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Almost strict total positivity and a class of Hurwitz polynomials

Authors: Dimitrov, D. K.; Pena, J. M.;

Almost strict total positivity and a class of Hurwitz polynomials

Abstract

A real \(n\times n\) matrix \(A:=[a_{ij}]\) is called totally positive (TP) if all its minors are nonnegative and strictly totally positive (STP) if they are positive. A TP matrix \(A\) is called almost strictly totally positive (ASTP) if \(A\) is nonsingular and, for each minor of \(A\) formed by consecutive rows and consecutive columns, the minor is positive if and only if its diagonal entries are positive. Let \(\delta=4.079\dots\) be the real root of \(x^{3}-5x^{2}+4x-1=0\). It has been shown by \textit{T. Craven} and \textit{G. Csordas} [Linear Multilinear Algebra 45, 19--34 (1998; Zbl 0922.15014)] that \(A\) is STP whenever its entries are positive and \(a_{ij}a_{i+1,j+1}\geq\delta a_{i,j+1}a_{i+1,j}\) for all \(i,j=1,\dots,n-1\). The main result of this paper is a generalization of the Craven-Csordas theorem in the case that not all entries of \(A\) are positive. It is known that for any nonsingular TP matrix \(A\) we have: \(a_{ii}>0\) for all \(i\); \(a_{hk}=0\) for all \(h\geq i\) and \(k\leq j\) whenever \(a_{ij}=0\) and \(i>j\); and \(a_{hk}=0\) for all \(h\leq i\) and \(k\geq j\) whenever \(a_{ij}=0\) and \(i0, \tag{*} \] then \(A\) is TP\(.\) Moreover if the inequality (*) is always strict, then \(A\) is ASTP. They apply this to show that if the coefficients of \(f(z):=c_{n}z^{n}+c_{n-1}z^{n-1}+\dots+c_{0}\) are positive real numbers and \(c_{k}c_{k+1}\geq\delta c_{k-1}c_{k+1}\) for \(k=1,\dots,n-2\) then \(f(z)\) is Hurwitz stable (that is, all the zeros of \(f(z)\) have negative real parts). They give similar conditions for an entire function to belong to the Laguerre-Pólya class of type I. The authors also show that the \(\delta\) which appears in these theorems cannot be universally replaced by a constant smaller than \(4\), but conjecture that for a fixed value of \(n\) the best constant is \(4\cos^{2}(\pi/(n+1))\).

Keywords

Mathematics(all), Numerical Analysis, Entire function in the Laguerre–Pólya class, Totally positive matrix, Applied Mathematics, Inequalities involving derivatives and differential and integral operators, Special classes of entire functions of one complex variable and growth estimates, strictly totally positive matrix, Hurwitz polynomial, Positive matrices and their generalizations; cones of matrices, Strictly totally positive matrix, totally positive matrix, shadows' lemma, entire function in the Laguerre-Polya class, Hurwitz stable polynomial, Laguerre-Pólya class, Shadows’ lemma, Analysis

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selected citations
These citations are derived from selected sources.
This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
BIP!Impulse provided by BIP!
15
Average
Top 10%
Average
Green
hybrid