See the analog A062821(n), with the number of divisors of phi(n). The corresponding order table is A216327.

LINKS

Table of n, a(n) for n=1..98.

FORMULA

a(n) = tau(delta(n)), n >= 1, with tau = A000005 (number of divisors), delta defined in a comment above and given as delta(n) = A055034(n).

EXAMPLE

a(8) = 3 because C(8,x) = x^4 - 4*x^2 + 2, with degree delta(8) = A055034(8) = 4, and the three divisors of 4 are 1, 2 and 4. tau(4) = A000005(4) = 3.

CROSSREFS

Cf. A062821 (analog).

Sequence in context: A057525 A331362 A139325 * A322868 A240975 A242166

Adjacent sequences:  A216322 A216323 A216324 * A216326 A216327 A216328

KEYWORD

nonn

AUTHOR

Wolfdieter Lang, Sep 27 2012

STATUS

approved

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Last modified August 12 11:44 EDT 2020. Contains 336439 sequences. (Running on oeis4.)