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ISSN Online: 2160-0384 ISSN Print: 2160-0368

DOI: 10.4236/apm.2019.91003 Jan. 25, 2019 43 Advances in Pure Mathematics

Some Result of Stability and Spectra Properties

on Semigroup of Linear Operator

Kamilu Rauf

*

, Akinola Yussuff Akinyele, Mfon Okon Etuk, Rafiu Obashola Zubair,

Moses Adebowale Aasa

Department of Mathematics, University of Ilorin, Ilorin, Nigeria

Abstract

This paper consists of some properties of a new subclass of semigroup of li-near operator. The stability and spectra analysis of ω-order preserving partial contraction mapping (ω-OCPn) are obtained. The results show that operators

on the proposed ω-OCPn are densely defined and closed. Several existing

re-sults in the literature are contained in this work.

Keywords

Contraction Mapping, Semigroup, Banach Space, Resolvent and Bounded Operator

1. Introduction

The theory of stability is important since stability plays a central role in the structural theory of operators such as semigroup of linear operator, contraction semigroup, invariant subspace theory and to mention but few. The theory of stability is rich in which concerns the methods and ideas, and this shall be one of the main points of this paper. The recent advances deeply interact with modern topics from complex function theory, harmonic analysis, the geometry of Ba-nach spaces, and spectra theory [1].

Another main focus of this paper is spectra analysis of a semigroup of linear operator, in which we use the resolvent to describe the relationship between the spectrum of A and of the semigroup operator

(

T t

( )

)

t0 and also determine the bounded linear operator A as the generators of one-parameter semigroups. Re-solvent operators are particularly useful in the analysis of Sturm-Liouville oper-ators and several others operoper-ators both bounded and unbounded.

Let X be a Banach space, XnX be a finite set,

(

T t

( )

)

t0 the C0-semigroup

which is strongly continuous one parameter semigroup of bounded linear

oper-How to cite this paper: Rauf, K., Akinyele, A.Y., Etuk, M.O., Zubair, R.O. and Aasa, M.A. (2019) Some Result of Stability and Spectra Properties on Semigroup of Linear Operator. Advances in Pure Mathematics, 9, 43-51.

https://doi.org/10.4236/apm.2019.91003

Received: October 17, 2018 Accepted: January 22, 2019 Published: January 25, 2019

Copyright © 2019 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0).

http://creativecommons.org/licenses/by/4.0/

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DOI: 10.4236/apm.2019.91003 44 Advances in Pure Mathematics

ator in X, ω-OCPn be ω-order-preserving partial contraction mapping

(semi-group of linear operator) which is an example of C0-semigroup. Similarly, let

( )

Mm  be a matrix, L X

( )

be a bounded linear operator on X, Pn a partial transformation semigroup, ρ

( )

A a resolvent set, σ

( )

A be spectrum and A is a generator of C0-semigroup.

This paper will focus on results of stability and spectra analysis of ω-OCPn on

Banach space as an example of a semigroup of linear called C0-semigroup, and

thereby establish the relationship between a semigroup, its generator and the resolvent as in Figure 1.

In [2], Batty obtained some spectral conditions for stability of one-parameter semigroup and also revealed some asymptotic behaviour of semigroup of opera-tor, see also, Batty et al.[3]. Chill and Tomilov [4] established some resolvent approach to stability operator semigroup. Räbiger and Wolf in [5] deduced some spectral and asymptotic properties of dominated operator. For relevant work on non-linear and one-parameter semigroups, see ([6] and [7]). The aim of this work is, therefore, to obtain stability and spectra analysis on a new subclass of semigroup of linear operator.

2. Preliminaries

The following definitions are crucial to the proof of our main results. Definition 2.1: (Stable Semigroup [8])

A strongly continuous semigroup

(

T t

( )

)

t0 is called

1) Uniformly exponentially stable if there exists >0 such that

( )

lim t 0

t→∞e T t =

(2.1)

2) Uniformly stable if

( )

lim 0

t→∞ T t = (2.2)

3) Strongly stable if

( )

lim x 0 .

