Cyclotomic polynomial
In mathematics, more specifically in algebra, the nth cyclotomic polynomial, for any positive integer n, is the unique irreducible polynomial with integer coefficients, which is a divisor of and is not a divisor of for any k < n. Its roots are all nth primitive roots of unity , where k runs over the positive integers not greater than n and coprime to n. In other words, the nth cyclotomic polynomial is equal to
It may also be defined as the monic polynomial with integer coefficients, which is the minimal polynomial over the field of the rational numbers of any primitive nthroot of unity ( is an example of such a root).
An important relation linking cyclotomic polynomials and primitive roots of unity is
showing that every root of is a dth primitive root of unity for some d that divides n.
Examples
If n is a prime number, then
If n=2p where p is an odd prime number, then
For n up to 30, the cyclotomic polynomials are:^{[1]}
The case of the 105th cyclotomic polynomial is interesting because 105 is the lowest integer that is the product of three distinct odd prime numbers and this polynomial is the first one that has a coefficient other than 1, 0, or 1:
Properties
Fundamental tools
The cyclotomic polynomials are monic polynomials with integer coefficients that are irreducible over the field of the rational numbers. Except for n equal to 1 or 2, they are palindromics of even degree.
The degree of , or in other words the number of nth primitive roots of unity, is , where is Euler's totient function.
The fact that is an irreducible polynomial of degree in the ring is a nontrivial result due to Gauss.^{[2]} Depending on the chosen definition, it is either the value of the degree or the irreducibility which is a nontrivial result. The case of prime n is easier to prove than the general case, thanks to Eisenstein's criterion.
A fundamental relation involving cyclotomic polynomials is
which means that each nth root of unity is a primitive dth root of unity for a unique d dividing n.
The Möbius inversion formula allows the expression of as an explicit rational fraction:
where is the Möbius function.
The Fourier transform of functions of the greatest common divisor together with the Möbius inversion formula gives:^{[3]}
The cyclotomic polynomial may be computed by (exactly) dividing by the cyclotomic polynomials of the proper divisors of n previously computed recursively by the same method:
(Recall that .)
This formula allows to compute on a computer for any n, as soon as integer factorization and division of polynomials are available. Many computer algebra systems have a built in function to compute the cyclotomic polynomials. For example, this function is called by typing cyclotomic_polynomial(n,'x')
in SageMath, numtheory[cyclotomic](n,x);
in Maple, and Cyclotomic[n,x]
in Mathematica.
Easy cases for computation
As noted above, if n is a prime number, then
If n is an odd integer greater than one, then
In particular, if n=2p is twice an odd prime, then (as noted above)
If n=p^{m} is a prime power (where p is prime), then
More generally, if n=p^{m}r with r relatively prime to p, then
These formulas may be applied repeatedly to get a simple expression for any cyclotomic polynomial in term of a cyclotomic polynomial of square free index: If q is the product of the prime divisors of n (its radical), then^{[4]}
This allows to give formulas for the nth cyclotomic polynomial when n has at most one odd prime factor: If p is an odd prime number, and h and k are positive integers, then:
For the other values of n, the computation of the nth cyclotomic polynomial is similarly reduced to that of where q is the product of the distinct odd prime divisors of n. To deal with this case, one has that, for p prime and not dividing n,^{[5]}
Integers appearing as coefficients
The problem of bounding the magnitude of the coefficients of the cyclotomic polynomials has been the object of a number of research papers.
If n has at most two distinct odd prime factors, then Migotti showed that the coefficients of are all in the set {1, −1, 0}.^{[6]}
The first cyclotomic polynomial for a product of 3 different odd prime factors is it has a coefficient −2 (see its expression above). The converse is not true: = only has coefficients in {1, −1, 0}.
If n is a product of more odd different prime factors, the coefficients may increase to very high values. E.g., = has coefficients running from −22 to 22, = , the smallest n with 6 different odd primes, has coefficients up to ±532.
