Exponential dichotomy of difference equations in lp phase spaces on the half line

IN

l

-PHASE SPACES ON THE HALF-LINE

Received 14 November 2005; Revised 12 March 2006; Accepted 17 May 2006

For a sequence of bounded linear operators{An}∞n=0on a Banach spaceX, we investigate the characterization of exponential dichotomy of the difference equationsvn+1=Anvn. We characterize the exponential dichotomy of difference equations in terms of the exis-tence of solutions to the equationsvn+1=Anvn+fninlp spaces (1≤p <∞). Then we

apply the results to study the robustness of exponential dichotomy of difference equa-tions.

Copyright © 2006 N. T. Huy and V. T. N. Ha. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, dis-tribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction and preliminaries

We consider the difference equation

xn+1=Anxn, n∈N, (1.1)

whereAn,n=0, 1, 2,..., is a sequence of bounded linear operators on a given Banach

spaceX,xn∈Xforn∈N.

One of the central interests in the asymptotic behavior of solutions to (1.1) is to find conditions for solutions to (1.1) to be stable, unstable, and especially to have an exponen-tial dichotomy (see, e.g., [1,5,7,12,16–20] and the references therein for more details on the history of this problem). One can also use the results on exponential dichotomy of difference equations to obtain characterization of exponential dichotomy of evolution equations through the discretizing processes (see, e.g., [4,7,9,18]).

One can easily see that ifAn=Afor alln∈N, then the asymptotic behavior of solu-tions to (1.1) can be determined by the spectra of the operatorA. However, the situation becomes more complicated if{An}n∈Nis not a constant sequence because, in this case,

the spectra of each operatorAncannot determine the asymptotic behavior of the solu-tions to (1.1). Therefore, in order to find the conditions for (1.1) to have an exponential dichotomy, one tries to relate the exponential dichotomy of (1.1) to the solvability of the

following inhomogeneous equation:

xn+1=Anxn+fn, n∈N, (1.2)

in some certain sequence spaces for each given f = {fn}. In other words, one wants to relate the exponential dichotomy of (1.1) to the surjectiveness of the operatorTdefined by

(Tx)n:=xn+1−Anxn forx=

xnbelonging to a relevant sequence space. (1.3)

In the infinite-dimensional case, in order to characterize the exponential dichotomy of (1.1) defined onN, beside the surjectiveness of the operatorT, one needs a priori con-dition that the stable space is complemented (see, e.g., [5]). In our recent paper, we have replaced this condition by the spectral conditions of related operators (see [9, Corollary 3.3]).

At this point, we would like to note that if one considers the difference equation (1.1) defined onZ, then the existence of exponential dichotomy of (1.1) is equivalent to the existence and uniqueness of the solution of (1.2) for a given f = {fn}n∈Z, or, in other

words, to the invertibility of the operatorT on suitable sequence spaces defined onZ. This means that one can drop the above priori condition in the case that the difference equations are defined onZ(see [7, Theorem 3.3] for the original result and see also [2,

3,11,15] for recent results on the exponential dichotomy of difference equations defined onZ).

However, if one considers the difference equation (1.1) defined only onN, then the situation becomes more complicated, because for a given f = {fn}n∈N, the solutions of

the difference equation (1.2) on Nare not unique even in the case that the difference equation (1.1) has an exponential dichotomy. Moreover, one does not have any informa-tion on the negative half-lineZ−:= {z∈Z:z≤0}of the difference equations (1.1) and (1.2) (we refer the readers to [8] for more details on the differences between the expo-nential dichotomy of the differential equations defined on the half-line and on the whole line). Therefore, one needs new ideas and new techniques to handle the exponential di-chotomy of difference equations defined only onN. For differential equations defined on the half-line, such ideas and techniques have appeared in [14] (see also [8,13]). Those ideas and techniques have been exploited to obtain the characterization of exponential dichotomy of difference equations defined onNwithl∞-phase space of sequences de-fined onN(see [9]). As a result, we have obtained a necessary and sufficient condition for difference equations to have an exponential dichotomy. This conditions related to the solvability of (1.2) inl∞spaces of sequences defined onN. In the present paper, we will characterize the exponential dichotomy of (1.1) by the solvability of (1.2) in lp spaces

(1≤p <∞) of sequences defined onN. Moreover, we also characterize the exponential dichotomy by invertibility of a certain appropriate difference operator derived from the operatorT. Consequently, we will use this characterization to prove the robustness of ex-ponential dichotomy under small perturbations. Our results are contained in Theorems

3.2,3.6,3.7, andCorollary 3.3.

usual, we denote byN,R,R+, andCthe sets of natural, real, nonnegative real, and com-plex numbers, respectively. Throughout this paper, for 1≤p <∞we will consider the following sequence spaces:

lp(N,X) :=

v=vnn∈N:vn∈X:

∞

n=0 _vnp

<∞

:=lp,

l0

p(N,X) :=v=vn:v∈lp;v0=0:=l0p

(1.4)

endowed with the normvp:=(∞_n=0vnp)1/p.