[image:2.595.211.540.358.692.2]

t→∞ T t = ∀ ∈x X (2.3)

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DOI: 10.4236/apm.2019.91003 45 Advances in Pure Mathematics

Definition 2.2: (C0-Semigroup [9])

A C0-Semigroup is a strongly continuous one parameter semigroup of

bounded linear operator on Banach space. Definition 2.3: (ω-OCPn[10])

A transformation α ∈Pn is called ω-order-preserving partial contraction mapping if ∀x y, ∈Dom :

α

x y≤ ⇒

α

x

α

y and at least one of its transforma-tion must satisfy αy y= such that T t s

(

+

)

=T t T s

( ) ( )

whenever t s, >0 and otherwise for T

( )

0 =I.

Definition 2.4: (Core [8])

Let A be a closed linear operator with domain D A

( )

and range R A

( )

in a Banach space X. A subspace D of D A

( )

is called a core if A is the closure of its restriction to D.

Definition 2.5: (Resolvent Set [11])

We define the resolvent set of A denoted by ρ

( )

A set of all λ∈ such that λI A is one-to-one with range equal to X.

Definition 2.6: (Spectrum [11])

The spectrum of A denoted by σ

( )

A is defined as the complement of the resolvent set.

Definition 2.7: (Hyperbolic [12])

A semigroup

(

T t

( )

)

t0 on a Banach space X is called hyperbolic if X can be written as direct sum X X= sXu of two

(

T t

( )

)

t0-invariant, closed sub-spaces Xs, Xu such that the restricted semigroups

(

T ts

( )

)

t0 on Xs and

( )

(

T tu

)

t0 on Xu satisfy the following conditions:

1) The semigroup

(

T ts

( )

)

t0 is uniformly exponentially stable on Xs.

2) The operator T tu

( )

are invertible on Xu, and

(

( )

1

)

0

u t

T t

≥ is uniformly

exponentially stable on Xu.

Some Basic Spectral Properties

1) To any linear operator A we associate its spectral bound defined by

( )

{

( )

}

Spec A =Sup Re : λ λ σ∈ A .

2) Resolvent set: ρ

( )

A =

{

λ∈: :λ−A D A

( )

X is bijective

}

. 3) Spectrum: σ

( )

A =/ρ

( )

A .

4) Resolvent:

(

) (

)

1

( )

;

R λ A λ A − λ ρ A

= − ∀ ∈ .

5) Resolvent equation: R

(

λ;A R

)

(

µ;A

) (

= µ λ−

) (

R λ;A R

) (

µ;A

)

. Example 1:

2 2× matrix Mm

( )

+ 

Suppose

1 2 2 2 A=  

 

and let T t

( )

=etA, then

2

2 2

t t tA

t t

e e e

e e

 

=  

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DOI: 10.4236/apm.2019.91003 46 Advances in Pure Mathematics

3 3× matrix Mm

( )

+ 

Suppose

1 2 3 1 2 2 2 3 A

 

 

=  

 

and let T t

( )

=etA, then

2 3

2 2

2 3

t t t

tA t t t

t t

e e e

e e e e

I e e

 

 

=  

 

 

Example 2:

2 2× matrix Mm

( )

, we have

for each λ>0 such that λ ρ∈

( )

A where ρ

( )

A is a resolvent set on X. Suppose we have

1 2 2 A=  

 

and let T t

( )

=etAλ , then

2

2

t t tA

t

e e e

I e λ

λ λ

λ

 

=  

 

Example 3:

Let X C= ub

( )

+ be the space of all bounded and uniformly continuous

function from + to  , endowed with the sup-norm ⋅∞ and let

( )

{

T t t; ≥0

}

L X

( )

be defined by

( ) ( )

(

)

T t f s = f t s+

 

 

For each f X and each t s, ∈+, one may easily verify that

{

T t t

( )

; ≥0

}

satisfies the example 1 and 2 above.

