1. Introduction
In this paper we present the first part, of a series of three works, on a new approach about the classification of the roots of real polynomials in one variable in the right half complex plane. This new idea arises from the need to obtain simple explicit criteria for the area of the complex plane not covered by the theory of Hurwitz polynomials (also known as stable polynomials). In fact, our results are natural extensions of the classical Theorems of Routh-Hurwitz and Hermite-Biehler for the complement zone;
.
In the literature we highlight as main references for the study of roots of real polynomials on the left half complex plane and its applications to system theory in a general framework the books of Gantmacher [2] [3], and the book of Iooss and Joseph [4]. Chappellat, Mansour and Brattacharyya present classic stability criteria with elementary demonstrations in their article [5] while new and interesting ideas about the demonstration of these results have been developed by Holtz in [6]. For a generalization to real polynomials in several variables we mention the work of Fettweis in [7]. The approach introduced in this work consist of a systematic use of the linear transformation
, on the properties that define the Hurwitz polynomials, which leads us to use and explore the notion originally introduced by Vergara-Hermosilla et al. in [1] about anti-Hurwitz polynomials. This notion can be recast as a dual result to the main necessary and sufficient conditions on stable polynomials. What is more, our Theorems and Propositions also depend on the coefficients of the polynomial in question which makes it more manipulable for applications in science and engineering. To this end, in Section 5 we will apply our results to a family of polynomials associated with a system of PDE's that describe interactions fluid-structure, for details see [1] or Vergara-Hermosilla [8]. With this preamble,, we are in a position of establish our first main result, which read as:
Proposition. Let
of degree
. Then
is an anti-Hurwitz polynomial, if and only if it satisfies the conditions:
1)
, for all
.
2)
, for all
.
As the practical use of of the Routh-Hurwitz criterion is usually limited, in the context of direct computations, to polynomials of low degrees (3rd, 4th, or 5th), we develop an alternatives result, which is more versatile and, as in the previous case, this extends to the dual zone the classical Boundary Crossing Theorems. More precisely, we will prove our second main result which is dual version of the Zero Exclusion Principle. Our second main result reads as:
Theorem. Let
the set of the real anti-Hurwitz polynomials. Suppose
is a family of real polinomials in the variable X wich depends continuously on the
, with
pathwise connected. Suppose moreover that the family
is of degree constant and there is at least one anti-Hurwitz polynomial. Then, the family
, if and only if
, for all
and
.
With this result, and by defining an appropriate property of anti-alternancy, we will demonstrate the third main result in this paper, which is a dual version of the Theorem of Hermite-Biehler. This third main result reads as:
Theorem. A real polinomial
is Anti-Hurwitz, if and only if satisfies the anti-alternancy property.
The outline of the paper is organized as follow. In Section 2 we state the main definitions and properties that describe the Hurwitz polynomials emphasizing the Hurwitz matrix and the Theorem of Routh-Hurwitz. In Section 3 we define the anti-Hurwitz polynomials, demonstrate our first main result, and establish explicit criteria for real polynomials of less than or equal order 4 and derivatives. In Section 4 we introduce the dual versions of the classical Boundary Crossing results and we proof our second and third main results. Finally, in Section 5 we apply our results for obtaining information about the behavior of the roots of the family of viscous polynomials defined in [1].
2. Hurwitz Polynomials
For
we denote by
the set of all degree n polynomials with real coefficients.
Definition 2.1. A polynomial
is Hurwitz if the real part of all its complex roots is negative i.e.,
for any
satisfying
.
Let
denote the set of all Hurwitz polynomials, and we set
. The set of all Hurwitz polynomials in
with positive coefficients is denoted by
.
Theorem 1 (Stodola condition). If a polynomial
is Hurwitz, then all its coefficients are of the same sign.
Proof. The roots of a real polynomial are symmetric with respect to the real line. For
, we can write
(2.1)
where each
are real roots, and
are complex roots of
with nonzero imaginary part. Note that
are negative. Since the expressions
and
have positive coefficients, their product has the same property.
Let
be a polynomial. The Hurwitz matrix of a polynomial, denoted as
, is the square matrix of size n defined as follows:
(2.2)
For every
, let
denote the square matrix of size k obtained from the first k rows and columns of
, and we set:
(2.3)
where
denotes the determinant of the square matrix
.
Theorem 2 (Routh-Hurwitz). A polynomial
with
is Hurwitz if and only if
for all
.
For a proof of this result see for instance [2] [4] or [6].
3. Anti-Hurwitz Polynomials
In this section we establish the definition of anti-Hurwitz polynomials and a dual criterion to the Theorem of Routh-Hurwitz. To this end, we introduce the following definition.
Definition 3.1. A polynomial
is said to be anti-Hurwitz if the real part of all its complex roots is positive, i.e.,
for all
satisfying
.
Lemma 3. A polynomial
is anti-Hurwitz if and only if
is Hurwitz.
