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Generalizing Tran's Conjecture
Stockholm University, Faculty of Science, Department of Mathematics.ORCID iD: 0000-0001-9439-2276
Stockholm University, Faculty of Science, Department of Mathematics.ORCID iD: 0000-0003-4500-4155
Stockholm University, Faculty of Science, Department of Mathematics.ORCID iD: 0000-0002-8438-3971
2020 (English)In: Electronic Journal of Mathematical Analysis and Applications, E-ISSN 2090-729X, Vol. 8, no 2, p. 346-351Article in journal (Refereed) Published
Abstract [en]

A conjecture of Khang Tran  claims that for an arbitrary pair of polynomials A(z) and B(z), every zero of every polynomial in the sequence {P_n(z)} satisfying the three-term recurrence relation of length k

P_n(z) + B(z)P_{n−1}(z) + A(z)P_{n−k}(z) = 0

with the standard initial conditions P_0(z) = 1, P_{−1}(z) = · · · = P_{−k+1}(z) = 0 which is not a zero of A(z) lies on the real (semi)-algebraic curve C  given by

Im(( B^k(z)/ A(z)) = 0 and 0 ≤ (−1)^k ≤ Re(( B^k(z)/ A(z)) ≤ k^k (k − 1)^{k−1}. In this short note, we show that for the recurrence relation (generalizing the latter recurrence of Tran) given by

P_n(z) + B(z)P_{n−l}(z) + A(z)P_{n−k}(z) = 0, with coprime k and l and the same standard initial conditions as above, every root of P_n(z) which is not a zero of A(z)B(z) belongs to the real algebraic curve C_{l,k} given by

Im(( B^k(z)/ A(z)) = 0.

Place, publisher, year, edition, pages
2020. Vol. 8, no 2, p. 346-351
Keywords [en]
recurrence, polynomial sequence, generating function, lattice paths
National Category
Mathematics
Research subject
Mathematics
Identifiers
URN: urn:nbn:se:su:diva-191105DOI: 10.48550/arXiv.2001.09248OAI: oai:DiVA.org:su-191105DiVA, id: diva2:1535397
Funder
Sida - Swedish International Development Cooperation Agency, 316Available from: 2021-03-08 Created: 2021-03-08 Last updated: 2022-04-07Bibliographically 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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Bøgvad, RikardNdikubwayo, InnocentShapiro, Boris

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