3. Main Results

In this section, results of stability and spectral properties on ω-OCPn in Banach

space and on C0-semigroup are considered:

Theorem 3.1

Suppose X is a Banach space. Then a linear operator A D A:

( )

XX is an infinitesimal generator of a strongly continuous semigroup

(

T t

( )

)

t0 on X is uniformly exponentially stable if and only if for all p

[ )

1,∞ one has

( )

0 d

p

T t x t

< ∞

for all x X and A∈ω-OCPn. Proof

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DOI: 10.4236/apm.2019.91003 47 Advances in Pure Mathematics

( )

lim 0.

t→∞ T t = (3.1)

So, we define for n, the operators

(

X L, p

(

,X

)

)

+

ℑ∈ by

[ ]0,

( ) ( )

nx φ n T

ℑ = ⋅ ⋅ (3.2)

Then by assumption, the set

{

:

}

p

(

,

)

nx n L + X

ℑ ∈ ⊂  is bounded for each

x X, hence by the uniform boundedness principle, there exists C>0 such

that

( )

0 d for all , 0.

t T r x p r C x p p x X t

On the other hand, there exist M ≥1 and w>0 such that

( )

wt for all 0

T tMe t

From the previous two inequalities, we obtain

( )

( ) (

)

(

)

0

0

1 d

d

, 0.

pwt p t p

pwt

t p p

p p p

e T t x e T r T t r x r

p

M T t r x r

M C x x X t

= −

≤ −

≤ ∀ ∈ ≥

(3.3)

Hence, there exists a constant L>0 such that

( )

0 T tL ∀ ≥t

Considering this, we conclude that

( )

(

) ( )

( )

0

0

d d

, 0

t

p p

t p p p p p

t T t x T t r T r x r L T r x r L C x x X t

= −

≤ ∀ ∈ ≥

(3.4)

and therefore

( )

p1 0. T t LCt t

≤ ∀ >

This implies

( )

lim 0

t→∞ T t = (3.5)

Hence the proof is complete. Proposition 3.2

Suppose X is a Banach space and A D A:

( )

XX where A∈ω-OCPn is the infinitesimal generator for a strongly continuous semigroup

(

T t

( )

)

t0, then the following assertions are equivalent.

1)

(

T t

( )

)

t0 is hyperbolic. 2)

(

T t

( )

)

t0Γ =φ for all

t

> 0

. Proof

The proof of implication 1) ⇒ 2) starts from the observation that

( )

(

T t

)

( )

As

( )

Au

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DOI: 10.4236/apm.2019.91003 48 Advances in Pure Mathematics

By assumption,

(

T ts

( )

)

t0 is uniformly exponentially stable; hence r T t

(

s

( )

)

<1

for t>0, and therefore

( )

(

T ts

)

σ Γ =φ (3.6)

By the same argument, we obtain that r T t

(

s

( )

1

)

1

< . Suppose

( )

(

)

{

1:

( )

1

}

,

u

T t A

σ = λ− λ σ (3.7)

we conclude that λ >1 for each λ σ∈

( )

Au ; hence σ

(

T tu

( )

)

Γ =φ.

To prove 2) ⇒ 1), we fix s>0 such that σ

(

T s

( )

)

Γ =φ and we use the existence at a spectral projection P corresponding to the spectral set

( )

(

T s

)

{

( )

A : 1 .

}

σ = λ σ∈ λ < (3.8)

Then the space X is the direct sum X X= sXu of the

(

T t

( )

)

t0-invariant

subspaces Xs =rgP and Xu =kerP, where AsXs and AuXu. Then the restriction T ts

( )

L X

( )

s of T(s) has spectrum

( )

(

T ss

)

{

( )

A : 1

}

σ = λ σ∈ λ < (3.9)

hence, spectral radius r T s

(

s

( )

)

<1 . It follows that the semigroup

( )

(

T ts

)

t0 =

(

PT t

( )

)

t0 is uniformly exponentially stable on Xs.

Similarly, the restriction T su

( )

L X

( )

u of T s

( )

in Xu has spectrum

( )

(

T su

)

{

( )

A : 1

}

σ = λ σ∈ λ > (3.10)

hence is invertible on Xu. Clearly this implies that T tu

( )

is invertible for

0≤ ≤t s, while for t s> we choose n N such that ns t> . Then

( )

n

( )

(

) ( )

( ) (

)

u u u u u

T s =T ns =T ns t T t− =T t T ns t− (3.11)

hence T tu

( )

is invertible, since T su

( )

is bijective.

Moreover, for the spectral radius, we have

(

1

( )

)

1

u

r Ts < , and again this im-plies uniformly exponentially stable for the semigroup

(

( )

1

)

0

u t

T t

≥ . Hence the

proof.

