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1.
It is shown that the exponent of convergence λ(f) of any solution f of with entire coefficients A0(z), …, Ak?2(z), satisfies λ(f) ? λ ∈ [1, ∞) if and only if the coefficients A0(z), …, Ak?2(z) are polynomials such that for j = 0, …, k ? 2. In the unit disc analogue of this result certain intersections of weighted Bergman spaces take the role of polynomials. The key idea in the proofs is W. J. Kim’s 1969 representation of coefficients in terms of ratios of linearly independent solutions. © 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim 相似文献
2.
Janne Heittokangas 《Constructive Approximation》2011,34(1):1-21
This research is partially a continuation of a 2007 paper by the author. Growth estimates for generalized logarithmic derivatives
of Blaschke products are provided under the assumption that the zero sequences are either uniformly separated or exponential.
Such Blaschke products are known as interpolating Blaschke products. The growth estimates are then proven to be sharp in a
rather strong sense. The sharpness discussion yields a solution to an open problem posed by E. Fricain and J. Mashreghi in
2008. Finally, several aspects are pointed out to illustrate that interpolating Blaschke products appear naturally in studying
the oscillation of solutions of a differential equation f″+A(z)f=0, where A(z) is analytic in the unit disc. In particular, a unit disc analogue of a 1988 result due to S. Bank on prescribed zero sequences
for entire solutions is obtained, and a more careful analysis of a 1955 example due to B. Schwarz on the case
A(z)=\frac1+4g2(1-z2)2A(z)=\frac{1+4\gamma^{2}}{(1-z^{2})^{2}} reveals that an infinite zero sequence is always a union of two exponential sequences. 相似文献
3.
Janne Heittokangas Risto Korhonen Jouni Rä ttyä 《Transactions of the American Mathematical Society》2008,360(2):1035-1055
Complex linear differential equations of the form with coefficients in weighted Bergman or Hardy spaces are studied. It is shown, for example, that if the coefficient of belongs to the weighted Bergman space , where , for all , then all solutions are of order of growth at most , measured according to the Nevanlinna characteristic. In the case when all solutions are shown to be not only of order of growth zero, but of bounded characteristic. Conversely, if all solutions are of order of growth at most , then the coefficient is shown to belong to for all and .
Analogous results, when the coefficients belong to certain weighted Hardy spaces, are obtained. The non-homogeneous equation associated to is also briefly discussed.
4.
Value sharing results for shifts of meromorphic functions, and sufficient conditions for periodicity 总被引:1,自引:0,他引:1
This research is a continuation of a recent paper due to the first four authors. Shared value problems related to a meromorphic function f(z) and its shift f(z+c), where c∈C, are studied. It is shown, for instance, that if f(z) is of finite order and shares two values CM and one value IM with its shift f(z+c), then f is a periodic function with period c. The assumption on the order of f can be dropped if f shares two shifts in different directions, leading to a new way of characterizing elliptic functions. The research findings also include an analogue for shifts of a well-known conjecture by Brück concerning the value sharing of an entire function f with its derivative f′. 相似文献
5.
Janne Heittokangas Katsuya Ishizaki Kazuya Tohge Zhi-Tao Wen 《Israel Journal of Mathematics》2018,227(1):397-421
An exponential polynomial of order q is an entire function of the form where the coefficients Pj(z),Qj(z) are polynomials in z such that It is known that the majority of the zeros of a given exponential polynomial are in domains surrounding finitely many critical rays. The shape of these domains is refined by showing that in many cases the domains can approach the critical rays asymptotically. Further, it is known that the zeros of an exponential polynomial are always of bounded multiplicity. A new sufficient condition for the majority of zeros to be simple is found. Finally, a division result for a quotient of two exponential polynomials is proved, generalizing a 1929 result by Ritt in the case q = 1 with constant coefficients. Ritt’s result is closely related to Shapiro’s conjecture that has remained open since 1958.
相似文献
$$g(z) = {P_1}(z){e^{{Q_1}(z)}} + ...{P_k}(z){e^{{Q_k}(z)}},$$
$$\max \{ deg({Q_j})\} = q.$$
6.
7.
In 1988, S. Bank showed that if {z
n
} is a sparse sequence in the complex plane, with convergence exponent zero, then there exists a transcendental entire A(z) of order zero such that f″+A(z)f=0 possesses a solution having {z
n
} as its zeros. Further, Bank constructed an example of a zero sequence {z
n
} violating the sparseness condition, in which case the corresponding coefficient A(z) is of infinite order. In 1997, A. Sauer introduced a condition for the density of the points in the zero sequence {z
n
} of finite convergence exponent such that the corresponding coefficient A(z) is of finite order. 相似文献
8.
Zhi-Tao Wen Gary G. Gundersen Janne Heittokangas 《Journal of Differential Equations》2018,264(1):98-114
We study linear differential equations with exponential polynomial coefficients, where exactly one coefficient is of order greater than all the others. The main result shows that a nontrivial exponential polynomial solution of such an equation has a certain dual relationship with the maximum order coefficient. Several examples illustrate our results and exhibit possibilities that can occur. 相似文献
9.
If φ: [0, 1) → (0,∞) is a non-decreasing unbounded function, then the φ-order of a meromorphic function f in the unit disc is defined as $$ \sigma _\phi (f) = \mathop {\lim \sup }\limits_{r \to 1^ - } \frac{{\log ^ + T(r,f)}} {{\log \phi (r)}}, $$ where T(r, f) is the Nevanlinna characteristic of f. In particular, $ \sigma _{\tfrac{1} {{1 - r}}} $ f is the order of f, and $ \sigma _{\log \tfrac{1} {{1 - r}}} $ f is the logarithmic order of f. Several results on the finiteness of the φ-order of solutions of $$ f^{(k)} + A_{k - 1} (z)f^{(k - 1)} + \cdots + A_1 (z)f' + A_0 (z)f = 0 $$ are obtained in the case when the coefficients A 0(z), ...,A k?1(z) are analytic functions in the unit disc. This paper completes some earlier results by various authors. 相似文献
10.
For f meromorphic in the complex plane and meromorphic in theunit disc, sharp upper bounds are obtained for
and
where k andj are integers satisfying k > j 0. The results generalizethe logarithmic derivative estimate due to Gol'dberg and Grinshteinto derivatives of higher order. 2000 Mathematics Subject Classification30D35. 相似文献