Let{An}n∈N be a sequence of bounded linear operators fromX toX which is

uni-formly bounded. This means that there existsM >0 such thatAnx ≤Mxfor all

n∈Nandx∈X. Next we define a discrete evolution familyᐁ=(Un,m)n≥m≥0associated with the sequence{An}n∈Nas follows:

Um,m=Id (the identity operator inX)

Un,m=An−1An−2···Am forn > m. (1.5)

The uniform boundedness of{An} yields the exponential boundedness of the evolu-tion family (Un,m)n≥m≥0. That is, there exist positive constantsK,αsuch thatUn,mx ≤ Keα(n−m)_{x;x∈X;n≥m≥0.}

Definition 1.1. Equation (1.1) is said to have anexponential dichotomy if there exist a family of projections (Pn)n∈NonXand positive constantsN,νsuch that

(1)AnPn=Pn+1An;

(2)An: kerPn→kerPn+1is an isomorphism and its inverse is denoted byA−_|n1; (3)Un,mx ≤Ne−ν(n−m)x;n≥m≥0;x∈PmX;

(4) denoteU|m,n=A−_|m1A−_|m1+1···A−|n1−1;n > m, andU|m,m=Id, then

_U|m,nx≤Ne−ν(n−m)_{x, n≥m≥0;x∈kerP}

n. (1.6)

The above family of projections (Pn)n∈Nis called the family of dichotomy projections.

We define a linear operatorTas follows:

Ifu=un∈lp set (Tu)n=un+1−Anun forn∈N. (1.7)

Foru= {un} ∈lp, we have(Tu)n = un+1−Anun ≤ un+1+Mun, henceTu∈lp

andTup≤(1 +M)up. This means thatTis a bounded linear operator fromlpinto lp. We denote the restriction ofTonl0pbyT0, this means thatD(T0)=l0pandT0u=Tu

foru∈l0

p. From the definition ofT, the following are obvious.

Remark 1.2. (i) kerT= {u= {un} ∈lp:un=Un,0u0,n∈N}.

(ii) It is easy to verify thatT0is injective. Indeed, letu= {un},v= {vn} ∈l0

pandT0u=

Recall that for an operatorBon a Banach spaceY, the approximate point spectrum Aσ(B) ofB is the set of all complex numbersλ such that for every>0, there exists y∈D(B) withy =1 and(λ−B)y ≤.

In order to characterize the exponential stability and dichotomy of an evolution family, we need the concept oflp-stable spaces defined as follows.

Definition 1.3. For a discrete evolution familyᐁ=(Um,n)m≥n≥0, m,n∈N, on Banach spaceXandn0∈N, define thelp-stable spaceX0(n0) by

X0 n0:=

x∈X: ∞

n=n0

_Un,n0xp_<∞.

(1.8)

An orbitUm,n0xform≥n0≥0 andx∈X0(n0) is called anlp-stable orbit.

2. Exponential stability

In this section we will give a sufficient condition for stability oflp-stable orbits of a

dis-crete evolution familyᐁ. The obtained results will be used in the next section to charac-terize the exponential dichotomy of (1.1).

Theorem2.1. Let the operatorT0defined as above satisfy the condition0∈Aσ(T0). Then everylp-stable orbit ofᐁis exponentially stable. Precisely, there exist positive constantsN,ν

such that for anyn0∈Nandx∈X0(n0), _Un,n0x≤Ne−ν(n−s)_U

s,n0x, n≥s≥n0. (2.1)

Proof. Since 0∈Aσ(T0), we have that there exists a constantη >0 such that

ηT0vp≥ vp forv∈l0p. (2.2)

To prove (2.1), we first prove that there is a positive constantlsuch that for anyn0∈N

andx∈X0(n0),

_{Un,n0x≤lUs,n}

0x, n≥s≥n0≥0. (2.3)

Fixn0∈N,x∈X0(n0), ands≥n0. Taking

v=vn withvn:=

⎧ ⎨ ⎩Un,n0x

forn > s,

0 for 0≤n≤s, (2.4)

we havev∈l0

p. By definition ofT0, we have (T0v)n=vn+1−Anvn. This yields

T0vn=

⎧ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎩

0 forn≤s−1, Us+1,n0x forn=s, 0 forn > s.

