# Transactions of the American Mathematical Society

Published by the American Mathematical Society since 1900, Transactions of the American Mathematical Society is devoted to longer research articles in all areas of pure and applied mathematics.

ISSN 1088-6850 (online) ISSN 0002-9947 (print)

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## Explicit merit factor formulae for Fekete and Turyn polynomialsHTML articles powered by AMS MathViewer

by Peter Borwein and Kwok-Kwong Stephen Choi
Trans. Amer. Math. Soc. 354 (2002), 219-234

## Abstract:

We give explicit formulas for the $L_{4}$ norm (or equivalently for the merit factors) of various sequences of polynomials related to the Fekete polynomials $f_{q}(z) := \sum ^{q-1}_{k=1} \left (\frac {k}{q}\right ) z^{k}$ where $\left (\frac {\cdot }{q}\right )$ is the Legendre symbol. For example for $q$ an odd prime, $\|f_{q}\|_{4}^{4} : = \frac {5q^{2}}{3}-3q+ \frac {4}{3} - 12 (h(-q))^{2}$ where $h(-q)$ is the class number of $\mathbb {Q}(\sqrt {-q})$. Similar explicit formulas are given for various polynomials including an example of Turyn’s that is constructed by cyclically permuting the first quarter of the coefficients of $f_{q}$. This is the sequence that has the largest known asymptotic merit factor. Explicitly, $R_{q}(z) := \sum ^{q-1}_{k=0} \left (\frac {k+[q/4] }{q}\right ) z^{k}$ where $[\cdot ]$ denotes the nearest integer, satisfies $\|R_{q}\|_{4}^{4} = \frac {7q^{2}}{6}- {q} - \frac {1}{6} - \gamma _{q}$ where $\gamma _{q}: = \begin {cases} h(-q) (h(-q)-4) & \text {if q \equiv 1,5 \pmod 8},\\ 12 (h(-q))^{2} & \text {if q \equiv 3 \pmod 8}, \\ 0 & \text {if q \equiv 7 \pmod 8}. \end {cases}$ Indeed we derive a closed form for the $L_{4}$ norm of all shifted Fekete polynomials $f_{q}^{t}(z) := \sum ^{q-1}_{k=0} \left (\frac {k+t}{q}\right ) z^{k}.$ Namely \begin{align*} \| f_{q}^{t} \|_{4}^{4} &= \frac {1}{3}(5q^{2}+3q+4)+8t^{2}-4qt-8t &\quad -\frac {8}{q^{2}}\left ( 1-\frac {1}{2} \left (\frac {-1}{q}\right ) \right ) \left |{\displaystyle \sum _{n=1}^{q-1}n \left (\frac {n+t}{q}\right )} \right |^{2}, \end{align*} and $\| f_{q}^{q-t+1} \|_{4}^{4}= \| f_{q}^{t} \|_{4}^{4}$ if $1 \le t \le (q+1)/2$.
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• Peter Borwein
• Affiliation: Department of Mathematics and Statistics, Simon Fraser University, Burnaby, B.C., Canada V5A 1S6
• Kwok-Kwong Stephen Choi
• Affiliation: Department of Mathematics and Statistics, Simon Fraser University, Burnaby, B.C., Canada V5A 1S6
• Received by editor(s): April 24, 2000
• Published electronically: August 20, 2001
• Additional Notes: Research of P. Borwein is supported, in part, by NSERC of Canada. K.K. Choi is a Pacific Institute of Mathematics Postdoctoral Fellow and the Institute’s support is gratefully acknowledged
• Journal: Trans. Amer. Math. Soc. 354 (2002), 219-234
• MSC (1991): Primary 11J54, 11B83, 12-04
• DOI: https://doi.org/10.1090/S0002-9947-01-02859-8
• MathSciNet review: 1859033