On hyperbolic polynomial-like functions and their derivatives
Published online by Cambridge University Press: 23 July 2007
Abstract
A real polynomial $P$ of degree $n$ in one real variable is hyperbolic if its roots are all real. A real-valued function $P$ is called a hyperbolic polynomial-like function of degree $n$ if it has $n$ real zeros and $P^{(n)}$ vanishes nowhere. Denote by $x_k^{(i)}$ the roots of $P^{(i)}$, $k=1,\dots,n-i$, $i=0,\dots,n-1$. Then, in the absence of any equality of the form
\begin{equation} x_i^{(j)}=x_k^{(l)} \tag{*} \label{*} \end{equation}
for all $i<j$ we have
\begin{equation} x_k^{(i)}<x_k^{(j)}<x_{k+j-i}^{(i)} \tag{**} \label{**} \end{equation}
(the Rolle theorem). For $n\geq4$ (respectively, for $n\geq5$) not all arrangements without equalities \eqref{*} of $\tfrac12n(n+1)$ real numbers $x_k^{(i)}$ and compatible with \eqref{**} are realizable by the roots of hyperbolic polynomials (respectively, of hyperbolic polynomial-like functions) of degree $n$ and of their derivatives. For $n=5$ and when
$$ x_1^{(1)}<x_1^{(3)}<x_2^{(1)}<x_3^{(1)}< x_2^{(3)}<x_4^{(1)} $$
we show that all such 102 arrangements are realizable by hyperbolic polynomial-like functions (of which 66 are obtained by hyperbolic polynomials and another 8 by perturbations of such).
- Type
- Research Article
- Information
- Proceedings of the Royal Society of Edinburgh Section A: Mathematics , Volume 137 , Issue 4 , August 2007 , pp. 819 - 845
- Copyright
- 2007 Royal Society of Edinburgh
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