By inequality (2.2), we have

By inequality (2.2), we obtain

we have that

b

n=a+1

_vn≤(b−a)1−1/p_{vp. (2.14)}

Substituting this into inequality (2.12), we obtain that

η(b−a)≥ b

n=a+1

_{vn. (2.15)}

Using now the estimates (2.9), we have

η(b−a)≥ b n=a+1

_vn= b n=a+1

n

k=a+1 _{un uk}

≥ b

n=a+1 1 2lua

n

k=a+1 1 lua=

(b−a)(b−a+ 1) 4l2 >

(b−a)2 4l2 .

(2.16)

This yieldsb−a <4ηl2_{. PuttingK:=4ηl}2_{, the inequality (2.1) follows.}

We finish by proving (2.1). Indeed, ifn≥s≥n0∈Nwritingn−s=n1K+rfor 0≤

r < K, andn1∈N, we have _{Un,n0x=Un−s+s,n0x=Un}

1K+r+s,n0x by (2.8)

≤ 1

2n1

_Ur+s,n0xby (2.3)

≤ l

2n1 _Us,n

0x≤2le−

((n−s)/K) ln 2_U

s,n0x.

(2.17)

TakingN:=2landν:=ln 2/K, the inequality (2.1) follows.

From this theorem, we obtain the following corollary.

Corollary2.2. Under the conditions ofTheorem 2.1, the spaceX0(n0)can be expressed as

X0 n0=x∈X:Un,n0x≤Ne−ν(n−n0)x;n≥n0≥0, (2.18)

for certain positive constantsN,ν. Hence,X0(n0)is a closed linear subspace ofX.

3. Exponential dichotomy and perturbations

We will characterize the exponential dichotomy of (1.1) by using the operatorsT0,T. In particular, we will also get necessary and sufficient conditions for exponential dichotomy in Hilbert spaces and finite-dimensional spaces. Moreover, using our characterization of the exponential dichotomy, we can prove the robustness of the exponential dichotomy of (1.1) under small perturbations. Then we start with the following lemma which has a history that can be traced back to [14, Lemma 4.2] and to [6] and beyond.

Proof. The proof is done in [9, Lemma 3.1]. We present it here for sake of completeness.

Now we come to our first main result. It characterizes the exponential dichotomy of (1.1) by properties of the operatorT.

Theorem3.2. Let{An}n∈Nbe a family of bounded linear and uniformly bounded operators

on the Banach spaceX. Then the following assertions are equivalent. (i)Equation (1.1) has an exponential dichotomy.

(ii)Tis surjective andX0(0)is complemented inX.

(ii)⇒(i). We prove this in several steps.

(A) LetZ⊆Xbe a complement ofX0(0) inX, that is,X=X0(0)⊕Z. SetX1(n)=

Un,0Z. Then

Un,sX0(s)⊆X0(n), Un,sX1(s)=X1(n), n≥s≥0. (3.5)

(B) There are constantsN,ν>0 such that _Un,0x≥Neν(n−s)_U

s,0x forx∈X1(0), n≥s≥0. (3.6)

In fact, letY:= {(vn)n∈N∈lp:v0∈X1(0)}endowed withlp-norm. ThenY is a closed subspace of the Banach spacelp, and henceYis complete. ByRemark 1.2, we have kerT:= {v∈lp:vn=Un,0xfor somex∈X0(0)}. SinceX=X0(0)⊕X1(0) andTis surjective, we

obtain that

T:Y−→lp (3.7)

is bijective and hence an isomorphism. Thus, by Banach isomorphism theorem, there is a constantη >0 such that

ηTvp≥ vp, forv∈Y. (3.8)

To prove (3.6), we first prove that there is a positive constantlsuch that

_{Un,0x≥lUs,0x forx∈X1(0),n≥s≥0,n,s∈N. (3.9)}

Indeed, fixx∈X1(0),x=0, andn≥s≥0. Ifn=0, there is nothing to do. So, assume thatn≥1. Now taking

v:=vm withvm:=

⎧ ⎨ ⎩Um,0x

for 0≤m≤n−1,

0 form > n−1, (3.10)

we have thatv∈Y. Then, by definition ofT, we obtain that

(Tv)m:=

⎧ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎩

0 form > n−1,

−Un,0x form=n−1, 0 form < n−1.