Let A(n) denote the maximum absolute value of the coefficients of Φ_{n}. It is known that for any positive k, the number of n up to x with A(n) > n^{k} is at least c(k)⋅x for a positive c(k) depending on k and x sufficiently large. In the opposite direction, for any function ψ(n) tending to infinity with n we have A(n) bounded above by n^{ψ(n)} for almost all n.^{[7]}
Gauss's formula
Let n be odd, squarefree, and greater than 3. Then^{[8]}^{[9]}
where both A_{n}(z) and B_{n}(z) have integer coefficients, A_{n}(z) has degree φ(n)/2, and B_{n}(z) has degree φ(n)/2 − 2. Furthermore, A_{n}(z) is palindromic when its degree is even; if its degree is odd it is antipalindromic. Similarly, B_{n}(z) is palindromic unless n is composite and ≡ 3 (mod 4), in which case it is antipalindromic.
The first few cases are
Lucas's formula
Let n be odd, squarefree and greater than 3. Then^{[10]}
where both U_{n}(z) and V_{n}(z) have integer coefficients, U_{n}(z) has degree φ(n)/2, and V_{n}(z) has degree φ(n)/2 − 1. This can also be written
If n is even, squarefree and greater than 2 (this forces n to be ≡ 2 (mod 4)),
where both C_{n}(z) and D_{n}(z) have integer coefficients, C_{n}(z) has degree φ(n), and D_{n}(z) has degree φ(n) − 1. C_{n}(z) and D_{n}(z) are both palindromic.
The first few cases are:
Cyclotomic polynomials over Z_{p}
For any prime number p which does not divide n, the cyclotomic polynomial is irreducible over Z_{p} if and only if p is a primitive root modulo n. That is, the p does not divide n, and its multiplicative order modulo n is (which is also the degree of ).
Polynomial values
If x takes any real value, then for every n ≥ 2 (this follow from the fact that the roots of a cyclotomic polynomial are all nonreal, for n ≥ 2).
For studying the values that a cyclotomic polynomial may take when x is given an integer value, it suffices to consider only the case n ≥ 2, as the cases n = 1 and n = 2 are trivial (one has and ).
For n ≥ 2, one has
 if n is not a prime power,
 if is a prime power with k ≥ 0.
The values that a cyclotomic polynomial may take for other integer values of x is strongly related with the multiplicative order modulo a prime number.
More precisely, given a prime number p and an integer b coprime with p, the multiplicative order of b modulo p, is the smallest positive integer n such that p is a divisor of For b ≥ 2, the multiplicative order of b modulo p is also the shortest period of the representation of 1/p in the numeral base b (see Unique prime; this explains the notation choice).
The definition of the multiplicative order implies that, if n is the multiplicative order of b modulo p, then p is a divisor of The converse is not true, but one has the following.
If n > 0 is a positive integer and b is an integer different from 0 and 1, then (see below for a proof)
where
 k is a nonnegative integer, always equal to 0 when b is even.
 g is an odd squarefree integer, which is a divisor of n.
 h is odd, coprime with n, and its prime factors are exactly the odd primes p such that n is the multiplicative order of b modulo p.
This implies that, if p is an odd prime divisor of then either n is a divisor of p − 1 or p is a divisor of n. In the latter case does not divides
Zsigmondy's theorem implies that the only cases where b > 1 and h = 1 are
 for k ≥ 0,
 for k > 0,
It follows from above factorization that the odd prime factors of
are exactly the odd primes p such that n is the multiplicative order of b modulo p. This fraction may be even only when b is odd. In this case, the multiplicative order of b modulo 2 is always 1.
There are many pairs (n, b) such that is prime. In fact, Bunyakovsky conjecture implies that, for every n, there are infinitely many b such that is prime. See A085398 for the list of the smallest b such that is prime. See also A206864 for the list of the smallest primes of the form with n > 1, and, more generally, A206942, for the smallest positive integers of this form.
