Mathematical Proceedings of the Cambridge Philosophical Society

Generalization of characterizations of the trigonometric functions

a1 Department of Mathematics, Sogang University, Seoul 121-742, Korea. e-mail:,
a2 Department of Mathematics and The Program of Integrated Biotechnology, Sogang University, Seoul 121-742, Korea. e-mail:

Article author query
kim j   [Google Scholar] 
chung y   [Google Scholar] 
chung s   [Google Scholar] 


Suppose that $P(D)$ is a linear differential operator with complex coefficients and $\{f_k\}_{k \in \mathbb{Z}}$ is a two-sided sequence of complex-valued functions on $\mathbb{R}$ such that $f_{k+1} \,{=}\, P(D)f_k$ with the following growth condition: there exist $a\,{\in}\,[0,1)$ and $N\,{\geq}\,0$ such that $|f_k(x)|\,{\leq}\,M_{|k|}\exp (N|x|^a)$, where the sequence $\{M_k\}_{k=0}^{\infty}$ satisfies that for any $\varepsilon\,{>}\,0$, the sequence $\{{M_k}/{(1+\varepsilon)^{k}}\}_{k=0}^{\infty}$ has a bounded subsequence. Then $f_0$ is an entire function of an exponential growth. This result is a generalization of those of Roe, Burkill and Howard.

(Received March 31 2005)
(Revised January 4 2006)