arXiv subjects

Olga Kushel

Publications and source records attributed to Olga Kushel.

3 recordsLinked to original sources

arXiv2019

Unifying matrix stability concepts with a view to applications

Multiplicative and additive $D$-stability, diagonal stability, Schur $D$-stability, $H$-stability are classical concepts which arise in studying linear dynamical systems. We unify these types of stability, as well as many others, in one concept of $({\mathfrak D}, {\mathcal G}, \circ)$-stability, which depends on a stability region ${\mathfrak D} \subset {\mathbb C}$, a matrix class ${\mathcal G}$ and a binary matrix operation $\circ$. This approach allows us to unite several well-known matrix problems and to consider common methods of their analysis. In order to collect these methods, we make a historical review, concentrating on diagonal and $D$-stability. We prove some elementary properties of $({\mathfrak D}, {\mathcal G}, \circ)$-stable matrices, uniting the facts that are common for many partial cases. Basing on the properties of a stability region $\mathfrak D$ which may be chosen to be a concrete subset of $\mathbb C$ (e.g. the unit disk) or to belong to a specified type of regions (e.g. LMI regions) we briefly describe the methods of further development of the theory of $({\mathfrak D}, {\mathcal G}, \circ)$-stability. We mention some applications of the theory of $({\mathfrak D}, {\mathcal G}, \circ)$-stability to the dynamical systems of different types.

math.SP↗

arXiv2015

Circulants and critical points of polynomials

We prove that for any circulant matrix $C$ of size $n\times n$ with the monic characteristic polynomial $p(z)$, the spectrum of its $(n-1)\times(n-1)$ submatrix $C_{n-1}$ constructed with first $n-1$ rows and columns of $C$ consists of all critical points of $p(z)$. Using this fact we provide a simple proof for the Schoenberg conjecture recently proved by R. Pereira and S. Malamud. We also prove full generalization of a higher order Schoenberg-type conjecture proposed by M. de Bruin and A. Sharma and recently proved by W.S. Cheung and T.W. Ng. in its original form, i.e. for polynomials whose mass centre of roots equals zero. In this particular case, our inequality is stronger than it was conjectured by de Bruin and Sharma. Some Schmeisser's-like results on majorization of critical point of polynomials are also obtained.

math.CA↗

arXiv2015

Generalized Hurwitz matrices, generalized Euclidean algorithm, and forbidden sectors of the complex plane

Given a polynomial \[ f(x)=a_0x^n+a_1x^{n-1}+\cdots +a_n \] with positive coefficients $a_k$, and a positive integer $M\leq n$, we define a(n infinite) generalized Hurwitz matrix $H_M(f):=(a_{Mj-i})_{i,j}$. We prove that the polynomial $f(z)$ does not vanish in the sector $$ \left\{z\in\mathbb{C}: |\arg (z)| < \fracπ{M}\right\} $$ whenever the matrix $H_M$ is totally nonnegative. This result generalizes the classical Hurwitz' Theorem on stable polynomials ($M=2$), the Aissen-Edrei-Schoenberg-Whitney theorem on polynomials with negative real roots ($M=1$), and the Cowling-Thron theorem ($M=n$). In this connection, we also develop a generalization of the classical Euclidean algorithm, of independent interest per se.

math.CA↗