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Criterion of the Reality of Zeros in a Polynomial Sequence Satisfying a Three-Term Recurrence Relation
Stockholm University, Faculty of Science, Department of Mathematics. Makerere University, Uganda.
Number of Authors: 12020 (English)In: Czechoslovak Mathematical Journal, ISSN 0011-4642, E-ISSN 1572-9141, Vol. 70, no 3, p. 793-804Article in journal (Refereed) Published
Abstract [en]

This paper establishes the necessary and sufficient conditions for the reality of all the zeros in a polynomial sequence {P-i}(i=1)(infinity) generated by a three-term recurrence relation P-i(x) + Q(1)(x)P-i (1)(x) + Q(2)(x)Pi-2(x) = 0 with the standard initial conditions P-0(x) = 1, P-1(x) = 0, where Q(1)(x) and Q(2)(x) are arbitrary real polynomials.

Place, publisher, year, edition, pages
2020. Vol. 70, no 3, p. 793-804
Keywords [en]
recurrence relation, polynomial sequence, support, real zeros
National Category
Mathematics
Identifiers
URN: urn:nbn:se:su:diva-181072DOI: 10.21136/CMJ.2020.0535-18ISI: 000570740200012OAI: oai:DiVA.org:su-181072DiVA, id: diva2:1427700
Available from: 2020-04-30 Created: 2020-04-30 Last updated: 2022-02-26Bibliographically approved
In thesis
1. Polynomial Sequences Generated by Linear Recurrences: Location and Reality of Zeros
Open this publication in new window or tab >>Polynomial Sequences Generated by Linear Recurrences: Location and Reality of Zeros
2021 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, we study the problem of location of the zeros of individual polynomials in sequences of polynomials generated by linear recurrence relations.

In paper I, we establish the necessary and sufficient conditions that guarantee hyperbolicity of all the polynomials generated by a three-term recurrence of length 2, whose coefficients are arbitrary real polynomials. These zeros are dense on the real intervals of an explicitly defined real semialgebraic curve.

Paper II extends Paper I to three-term recurrences of length greater than 2. We prove that there always exist non-hyperbolic polynomial(s) in the generated sequence. We further show that with at most finitely many known exceptions, all the zeros of all the polynomials generated by the recurrence lie and are dense on an explicitly defined real semialgebraic curve which consists of real intervals and non-real segments. The boundary points of this curve form a subset of zero locus of the discriminant of the characteristic polynomial of the recurrence.

Paper III discusses the zero set for polynomials generated by three-term recurrences of lengths 3 and 4 with arbitrary polynomial coefficients. We prove that except the zeros of the polynomial coefficients, all the zeros of every generated polynomial lie on an explicitly defined real semialgebraic curve.

Paper IV extends the results in paper III and generalizes a conjecture by K. Tran [2]. We consider a three-term recurrence relation of any length whose coefficients are arbitrary complex polynomials and prove that with the exception of the zeros of the polynomial coefficients, all the zeros of every generated polynomial lie on a real algebraic curve. We derive the equation of this curve.

Paper V establishes the necessary and sufficient conditions guaranteeing the reality of all the zeros of every polynomial generated by a special five-term recurrence with real coefficients. We put the problem in the context of banded Toeplitz matrices whose associated Laurent polynomial is holomorphic in the punctured plane. We interpret the conditions in terms of the positivity/negativity of the discriminant of a certain polynomial whose coefficients are explicit functions of the parameters in the recurrence.

Place, publisher, year, edition, pages
Stockholm: Department of Mathematics, Stockholm University, 2021. p. 35
Keywords
real-rooted polynomials, generating functions, discriminants, Tran's conjecture, Toeplitz matrices
National Category
Mathematical Analysis
Research subject
Mathematics
Identifiers
urn:nbn:se:su:diva-191522 (URN)978-91-7911-462-6 (ISBN)978-91-7911-463-3 (ISBN)
Public defence
2021-05-14, sal 14 (Gradängsalen), hus 5, Kräftriket, Roslagsvägen 101 and online via Zoom, public link is available at the department website, Stockholm, 15:00 (English)
Opponent
Supervisors
Funder
Sida - Swedish International Development Cooperation Agency, 316
Available from: 2021-04-21 Created: 2021-03-24 Last updated: 2022-02-25Bibliographically approved

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