Proof. Let
be an anti-Hurwitz polynomial and u a complex root of
. Then
and
, i.e.,
. Therefore
is Hurwitz. On the other hand, if
is a Hurwitz polynomial and u a complex root of
, then
. In this case,
, i.e.,
. Hence,
is anti-Hurwitz.
Lemma 4. Let
be a polynomial of degree n and
the determinant of the Hurwitz submatrix
, for
. Then we have
(3.1)
where
is the determinant of i-th Hurwitz submatrix
.
Proof. The matrix for
is written as
(3.2)
Comparing it with the matrix of
, we immediately see that
(3.3)
Proposition 5. Let
of degree
. Then
is an anti-Hurwitz polynomial, if and only if it satisfies the conditions:
1)
, for all
.
2)
, for all
.
Proof. By lemma (3), we know that
is an anti-Hurwitz polynomial if and only if
is a Hurwitz polynomial. In this case, the coefficient of
in
is
. Without loss of generality, we may suppose that
. Now the Stodola Condition (1) and Theorem (2), gives us that
, for
and
. Hence, we conclude by Lemma (4).
In the following we establish simple criteria on the property of anti-Hurwitz, applicable to real polynomials of less than or equal order 4 and derivatives of polynomials. To this end we consider a polynomial
and the necessary and sufficient conditions developed in the Proposition 5. The criteria read as:
• The polynomial
is an anti-Hurwitz polynomial, if and only if
(3.4)
• The polynomial
is an anti-Hurwitz polynomial, if and only if
(3.5)
• The polynomial
is an anti-Hurwitz polynomial, if and only if
(3.6)
• Let
be an anti-Hurwitz polynomial of degree n and let
denote the first-order derivative of
with respect to X. Then
is again an anti-Hurwitz polynomial.
4. A Dual Version of the Theorem of Hermite-Biehler
In this Section we establish a dual version of the Theorem of Hermite-Biehler for anti-Hurwitz polynomials. To this end, we need to introduce dual versions of Boundary Crossing Theorems. We begin the Section with the following definition.
Definition 4.1. Let
and
. The argument of
is called the phase of
.
Lemma 6. Let
be an anti-Hurwitz polynomial of degree n. Then,
is a strictly decreasing function. Moreover, the net change in the phase from
to
is
(4.1)
Proof. By the fundamental theorem of algebra, we can write
as a product of its roots
Plugging
, we get
and so, we obtain
(4.2)
Differentiating the above expression with respect to w, we get
Since
is an anti-Hurwitz polynomial, we have that
for
. Therefore,
is decreasing is a decreasing function in w. Now, from Equation (4.2), we have
The claim now follows.
In the following we enunciate two classic results on stability, whose demonstrations can be consulted in the article of Chappellat et al. [5].
Proposition 7. Let
,
, and
. Consider the circle
of radius
centered at
. Let
be fixed such that
, for
. Then, there exists an
such that for all
,
has precisely
zeros inside the circle
.
Corollary 1. Fix m circles
that are pairwise disjoint and centered at
respectively. Then, by repeatedly applying Theorem (7), it is always possible to find an
such that for any
,
has precisely
zeros inside each of the circles
.
Remark. In the previous Corollary, we note that
always has
zeros and must therefore remain of degree n, so necessarily we have
.
In the following we denote the set of anti-Hurwitz polynomials of degree n by
.
Remark. By Proposition (7), Corollary (1) and Remark (4), we see that if
, then there exists an
such that for all
, the polynomial
.
Boundary Crossing Theorems
Let
be a family of degree n polynomials with real coefficients, which is continuous with respect to
. In other words,
can be written as
where
are continuous functions in
and
for all
.
Theorem 8. Suppose that
has all its roots in
, where
has at leat one root in
. Then, there exist at least one
such that
1)
has all its roots in
.
2)
has at least one root in
.
The proof of the Theorem above can be see in [5]. A direct consequence of the last Theorem relevant for the case of Families of anti-Hurwitz polinomials is given in the following Corollary:
Corollary 2. Suppose
is a family of real polinomials in the variable X which depends continuously on the
and that the family is of degree constant. If
has all its roots on
and
has at least one root on
, then there exist
such that
1)
has all its roots in
.
2)
has at least one root in
.
Theorem 9. Let
be a sequence of anti-Hurwitz polynomials of degree least or equal to N such that
. Then, the roots of
remain in
.
Proof. We consider the polynomials
, and
. We suppose that
has a root
. We know that there is a circle C with center
such that
. Then, by Theorem 7 there is
such that if
, for all
, then
has at least one root inside of C. How
, then there is
such that
. Then, the following polynomial
has a root in
, which is a contradiction with the fact that
is a sequence of anti-Hurwitz polynomials.
Theorem 10 (Zero exclusion principle). Let
the set of the real anti-Hurwitz polynomials. Suppose
is a family of real polinomials in the variable X wich depends continuously on the
, with
pathwise connected. Suppose moreover that the family
is of degree constant and there is at least one anti-Hurwitz polynomial. Then, the family
, if and only if
, for all
and
.
Proof.
• This is a direct consequence of Theorem 6.
