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On the number of linearly independent admissible solutions to linear differential and linear difference equations

Part of: Differential equations in the complex domain Entire and meromorphic functions, and related topics

Published online by Cambridge University Press:  30 July 2020

Janne Heittokangas ,
Hui Yu  and
Mohamed Amine Zemirni
Janne Heittokangas
Affiliation:
Department of Physics and Mathematics, University of Eastern Finland, 80101Joensuu, P.O. Box 111, Finland e-mail: [email protected]@uef.fi
Hui Yu*
Affiliation:
Department of Physics and Mathematics, University of Eastern Finland, 80101Joensuu, P.O. Box 111, Finland e-mail: [email protected]@uef.fi
Mohamed Amine Zemirni
Affiliation:
Department of Physics and Mathematics, University of Eastern Finland, 80101Joensuu, P.O. Box 111, Finland e-mail: [email protected]@uef.fi
*
e-mail: [email protected]
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Abstract

A classical theorem of Frei states that if $A_p$ is the last transcendental function in the sequence $A_0,\ldots ,A_{n-1}$ of entire functions, then each solution base of the differential equation $f^{(n)}+A_{n-1}f^{(n-1)}+\cdots +A_{1}f'+A_{0}f=0$ contains at least $n-p$ entire functions of infinite order. Here, the transcendental coefficient $A_p$ dominates the growth of the polynomial coefficients $A_{p+1},\ldots ,A_{n-1}$ . By expressing the dominance of $A_p$ in different ways and allowing the coefficients $A_{p+1},\ldots ,A_{n-1}$ to be transcendental, we show that the conclusion of Frei’s theorem still holds along with an additional estimation on the asymptotic lower bound for the growth of solutions. At times, these new refined results give a larger number of linearly independent solutions of infinite order than the original theorem of Frei. For such solutions, we show that $0$ is the only possible finite deficient value. Previously, this property has been known to hold for so-called admissible solutions and is commonly cited as Wittich’s theorem. Analogous results are discussed for linear differential equations in the unit disc, as well as for complex difference and complex q-difference equations.

Keywords

Deficient valuesentire functionsFrei’s theoremlinear difference equationslinear differential equationsWittich’s theorem

MSC classification

Primary: 34M10: Oscillation, growth of solutions
Secondary: 30D35: Distribution of values, Nevanlinna theory
Type
Article
Information
Canadian Journal of Mathematics , Volume 73 , Issue 6 , December 2021 , pp. 1556 - 1591
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Copyright
© Canadian Mathematical Society 2020

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