Proofs 


Applications
Using , one can give an elementary proof for the infinitude of primes congruent to 1 modulo n,^{[11]} which is a special case of Dirichlet's theorem on arithmetic progressions.
Let be a finite list of primes congruent to . Let and consider . Let be a prime factor of (to see that is not , decompose it into linear factors and note that 1 is the closest root of unity to ). Since , we know that is a new prime not in the list. We will show that .
Let be the order of . Since we have . Thus . We will show that .
Assume for contradiction that . Since we have for some . Then is a double root of . Thus must be a root of the derivative so . But and therefore . This is a contradiction so . The order of , which is , must divide . Thus .
See also
Notes
 ↑ (sequence A013595 in the OEIS)
 ↑ Lang, Serge (2002), Algebra, Graduate Texts in Mathematics, 211 (Revised third ed.), New York: SpringerVerlag, ISBN 9780387953854, MR 1878556
 ↑ Schramm, Wolfgang (2015). "Eine alternative Produktdarstellung für die Kreisteilungspolynome". Elemente der Mathematik. Swiss Mathematical Society. 70 (4): 137–143. Retrieved 20151010.
 ↑ Cox, David A. (2012), "Exercise 12", Galois Theory (2nd ed.), John Wiley & Sons, p. 237, doi:10.1002/9781118218457, ISBN 9781118072059.
 ↑ Weisstein, Eric W. "Cyclotomic Polynomial". Retrieved 12 March 2014.
 ↑ Isaacs, Martin (2009). Algebra: A Graduate Course. AMS Bookstore. p. 310. ISBN 9780821847992.
 ↑ Meier (2008)
 ↑ Gauss, DA, Articles 356357
 ↑ Riesel, pp. 315316, p. 436
 ↑ Riesel, pp. 309315, p. 443
 ↑ S. Shirali. Number Theory. Orient Blackswan, 2004. p. 67. ISBN 8173714541
References
Gauss's book Disquisitiones Arithmeticae has been translated from Latin into English and German. The German edition includes all of his papers on number theory: all the proofs of quadratic reciprocity, the determination of the sign of the Gauss sum, the investigations into biquadratic reciprocity, and unpublished notes.
 Gauss, Carl Friedrich (1986) [1801]. Disquisitiones Arithmeticae. Translated into English by Clarke, Arthur A. (2nd corr. ed.). New York: Springer. ISBN 0387962549.
 Gauss, Carl Friedrich (1965) [1801]. Untersuchungen uber hohere Arithmetik (Disquisitiones Arithmeticae & other papers on number theory). Translated into German by Maser, H. (2nd ed.). New York: Chelsea. ISBN 0828401918.
 Lemmermeyer, Franz (2000). Reciprocity Laws: from Euler to Eisenstein. Berlin: Springer. doi:10.1007/9783662128930. ISBN 9783642086281.
 Maier, Helmut (2008), "Anatomy of integers and cyclotomic polynomials", in De Koninck, JeanMarie; Granville, Andrew; Luca, Florian, Anatomy of integers. Based on the CRM workshop, Montreal, Canada, March 1317, 2006, CRM Proceedings and Lecture Notes, 46, Providence, RI: American Mathematical Society, pp. 89–95, ISBN 9780821844069, Zbl 1186.11010
 Riesel, Hans (1994). Prime Numbers and Computer Methods for Factorization (2nd ed.). Boston: Birkhäuser. ISBN 0817637435.
External links
 Hazewinkel, Michiel, ed. (2001), "Cyclotomic polynomials", Encyclopedia of Mathematics, Springer, ISBN 9781556080104
 "Sloane's A013595 : Triangle of coefficients of cyclotomic polynomial Phi_n(x) (exponents in increasing order)". The OnLine Encyclopedia of Integer Sequences. OEIS Foundation.
 "Sloane's A013594 : Smallest order of cyclotomic polynomial containing n or −n as a coefficient". The OnLine Encyclopedia of Integer Sequences. OEIS Foundation.