Bézout matrix
Matrix whose determinant is a resultant From Wikipedia, the free encyclopedia
In mathematics, a Bézout matrix (or Bézoutian or Bezoutiant) is a special square matrix associated with two polynomials, introduced by James Joseph Sylvester in 1853 and Arthur Cayley in 1857 and named after Étienne Bézout.[1][2] Bézoutian may also refer to the determinant of this matrix, which is equal to the resultant of the two polynomials. Bézout matrices are sometimes used to test the stability of a given polynomial.
Definition
Summarize
Perspective
Let and be two complex polynomials of degree at most n,
(Note that any coefficient or could be zero.) The Bézout matrix of order n associated with the polynomials f and g is
where the entries result from the identity
It is an n × n complex matrix, and its entries are such that if we let and for each , then:
To each Bézout matrix, one can associate the following bilinear form, called the Bézoutian:
Examples
- For n = 3, we have for any polynomials f and g of degree (at most) 3:
- Let and be the two polynomials. Then:
The last row and column are all zero as f and g have degree strictly less than n (which is 4). The other zero entries are because for each , either or is zero.
Properties
Applications
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Perspective
An important application of Bézout matrices can be found in control theory. To see this, let f(z) be a complex polynomial of degree n and denote by q and p the real polynomials such that f(iy) = q(y) + ip(y) (where y is real). We also denote r for the rank and σ for the signature of . Then, we have the following statements:
- f(z) has n − r roots in common with its conjugate;
- the left r roots of f(z) are located in such a way that:
- (r + σ)/2 of them lie in the open left half-plane, and
- (r − σ)/2 lie in the open right half-plane;
- f is Hurwitz stable if and only if is positive definite.
The third statement gives a necessary and sufficient condition concerning stability. Besides, the first statement exhibits some similarities with a result concerning Sylvester matrices while the second one can be related to Routh–Hurwitz theorem.
Citations
References
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