Theorem 3.3

Suppose A∈ω-OCPn and ω-OCPnL X

( )

. Let A D A:

( )

XX be a

linear operator which satisfies:

a) A is densely defined and closed; and

b)

(

0,+∞ ⊆

)

ρ

( )

A and for each λ>0, we have

(

,

)

L X( ) 1. R λ A

λ ≤

Then:

1) limλ→∞λ λR

(

,A x x

)

= for each x X∈ ,

2) A x 2R

(

,A x

)

x

λ =

λ

λ

λ

for each x X∈ ,

3) limλ→∞A x Axλ = for each x D A

( )

and,

4) Aλ is the infinitesimal generator of a uniformly continuous semigroup

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DOI: 10.4236/apm.2019.91003 49 Advances in Pure Mathematics

( ) 1

tA L X

e λ

for each t≥0. In addition for each x X and

λ µ

, >0, we have

.

tA tA

e x e x t A x A xλ µ

λ µ

− ≤ −

Proof

Let x D A

( )

and λ>0. Then we have

(

,

)

(

,

)

1

R A x x AR A Ax Ax

λ λ λ

λ

− = ≤ (3.12)

and as a result

(

)

lim R ,A x x

λ→∞λ λ =

for each x D A

( )

.

Since D A

( )

is dense in X and

(

,

)

L X( ) 1, R A

λ λ ≤

and from (3.12), we deduce

(

)

lim R ,A x x.

λ→∞λ λ =

To show 2). Let us remark that we have successively

(

)

(

) (

) (

)

(

)

2R ,A I 2R ,A I A R ,A AR ,A A

λ

λ

λ

λ

=

λ

λ

λ λ

λ

=

λ

λ

= (3.13)

So, if x D A

( )

, by 1), we have

(

)

(

)

limA xλ lim R ,A x lim R ,A Ax Ax,

λ→∞ =λ→∞λ λ =λ→∞λ λ = (3.14)

which complete the proof of 2) and 3).

To show that tA ( ) 1

L X

e λ for each t0 . Since

-n Aλ∈ω OCP and

( )

-OCPn L X

ω ∈ , then by theorem of uniformly continuous semigroup, it fol-lows that its generates a uniformly semigroup

{

etAλ;t≥0

}

.

In order to show that tA ( ) 1 L X

e λ , let us remark that, by virtue of

(

)

2 ,

A xλ =

λ

R

λ

A x

λ

x for each x X∈ and b), we have

( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) 2 2 2 , , , 1 L X

t R A t I t R A

tA t I

L X

L X L X L X

t R A t I t I t I

e e e e

e e e e

λ λ λ λ λ λ λ

λ λ λ λ λ

− −

− −

= ≤

≤ ≤ =

(3.15)

Since Aλ, Aµ, etAλ and etAµ commute each to another for each λ µ ρ, ∈

( )

A

and

λ µ

, >0, we have

( )

(

)

( )

(

)

1 1 0 1 1 0 d d d d

tA s tA

tA stA

s tA stA

e x e x e e x s

s

t e e A x A x s

t A x A x

µ µ λ λ µ λ λ µ λ µ − − − = ≤ − ≤ −

(3.16)

Hence the proof is complete. Theorem 3.4

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satis-DOI: 10.4236/apm.2019.91003 50 Advances in Pure Mathematics

fying both

(

0,+∞ ⊆

)

σ

( )

A and

(

)

( )

, n

n

L X

R A M

λ λ ≤

for each n N and λ>0 and if

λ µ

, are regular values, i.e. λ µ ρ, ∈

( )

A and R

(

λ, ,A R

) (

µ,A

)

L X

( )

, then there exist:

1) R

(

λ,A R

)

(

µ,A

) (

= µ λ−

) (

R λ,A R

) (

µ,A

)

. 2) xx M x.

3) λ λR

(

,A x

)

x for each x X and λ>0.

