Hostname: page-component-7479d7b7d-8zxtt Total loading time: 0 Render date: 2024-07-11T03:34:24.390Z Has data issue: false hasContentIssue false

On the Smallest and Largest Zeros of Müntz-Legendre Polynomials

Published online by Cambridge University Press:  20 November 2018

Úlfar F. Stefánsson*
Affiliation:
School of Mathematics, Georgia Institute of Technology, Atlanta, GA 30332-0160, USA e-mail: ustefans@math.gatech.edu
Rights & Permissions [Opens in a new window]

Abstract

Core share and HTML view are not available for this content. However, as you have access to this content, a full PDF is available via the ‘Save PDF’ action button.

Müntz-Legendre polynomials ${{L}_{n}}\left( \Lambda ;\,x \right)$ associated with a sequence $\Lambda \,=\,\left\{ {{\lambda }_{k}} \right\}$ are obtained by orthogonalizing the system $\left( {{x}^{{{\lambda }_{0}}}},{{x}^{{{\lambda }_{1}}}},{{x}^{{{\lambda }_{2}}}},... \right)$ in ${{L}_{2}}\left[ 0,1 \right]$ with respect to the Legendre weight. If the ${{\lambda }_{k}}\text{ }\!\!'\!\!\text{ s}$ are distinct, it is well known that ${{L}_{n}}\left( \Lambda ;\,x \right)$ has exactly $n$ zeros ${{l}_{n,n}}\,<\,{{l}_{n-1,n}}\,<\,\cdot \cdot \cdot \,<\,{{l}_{2,n}}\,<\,{{l}_{1,n}}$ on $\left( 0,1 \right)$.

First we prove the following global bound for the smallest zero,

$$\exp \left( -4\sum\limits_{j=0}^{n}{\frac{1}{2\text{ }\!\!\lambda\!\!\text{ j}\,\text{+}\,\text{1}}} \right)\,<\,{{l}_{n,n}}.$$

An important consequence is that if the associated Müntz space is non-dense in ${{L}_{2}}\left[ 0,1 \right]$, then

$$\underset{n}{\mathop{\inf }}\,\,{{x}_{n,n}}\,\ge \,\exp \,\left( -4\,\sum\limits_{j=0}^{\infty }{\frac{1}{2{{\text{ }\!\!\lambda\!\!\text{ }}_{j}}\,+\,1}} \right)\,>\,0,$$

so the elements ${{L}_{n}}\left( \Lambda ;\,x \right)$ have no zeros close to 0.

Furthermore, we determine the asymptotic behavior of the largest zeros; for $k$ fixed,

$$\underset{n\to \infty }{\mathop{\lim }}\,\,\left| \log \,{{l}_{k,n}} \right|\,\sum\limits_{j=0}^{n}{\left( 2{{\text{ }\!\!\lambda\!\!\text{ }}_{j}}\,+\,1 \right)}\,=\,{{\left( \frac{jk}{2} \right)}^{2}},$$

where ${{j}_{k}}$ denotes the $k$-th zero of the Bessel function ${{J}_{0}}.$

Type
Research Article
Copyright
Copyright © Canadian Mathematical Society 2013

References

[1] Almira, J. M., Möntz type theorems I. Surv. Approx. Theory 3 (2007), 152194.Google Scholar
[2] Borwein, P. and Erdélyi, T., Polynomials and polynomial inequalities. Graduate Texts in Mathematics, 161, Springer-Verlag, New York, 1995.Google Scholar
[3] Borwein, P., Erdélyi, T., and Zhang, J., Müntz systems and Müntz-Legendre polynomials. Trans. Amer. Math. Soc. 342 (1994), no. 2, 523542. http://dx.doi.org/10.2307/2154639 Google Scholar
[4] Gurariy, V. I. and Lusky, W., Geometry of Müntz spaces and related questions. Lecture Notes in Mathematics, 1870, Springer-Verlag, Berlin, 2005.Google Scholar
[5] Lubinsky, D. S. and Saff, E. B., Zero distribution of Müntz extremal polynomials in Lp [0, 1]. Proc. Amer. Math. Soc. 135 (2007), no. 2, 427435. http://dx.doi.org/10.1090/S0002-9939-06-08694-1 Google Scholar
[6] Olver, F.W. J., Asymptotics and special functions. A K Peters ,Wellesley, MA, 1997.Google Scholar
[7] Stef ánsson, Ú. F., Asymptotic behavior of Müntz orthogonal polynomials. Constr. Approx. 32 (2010), no. 2, 193220. http://dx.doi.org/10.1007/s00365-009-9059-x Google Scholar
[8] Stef ánsson, Ú. F., Endpoint asymptotics for Müntz-Legendre polynomials. Acta. Math. Hungar. 130 (2011), no. 4, 372381. http://dx.doi.org/10.1007/s10474-010-0013-y Google Scholar
[9] Stef ánsson, Ú. F., Zero spacing of Möntz orthogonal polynomials. Comput. Methods Funct. Theory. 11 (2011), no. 1, 4557 Google Scholar
[10] Szegö, G., Orthogonal polynomials. American Mathematical Society, Colloquium Publications, 23, American Mathematical Society, Providence, RI, 1975.Google Scholar