User:Holmansf/Quasi-analytic classes

A quasi-analytic class of functions is a generalization of the class of analytic functions based upon the following fact. If f is an analytic function on an interval $[a,b]\subset \mathbb {R}$, and at some point f and all of its deriviates are zero, then f is identically zero on all of $[a,b]$. Quasi-analytic classes are broader classes of functions for which this statement still holds true.

Definitions

Let $M=\{M_{k}\}_{k=0}^{\infty }$ be a sequence of positive real numbers with $M_{0}=1$. Then we define the class of functions $C^{M}([a,b])$ to be those $f\in C^{\infty }([a,b])$ which satisfy

$\left|{\frac {d^{k}f}{dx^{k}}}(x)\right|\leq C^{k+1}M_{k}$

for some constant C and all non-negative integers k. If $M_{k}=k!$ this is exactly the class of real-analytic functions on $[a,b]$. The class $C^{M}([a,b])$ is said to be quasi-analytic if whenever $f\in C^{M}([a,b])$ and

${\frac {d^{k}f}{dx^{k}}}(x)=0$

for some point $x\in [a,b]$ and all k, f is identically equal to zero. The Denjoy-Carleman theorem gives criteria on the sequence M under which $C^{M}([a,b])$ is a quasi-analytic class.

A function f is called a quasi-analytic function if f is in some quasi-analytic class.

References

• Hörmander, Lars (1990). The Analysis of Linear Partial Differential Operators I. Springer-Verlag. ISBN 3-540-00662. {{cite book}}: Check |isbn= value: length (help)
• Cohen, P. J. (1968). "A simple proof of the Denjoy-Carleman theorem". Amer. Math. Monthly. 75: 26–31.