• Let
an arbitrary polynomial and
the anti-Hurwitz polynomial on
. We conside the path
such that
and
and the subfamily:
. We can see that
is anti-Hurwitz. Suppose that
does not an anti-Hurwitz polynomial, and hence has a root in
. If
is a contradiction. If
has a root in
, then by Theorem 8 there is
such that
1)
has all its roots in
.
2)
has at least one root in
.
By 2) there is
such that
, but this is a contradiction. Therefore
is anti-Hurwitz for all
.
Given a real polinomial
, we note that
By evaluate
, we obtain
Considering this, we consider the following notations
•
.
•
.
•
.
•
.
Definition 4.2. A real polynomial
satisfies the anti-alternancy property if and only if
1) The principal coeffients of
and
has different sign.
2) All the roots of
and
are reals and its negatives roots are interspersed, i.e.
Theorem 11 (Dual version of the Theorem of Hermite-Biehler). A real polinomial
is Anti-Hurwitz, if and only if satisfies the anti-alternancy property.
Proof. By Theorem 6 that the phase of
strictly decreases for
from
to
and the change in the phase is
, which is equivalent
tom turns in
, or m/2 turns on
. We note that for
the roots of
and
must be ordering in the following manner:
(4.3)
In fact, in every turn it goes through by two roots of
, and by two roots of
. Then, in m/2 turns it goes through by m roots of of
and by m roots of
. We note that, everyone is real and negative, and then, we obtain part (2) of property of anti-alternancy. For the converse, assume that
satisfies the anti-intelacing property, and suposes without loss of generality of p is of degree
and that the coefficient
is positive. Let us consider the roots of
and
in the form
(4.4)
Now, let us consider a polynomial
that is known to be anti-Hurwitz, of the same degree 2m, and with its leader coefficients positive. With this assumption on
, we know from the first part of that
satisfies the anti-interlacing Theorem so that
has m negatives roots and
has
negative roots, both set of roots such that
(4.5)
We note that for
, it has no imaginary roots, then for any
,
. By taking
, we have
Consider now the polynomial
given by
We can see that the coefficients of
are a family of polynomial functions in
, wich are continuous on
. Moreover, the coefficient of the leader degree term of
remains positive as
. Moreover, we note that for
, we have
. Then, how q is an anti-Hurwitz polynomial. This implies that the family
has al least an element that is anti-Hurwitz. Hence, by the principle of exclusion of zero all the elements of the family are anti-Hurwitz polynomials, in particular
.
5. Applications
In this section we consider a family of reals polynomials called viscous polynomials introduced by Vergara-Hermosilla et al. in [1].
(5.1)
where l and
are free parameters in
. The viscous polynomials arises naturally when considering the transfer function of a system that models the vertical movement of a solid floating in a viscous fluid, studied by Vergara-Hermosilla et al. in [9] and [1], in fact, the name of the family of polynomials is originally due to the fact that the parameters l and
represent a measure associated with the size of the floating structure and the viscosity coefficient, respectively. Our objective in this section is to use the criteria developed in Section 2 to obtain information on the location of the roots.
To this end, we can check easily that:
1) When dividing
by the coefficient of the term with exponent 4, we obtain the equivalent polynomial.
Figure 1. Evolution of the four roots
in the complex plane, as a function of
. (a): global picture with 4 trajectories; (b): zoom in the right-half plane
, here 2 trajectories are crossing the segment
for a critical value of
referent to the viscosity coefficient.
(5.2)
In this polynomial we can see that the coefficients of the terms with exponents 3 and 2 have the same sign, by considering the criteria developed in Section 3, we can conclude that the viscous polynomial is not anti-Hurwitz.
2) In a similar form, we can see that the there are coefficients in
with different sign, then using the Stodola condition give in Theorem 1, we conclude that the viscous polynomial is not Hurwitz.
In conclusion, due to the polynomial
have degree 4, is not Hurwitz and is not anti-Hurwitz, we will always have two roots in the right complex half plane and two roots in the left complex half plane. In fact, in the Figure 1 we can see numerical evidence about the behavior of the roots of the viscous polynomial with suitable parameters.
6. Conclusions
In this paper we present simple explicit criteria for determining the classification of the roots of real polynomials in one variable in the right half complex plane. These results appear as natural extensions of the classical theory of Hurwitz polynomials over the family of anti-Hurwitz polynomials introduced in [1]. More precisely, the results introduced in this work follow an implicit use of the linear transformation
into the properties that define the theory of Hurwitz polynomials, and define our notion of duality. Considering this, we can summarize our contribution in two important results: A dual version of the Theorem of Routh-Hurwitz and a version dual of the Boundary Crossing Theorems. These ideas are applied on a family of polynomials associated to a system that describes the vertical movement of a solid floating in a viscous fluid, called viscous polynomials.
In a subsequent work we will extend the ideas developed in this paper in order to explore the classification of roots of real polynomials on subregions of the complex plane limited for the intersection of finite number of graphs of convex functions.
Acknowledgements
The author appreciates the conversations and helps provided by Professor Florian Luca.
The author has been supported by the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 765579.