Square-free polynomial

In mathematics, a square-free polynomial is a polynomial defined over a field (or more generally, an integral domain) that does not have as a divisor any square of a non-constant polynomial. A univariate polynomial is square free if and only if it has no multiple root in an algebraically closed field containing its coefficients. This motivates that in applications in physics and engineering, a square-free polynomial is commonly called a polynomial with no repeated roots.
In the case of univariate polynomials, the product rule implies that, if divides , then divides the formal derivative of . The converse is also true in characteristic zero, and for polynomials over a finite field (or, more generally, over a perfect field). That is, in these cases, a polynomial is square free if and only if is a greatest common divisor of the polynomial and its derivative.
A square-free decomposition or square-free factorization of a polynomial is a factorization into powers of square-free polynomials
where those of the that are non-constant are pairwise coprime square-free polynomials (here, two polynomials are said "coprime" is their greatest common divisor is a constant; in other words that is the coprimality over the field of fractions of the coefficients that is considered). Every non-zero polynomial a square-free factorization, which is unique up to the multiplication and division of the factors by non-zero constants. The square-free factorization is much easier to compute than the complete factorization into irreducible factors, and is thus often preferred when the complete factorization is not really needed, as for the partial fraction decomposition and the symbolic integration of rational fractions. Square-free factorization is the first step of the polynomial factorization algorithms that are implemented in computer algebra systems. Therefore, the algorithm of square-free factorization is basic in computer algebra.
Over a field of characteristic 0, the quotient of formula_2 by its GCD with its derivative is the product of the formula_3 in the above square-free decomposition. Over a perfect field of non-zero characteristic , this quotient is the product of the formula_3 such that is not a multiple of . Further GCD computations and exact divisions allow computing the square-free factorization (see square-free factorization over a finite field). In characteristic zero, a better algorithm is known, Yun's algorithm, which is described below. Its computational complexity is, at most, twice that of the GCD computation of the input polynomial and its derivative. More precisely, if formula_5 is the time needed to compute the GCD of two polynomials of degree formula_6 and the quotient of these polynomial by the GCD, then formula_7 is an upper bound for the time needed to compute the square free decomposition.
There are also known algorithms for the computation of the square-free decomposition of multivariate polynomials, that proceed generally by considering a multivariate polynomial as a univariate polynomial with polynomial coefficients, and applying recursively a univariate algorithm.
Yun's algorithm.
This section describes Yun's algorithm for the square-free decomposition of univariate polynomials over a field of characteristic 0. It proceeds by a succession of GCD computations and exact divisions.
The input is thus a non-zero polynomial "f", and the first step of the algorithm consists of computing the GCD "a"0 of "f" and its formal derivative "f"'. 
If
is the desired factorization, we have thus 
and 
If we set formula_12, formula_13 and formula_14, we get that
and
Iterating this process until formula_18 we find all the formula_19
This is formalized into an algorithm as follows:
The degree of formula_20 and formula_21 is one less than the degree of formula_22 As formula_2 is the product of the formula_24 the sum of the degrees of the formula_25 is the degree of formula_26 As the complexity of GCD computations and divisions increase more than linearly with the degree, it follows that the total running time of the "repeat" loop is less than the running time of the first line of the algorithm, and that the total running time of Yun's algorithm is upper bounded by twice the time needed to compute the GCD of formula_2 and formula_28 and the quotient of formula_2 and formula_28 by their GCD.
Square root.
In general, a polynomial has no square root. More precisely, most polynomials cannot be written as the square of another polynomial.
A polynomial has a square root if and only if all exponents of the square-free decomposition are even. In this case, the square root is obtained by dividing by 2 these exponents.
Thus the problem of deciding if a polynomial has a square root, and of computing it if it exists, is a special case of square-free factorization. 