Proof

To prove 1), let us observe that

(

)

(

)(

) (

)

(

) (

{

) (

)

}

(

)

(

) (

) (

)

(

)

, , ,

, ,

, , ,

R A R A I A R A

R A I I A R A

R A R A R A

λ λ µ µ

λ µ λ λ µ

µ λ λ µ µ

= −

= − + −

= − +

so that

(

,

)

(

,

) (

) (

,

) (

,

)

R λ A R− µ A = µ λ− R λ A R µ A (3.17)

and this complete the proof of 1).

To prove 2), we assume for

µ

>0 and let us define ⋅µ:D A

( )

X →+

by

(

)

Sup n , n ,

n

xµ = µ R µ A x (3.18)

it’s obvious that

xxµM x (3.19)

and

(

,

)

.

R A xµ xµ

µ µ ≤

We want to prove that

(

,

)

R A xµ xµ

λ λ ≤ (3.20)

for each λ∈

(

0,µ

]

.

So by resolvent Equation (3.17), we have

(

,

)

(

,

)

(

(

) (

,

)

)

R λ A x R= µ A x+ µ λ− R µ A x

and therefore

(

,

)

1 1

(

,

)

.

R λ A xµ xµ λ R λ A xµ

µ µ

 

≤ + −

 

Consequently λ R

(

λ,A x

)

µxµ which proves (3.20).

From (3.19) and (3.20), we deduced that, for each n and λ∈

(

0,µ

]

, we have

(

,

)

n

(

,

)

n

nR A x nR A x x

µ

λ λ ≤ λ λ ≤ (3.21)

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DOI: 10.4236/apm.2019.91003 51 Advances in Pure Mathematics

now get xλxµ for each λ∈

(

0,µ

]

. We can now define

0

lim x xµ

µ→

= (3.22)

Since 2) readily follows from (3.19), and 3) from (3.21) by taking n=1, we

have

(

,

)

R A x x

λ λ ≤

Hence the proof.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this pa-per.

References

[1] Banach, S. (1922) Surles Operation Dam Les Eusembles Abstracts et lear Applica-tion Aus EquaApplica-tion Integrals. Fundamenta Mathematicae, 3, 133-181.

https://doi.org/10.4064/fm-3-1-133-181

[2] Batty, C.J.K. (1996) Spectral Condition for Stabilty of One-Parameter Semigroup. Journal of Differential Equations, 127, 87-96.

https://doi.org/10.1006/jdeq.1996.0062

[3] Batty, C.J.K., Chill, R. and Tomilov, Y. (2002) Strong Stability of Bounded Evolution Families and Semigroup. Journal of Functional Analysis, 193, 116-139.

https://doi.org/10.1006/jfan.2001.3917

[4] Chill, R. and Tomilov, Y. (2007) Stability Operator Semigroup. Banach Center Pub-lication 75, Polish Academy of Sciences, Warsaw, 71-73.

[5] Räbiger, F. and Wolf, M.P.H. (2000) Spectral and Asymptotic Properties of Resol-vent Dominated-Operators. Journal of the Australian Mathematical Society Series A, 68, 181-201. https://doi.org/10.1017/S1446788700001944

[6] Ambrozie, C. and Müller, V. (2004) Invariant Subspaces for Polynomially Bounded Operators. Journal of Functional Analysis, 213, 321-345.

https://doi.org/10.1016/j.jfa.2003.12.004

[7] Balakrishnan, A.V. (1960) Fractional Powers of Closed Operators and Semigroups Generated by Them. Pacific Journal of Mathematics, 10, 419-437.

https://doi.org/10.2140/pjm.1960.10.419

[8] Ioan, I.V. (2003) C0-Semigroups and Applications. Mathematics Studies, 191,

Elser-vier, North-Holland.

[9] Engel, K. and Nagel, R. (1999) One-Parameter Semigroups for Linear Equations. Graduate Texts in Mathematics, 194, Springer, New York.

[10] Rauf, K. and Akinyele, A.Y. (2019) Properties of ω-Order-Preserving Partial Con-traction Mapping and Its Relation to C0-Semigroup. International Journal of

Ma-thematics and Computer Science, 14, 61-68.

[11] Bellem-Morante, A. and McBride, A. (1998) Applied Non-Linear Semigroups. Ma-thematics Methods in Practices. John Wiley and Sons, Chichester.

Figure

Figure 1. Diagrammatical representation of relationship between a semigroup, its gene-rator and its resolvent [8]

References

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