(3.11)

Inequality (3.8) yields

ηUn,0x≥

⎛ ⎝n−1

k=0

_Uk,0xp

⎞ ⎠

1/p

≥Us,0x ∀0≤s≤n−1. (3.12)

We now show that there is a numberK=K(η,l)>0 such that

By inequality (3.8), we obtain

η(b−a)1/p≥ vp. (3.17)

Substituting this into inequality (3.17), we obtain that

η(b−a)≥

b−1

n=a

Since v−w∈lp, we have that x−wn0∈X0(n0). On the other hand, since w0=w 0₊ w1 _withwk_∈Xk(0),wn

0=Un0,0w 0_+U

n0,0w1, and by (3.5), we have Un0,0wk∈Xk(n0),

k=0, 1. Hencex=x−wn0+wn0=x−wn0+Un0,0w 0_+U

n0,0w1∈X0(n0) +X1(n0). This

proves (C).

(D) LetPnbe the projections fromXontoX0(n) with kernelX1(n),n≥0. Then (3.5)

implies thatPn+1Un+1,n=Un+1,nPn, orAnPn=Pn+1Anforn≥0. From (3.5), (3.6), and

An=Un+1,n, we obtain thatAn: kerPn→kerPn+1,n≥0 is an isomorphism. Finally, by (3.6),Theorem 2.1, and the assumption 0∈Aσ(T0), there exist constantsN,ν>0 such that

_Un,mx≤Ne−ν(n−m)_{x forx∈P}

mX,n≥m≥0,

_U|m,nx≤Ne−ν(n−m)_{x forx∈kerP}

n,n≥m≥0.

(3.26)

Thus (1.1) has an exponential dichotomy.

IfXis a Hilbert space, we need only the closedness ofX0(0). Therefore, we obtain the following corollary.

Corollary3.3. IfXis a Hilbert space, then the conditions that0∈Aσ(T0)andTis sur-jective are necessary and sufficient for (1.1) to have an exponential dichotomy.

This can be restated as follows.

IfXis a Hilbert space, then the conditions

(1)for allf ∈lp, there exists a solutionx∈lpof (1.2);

(2)there exists a constantc >0such that all bounded solutionsx= {xn}(withx0=0 andx∈lp) of (1.2) (with f ∈lp) satisfy∞n=0xnp≤c

_∞

n=0fnp are necessary and sufficient for (1.1) to have an exponential dichotomy.

Proof. The corollary is obvious in view ofCorollary 2.2andTheorem 3.2.

IfX is a finite-dimensional space, then every subspace of X is closed and comple-mented. Hence, byTheorem 3.2we have the following corollary.

Corollary3.4. IfXis a finite-dimensional space, then the condition thatTis surjective is necessary and sufficient for existence of exponential dichotomy of (1.1).

In our next result, we will characterize the exponential dichotomy of (1.1) using in-vertibility of a certain operator derived from the operatorT. In order to obtain such a characterization, we have to know the subspace kerP0in advance (see Theorem 3.6). Consequently, the exponential dichotomy of evolution family will be characterized by the invertibility of the restriction ofT to a certain subspace oflp. This restriction will be defined as follows.

Definition 3.5. For a closed linear subspaceZofX, define

lZ

p:=f =fn∈lp:f0∈Z. (3.27)

Then,lZ_p is a closed subspace of (lp, · p). Denote by TZ the part ofT inlZp, that is,

With these notations, we obtain the following characterization of exponential dicho-tomy of (1.1).

Theorem3.6. Let{An}n∈Nbe a family of bounded linear and uniformly bounded

opera-tors on the Banach spaceX and letZ be a closed linear subspace ofX. Then the following assertions are equivalent.

(i)Equation (1.1) has an exponential dichotomy with the family of dichotomy projec-tions{Pn}n∈NsatisfyingkerP0=Z.

(ii)TZ:lZp→lpis invertible.

Proof. We first note that the following proof is inspired by the proof of [14, Theorem 4.5]. (i)⇒(ii). LetPn,n∈N, be a family of projections given by the exponential dichotomy

of (1.1) such that kerP0=Z. ThenP0X=X0(0) andX=X0(0)⊕Z. Fix f = {fn} ∈lp. By

Theorem 3.2, there isv= {vn} ∈D(T) such thatTv= f. On the other hand, by definition ofX0(0), the sequenceu= {un}defined byun=Un,0P0v0belongs tolp. By definition ofT,

we obtain thatTu=0. Moreover,v0−u0=v0−P0v0∈ZsinceX=P0X⊕Z. Therefore, v−u∈lZ

pandTZ(v−u)=T(v−u)=Tv=f. Hence,TZ:D(TZ)→lpis surjective.

