Notes on the Propagators of Evolution Equations
© Yu Lin et al. 2010
Received: 8 January 2010
Accepted: 1 February 2010
Published: 16 February 2010
We consider the propagator of an evolution equation, which is a semigroup of linear operators. Questions related to its operator norm function and its behavior at the critical point for norm continuity or compactness or differentiability are studied.
As it is well known, each well-posed Cauchy problem for first-order evolution equation in Banach spaces
gives rise to a well-defined propagator, which is a semigroup of linear operators, and the theory of semigroups of linear operators on Banach spaces has developed quite rapidly since the discovery of the generation theorem by Hille and Yosida in 1948. By now, it is a rich theory with substantial applications to many fields (cf., e.g., [1–6]).
In this paper, we pay attention to some basic problems on the semigroups of linear operators and reveal some essential properties of theirs.
Let be a Banach space.
, with ( being the identity operator on X) ,
the map is continuous on for every .
The infinitesimal generator of is defined as
For a comprehensive theory of -semigroups we refer to .
2. Properties of the Function
, for ;
Furthermore, we can infer from the strong continuity of that
is lower-semicontinuous, that is,(2.1)
holds for all with . Thus, taking the supremum for all with on the left-hand side leads to (2.1).
We ask the following question
For every function satisfying , , and , does there exist a semigroup on some Banach space such that for all ?
We show that this is not true even if is a finite-dimensional space.
Then satisfies (I), (II), and (III), and there exists no semigroup on such that for all .
First, we show that satisfies (I), (II), and (III). (I) is clearly satisfied.
To show (III) and (II), we write
hence satisfies (III).
For (II), suppose , and consider the following four cases.
Case 1 ( and ).
Case 2 ( and ).
that is, .
Case 3 ( , but and ).
Case 4 ( ).
Next, we prove that there does not exist any semigroup on such that . Suppose for some semigroup on and let be its infinitesimal generator.
First we note from (2.3) that
which is a contradiction to (2.16).
Open Problem 1.
Is it possible that there exists an with and a semigroup on such that for all ?
3. The Critical Point of Norm-Continuous (Compact, Differentiable) Semigroups
A -semigroup is called norm-continuous for if is continuous in the uniform operator topology for .
A -semigroup is called compact for if is a compact operator for .
A -semigroup is called differentiable for if for every , is differentiable for .
It is known that if a -semigroup is norm continuous (compact, differentiable) at , then it remains so for all . For instance, the following holds.
If the map is right differentiable at , then it is also differentiable for .
Therefore, if we write
and suppose ( , ), then ( , ) takes the form of for a nonnegative real number . In other words, if ( , ), then is norm continuous (compact, differentiable) on the interval but not at any point in . We call the critical point of the norm continuity (compactness, differentiability) of operator semigroup .
A natural question is the following
Suppose that is the critical point of the norm continuity compactness, differentiability of the operator semigroup . Is also norm continuous (compact, differentiable) at ? Of course, concerning norm continuity or differentiability at we only mean right continuity or right differentiability.
We show that the answer is "yes" in some cases and "no" for other cases.
Therefore, in this case we have . Since , we see that is compact and is differentiable at from the right.
In view of that is unbounded, we know that is not norm continuous at .
For differentiability, we note that is differentiable at if and only if for each . From
it follows that when , for every . On the other hand, when and is any nonzero constant sequence, . Therefore the critical point for differentiability is . But is not differentiable at .
The authors acknowledge the support from the NSF of China (10771202), the Research Fund for Shanghai Key Laboratory of Modern Applied Mathematics (08DZ2271900), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (2007035805).
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