Abstract
We generalise a necessary and sufficient condition given by Cohn for all the zeros of a self-inversive polynomial to be on the unit circle. Our theorem implies some sufficient conditions found by Lakatos, Losonczi and Schinzel. We apply our result to the study of a polynomial family closely related to Ramanujan polynomials, recently introduced by Gun, Murty and Rath, and studied by Murty, Smyth and Wang as well as by Lalín and Rogers. We prove that all polynomials in this family have their zeros on the unit circle, a result conjectured by Lalín and Rogers on computational evidence.
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The first author is supported by NSERC Discovery Grant 355412-2008, FQRNT Subvention établissement de nouveaux chercheurs 144987, and a start-up grant from the Université de Montréal.
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Lalín, M.N., Smyth, C.J. Unimodularity of zeros of self-inversive polynomials. Acta Math Hung 138, 85–101 (2013). https://doi.org/10.1007/s10474-012-0225-4
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DOI: https://doi.org/10.1007/s10474-012-0225-4