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Notes on the Propagators of Evolution Equations
Advances in Difference Equations volumeÂ 2010, ArticleÂ number:Â 795484 (2010)
Abstract
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.
1. Introduction
As it is well known, each wellposed Cauchy problem for firstorder evolution equation in Banach spaces
gives rise to a welldefined 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., [1â€“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.
A oneparameter family of bounded linear operators on is called a strongly continuous semigroup (or simply semigroup) if it satisfies the following conditions:

(i)
, with ( being the identity operator on X) ,

(ii)
for ,

(iii)
the map is continuous on for every .
The infinitesimal generator of is defined as
with domain
For a comprehensive theory of semigroups we refer to [2].
2. Properties of the Function Â
Let be a semigroup on and define for . Clearly, from Definition 1.1 we see that

(I)
, for ;

(II)
for .
Furthermore, we can infer from the strong continuity of that

(III)
is lowersemicontinuous, that is,
(2.1)In fact,
(2.2)holds for all with . Thus, taking the supremum for all with on the lefthand side leads to (2.1).
We ask the following question
For every functionsatisfying,, and, does there exist asemigroupon some Banach spacesuch thatfor all?
We show that this is not true even if is a finitedimensional space.
Theorem 2.1.
Let be an dimensional Banach space with . Let
Then satisfies (I), (II), and (III), and there exists no semigroup on such that for all .
Proof.
First, we show that satisfies (I), (II), and (III). (I) is clearly satisfied.
To show (III) and (II), we write
Then
hence satisfies (III).
For (II), suppose , and consider the following four cases.
Case 1 ( and ).
In this case
that is,
Case 2 ( and ).
Let
Then
and is a convex function on . So by Jensen's inequality, we have
that is,
Therefore
that is, .
Case 3 (, but and ).
It follows from Case 2 that
Case 4 ().
Again we have
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
for every , while
By the wellknown Lyapunov theorem [2, Chapter I, Theorem ], all eigenvalues of (the infinitesimal generator of ) have negative parts for every . Letting be the eigenvalues of , we then have
and this implies that
It is known that there is an isomorphism of onto such that
where is the Jordan block corresponding to . Therefore
Set
where is the order of . Then is a th nilpotent matrix with for each . According to (2.20) and (2.18), we have
Observing
we see that
Thus,
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 NormContinuous (Compact, Differentiable) Semigroups
The following definitions are basic [1â€“6].
Definition 3.1.
A semigroup is called normcontinuous for if is continuous in the uniform operator topology for .
Definition 3.2.
A semigroup is called compact for if is a compact operator for .
Definition 3.3.
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.
Proposition 3.4.
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 thatis the critical point of the norm continuitycompactness, differentiabilityof the operator semigroup. Isalso 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.
Example 3.5.
Let and
Then clearly for . Moreover, is not norm continuous (not compact, not differentiable) for any since
for sufficiently small , where
Therefore, in this case we have . Since , we see that is compact and is differentiable at from the right.
Example 3.6.
Let
with supremum norm. For any set
Then, is compact (hence normcontinuous) for since is the operatornorm limit of a sequence of finiterank operators:
where
So the critical point for compactness and norm continuity is . However, the infinitesimal generator of is given by
with
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 .
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Acknowledgments
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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Lin, Y., Xiao, TJ. & Liang, J. Notes on the Propagators of Evolution Equations. Adv Differ Equ 2010, 795484 (2010). https://doi.org/10.1155/2010/795484
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DOI: https://doi.org/10.1155/2010/795484