# Fast calculation of cyclotomic polynomials¶

This module provides a function cyclotomic_coeffs(), which calculates the coefficients of cyclotomic polynomials. This is not intended to be invoked directly by the user, but it is called by the method cyclotomic_polynomial() method of univariate polynomial ring objects and the top-level cyclotomic_polynomial() function.

sage.rings.polynomial.cyclotomic.bateman_bound(nn)

Reference:

Bateman, P. T.; Pomerance, C.; Vaughan, R. C. On the size of the coefficients of the cyclotomic polynomial.

sage.rings.polynomial.cyclotomic.cyclotomic_coeffs(nn, sparse=None)

This calculates the coefficients of the n-th cyclotomic polynomial by using the formula

$\Phi_n(x) = \prod_{d|n} (1-x^{n/d})^{\mu(d)}$

where $$\mu(d)$$ is the Möbius function that is 1 if d has an even number of distinct prime divisors, -1 if it has an odd number of distinct prime divisors, and 0 if d is not squarefree.

Multiplications and divisions by polynomials of the form $$1-x^n$$ can be done very quickly in a single pass.

If sparse is True, the result is returned as a dictionary of the non-zero entries, otherwise the result is returned as a list of python ints.

EXAMPLES:

sage: from sage.rings.polynomial.cyclotomic import cyclotomic_coeffs
sage: cyclotomic_coeffs(30)
[1, 1, 0, -1, -1, -1, 0, 1, 1]
sage: cyclotomic_coeffs(10^5)
{0: 1, 10000: -1, 20000: 1, 30000: -1, 40000: 1}
sage: R = QQ['x']
sage: R(cyclotomic_coeffs(30))
x^8 + x^7 - x^5 - x^4 - x^3 + x + 1


Check that it has the right degree:

sage: euler_phi(30)
8
sage: R(cyclotomic_coeffs(14)).factor()
x^6 - x^5 + x^4 - x^3 + x^2 - x + 1


The coefficients are not always +/-1:

sage: cyclotomic_coeffs(105)
[1, 1, 1, 0, 0, -1, -1, -2, -1, -1, 0, 0, 1, 1, 1, 1, 1, 1, 0, 0, -1, 0, -1, 0, -1, 0, -1, 0, -1, 0, 0, 1, 1, 1, 1, 1, 1, 0, 0, -1, -1, -2, -1, -1, 0, 0, 1, 1, 1]


In fact the height is not bounded by any polynomial in n (Erdos), although takes a while just to exceed linear:

sage: v = cyclotomic_coeffs(1181895)
sage: max(v)
14102773


The polynomial is a palindrome for any n:

sage: n = ZZ.random_element(50000)
sage: factor(n)
3 * 10009
sage: v = cyclotomic_coeffs(n, sparse=False)
sage: v == list(reversed(v))
True


AUTHORS:

• Robert Bradshaw (2007-10-27): initial version (inspired by work of Andrew Arnold and Michael Monagan)
sage.rings.polynomial.cyclotomic.cyclotomic_value(n, x)

Return the value of the $$n$$-th cyclotomic polynomial evaluated at $$x$$.

INPUT:

• n – an Integer, specifying which cyclotomic polynomial is to be evaluated.
• x – an element of a ring.

OUTPUT:

• the value of the cyclotomic polynomial $$\Phi_n$$ at $$x$$.

ALGORITHM:

• Reduce to the case that $$n$$ is squarefree: use the identity
$\Phi_n(x) = \Phi_q(x^{n/q})$

where $$q$$ is the radical of $$n$$.

• Use the identity
$\Phi_n(x) = \prod_{d | n} (x^d - 1)^{\mu(n / d)},$

where $$\mu$$ is the Möbius function.

• Handles the case that $$x^d = 1$$ for some $$d$$, but not the case that $$x^d - 1$$ is non-invertible: in this case polynomial evaluation is used instead.

EXAMPLES:

sage: cyclotomic_value(51, 3)
1282860140677441
sage: cyclotomic_polynomial(51)(3)
1282860140677441


It works for non-integral values as well:

sage: cyclotomic_value(144, 4/3)
79148745433504023621920372161/79766443076872509863361
sage: cyclotomic_polynomial(144)(4/3)
79148745433504023621920372161/79766443076872509863361