If noww={wn} ∈kerTZthen, by definition ofTZ,wn=Un,0w0withw0∈Z∩X0(0)= {0}. Thus,w=0, that is,TZis injective.

(ii)⇒(i). LetTZ:lZp→lpbe invertible. SinceTZ is the restriction ofTtolZp, it follows

thatTis surjective. The boundedness ofTZimplies thatT_Z−1is bounded, and hence there isη >0 such thatηTvp=ηTZvp≥ vp for allv∈lZp. SinceT0 is the part ofTZ

inl0

p= {f ∈lp: f0=0}, we obtain thatηT0vp≥ vp for allv∈D(T0). Hence, 0∈

Aσ(T0). ByCorollary 2.2,X0(0) is closed. We now prove thatX=X0(0)⊕Z. Let nowx∈ X. IfUn,0x=0 for somen=n0>0, thenUn,0x=Un,n0Un0,0x=0 for alln≥n0yielding x∈X0(0). Otherwise,Un,0x=0 for alln∈N. Setu= {un}with

un:=

⎧ ⎨ ⎩x

forn=0

0 forn >0, f =

fn with fn:=

⎧ ⎨ ⎩−A0x

forn=0

0 forn >0. (3.28)

Clearly,u∈lp,f ∈lp, and fn=un+1−Anun. Therefore,Tu= f. On the other hand, since

TZis invertible, there existsv∈lZ_psuch thatTZv=f =Tv. Thus,u−v∈kerT, and hence

(u−v)n=Un,0 u0−v0

=Un,0 x−v0

, n∈N. (3.29)

Sinceu−v∈lp, this implies thatx−v0∈X0(0). Thusx=x−v0+v0∈X0(0) +Z.

If now y∈X0(0)∩Z, then the sequence w= {wn} defined bywn:=Un,0y, n∈N, belongs tolZ_p∩kerT(see definitions ofX0(0) andT). Hence,TZw=0 and by invertibility

of TZ, we have thatw=0. Thus y=w0=0, that is,X0(0)∩Z= {0}. This yields that

X=X0(0)⊕Z. The assertion now follows fromTheorem 3.2. Using the above characterization of exponential dichotomy, we now study the robust-ness of the exponential dichotomy of (1.1) under small perturbations. Precisely, we have the following perturbation theorem.

Theorem3.7. Let (1.1) have an exponential dichotomy and let{Bn}n∈Nbe a sequence of

M >0such thatBn ≤M for alln∈N). Then, ifH:=sup_n∈NBnis sufficiently small, the equation

un+1= An+Bn

un (3.30)

has an exponential dichotomy as well.

Proof. Let (1.1) have an exponential dichotomy with the corresponding dichotomy pro-jections (Pn)n∈N. Put kerP0=Z. Then,Zis a closed subspace ofX. ByTheorem 3.6, we

have that the operatorTZ defined byDefinition 3.5is invertible. Let nowTB,Zbe the

op-erator corresponding to the perturbed difference equation (3.30). That is,TB,Z :lZ_p →lp is defined by (TB,Zu)n=un+1−(An+Bn)un. We now define the operatorBby (Bf)n:= Bnfnfor f = {fn}and alln∈N. We then prove thatB:lp→lpis a bounded linear

oper-ator andB ≤H. Indeed, take f ∈lp. Then(Bf)n = Bnfn ≤Hfnfor alln∈N. It follows that the sequenceBf belongs tolpandBfp≤Hfp. We thus obtain that

B:lp→lpis a bounded linear operator andB ≤H.

It is clear thatTB,Z =TZ−B. Since TZ is invertible, by a perturbation theorem of Kato [10, IV.1.16], we obtain that ifBis sufficiently small thenTB,Z=TZ−Bis also invertible. ByTheorem 3.6we have that (3.30) has an exponential dichotomy.

Acknowledgment

The second author would like to thank Professor Heinrich Begehr for moral support.

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Nguyen Thieu Huy: Department of Applied Mathematics, Hanoi University of Technology, Khoa Toan Ung Dung, Dai Hoc Bach Khoa Ha Noi, 1 Dai Co Viet Street, Hanoi, Vietnam

E-mail address:[email protected]

Vu Thi Ngoc Ha: Department of Applied Mathematics, Hanoi University of Technology, Khoa Toan Ung Dung, Dai Hoc Bach Khoa Ha Noi, 1 Dai Co Viet Street, Hanoi, Vietnam