• No results found

Compact operators on some Fibonacci difference sequence spaces


Academic year: 2020

Share "Compact operators on some Fibonacci difference sequence spaces"


Loading.... (view fulltext now)

Full text



Open Access

Compact operators on some Fibonacci

difference sequence spaces

Abdullah Alotaibi


, Mohammad Mursaleen


, Badriah AS Alamri


and Syed Abdul Mohiuddine



mursaleenm@gmail.com 1Operator Theory and Applications Research Group, Department of Mathematics, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah, 21589, Saudi Arabia 2Department of Mathematics,

Aligarh Muslim University, Aligarh, 202 002, India


In this paper, we characterize the matrix classes (


p(F)) (1≤p<∞), where

p(F) is

some Fibonacci difference sequence spaces. We also obtain estimates for the norms of the bounded linear operatorsLAdefined by these matrix transformations and find

conditions to obtain the corresponding subclasses of compact matrix operators by using the Hausdorff measure of noncompactness.

MSC: 46A45; 11B39; 46B50

Keywords: sequence spaces; Fibonacci numbers; compact operators; Hausdorff measure of noncompactness

1 Introduction and preliminaries

LetN={, , , . . .}andRbe the set of all real numbers. We shall writelimk,supk,infk, and

kinstead oflimk→∞,supk∈N,infk∈N, and ∞

k=, respectively. Letωbe the vector space

of all real sequencesx= (xk)k∈N. By the termsequence space, we shall mean any linear

subspace ofω. Letϕ,∞,c, andcdenote the sets of all finite, bounded, convergent and

null sequences, respectively. We writep={xω:

k|xk|p<∞}for ≤p<∞. Also, we

shall use the conventions thate= (, , . . .) ande(n)is the sequence whose only non-zero

term is  in thenth place for eachn∈N. For any sequencex= (xk), letx[n]= n


be itsn-section. Moreover, we writebsandcsfor the sets of sequences with bounded and convergent partial sums, respectively.

AB-space is a complete normed space. A topological sequence space in which all coor-dinate functionalsπk,πk(x) =xk, are continuous is called aK-space. ABK-space is defined

as aK-space which is also aB-space, that is, aBK-space is a Banach space with continu-ous coordinates. ABK-spaceXϕis said to haveAKif every sequencex= (xk)∈Xhas

a unique representationx=kxke(k). For example, the spacep(≤p<∞) isBK-space

withxp= (

k|xk|p)/pandc,c, and∞areBK-spaces withx∞=supk|xk|. Further,

theBK-spacescandphaveAK, where ≤p<∞(cf.[]).

A sequence (bn) in a normed space X is called a Schauder basisfor X if for every

xX there is a unique sequence (αn) of scalars such that x=

nαnbn, i.e., limmxm

n=αnbn= .

Theβ-dual of a sequence spaceXis defined by

=a= (ak)∈ω:ax= (akxk)∈csfor allx= (xk)∈X



LetA= (ank)∞n,k=be an infinite matrix of real numbersank, wheren,k∈N. We writeAn

for the sequence in thenth row ofA, that is,An= (ank)∞k=for everyn∈N. In addition,

ifx= (xk)∞k=ωthen we define theA-transform of xas the sequenceAx={An(x)}∞n=,


An(x) = ∞


ankxk (n∈N)

provided the series on the right side converges for eachn∈N.

For arbitrary subsetsXandY ofω, we write (X,Y) for the class of all infinite matrices that mapXintoY. Thus,A∈(X,Y) if and only ifAnfor alln∈NandAxYfor all


The matrix domainXAof an infinite matrixAin sequence spaceXis defined by


x= (xk)∈ω:AxX

which is a sequence space.

The following results are very important in our study [, ].

Lemma . Let Xφand Y be BK -spaces.

(a) Then we have(X,Y)⊂B(X,Y),that is,every matrixA∈(X,Y)defines an operator

LAB(X,Y)byLA(x) =Axfor allxX.

(b) IfXhasAK,thenB(X,Y)⊂(X,Y),that is,for every operatorLB(X,Y)there

exists a matrixA∈(X,Y)such thatL(x) =Axfor allxX.

TheFibonacci numbersare the sequence of numbers{fn}∞n=defined by the linear

recur-rence equations

f=f=  and fn=fn–+fn–; n≥.

Fibonacci numbers have many interesting properties and applications in arts, sciences and architecture. For example, the ratio sequences of Fibonacci numbers converge to the golden ratio which is important in sciences and arts. Also, some basic properties of Fi-bonacci numbers can be found in [].

Throughout, let ≤p≤ ∞andq denote the conjugate ofp, that is,q=p/(p– ) for  <p<∞,q=∞forp=  orq=  forp=∞.

Recently, in [] and [], the Fibonacci difference sequence spacesp(F) and∞(F) have

been defined as follows:

p(F) =

x= (xn)∈ω:

n fn






p<∞ ; ≤p<∞


∞(F) =

x= (xn)∈ω:sup n∈N







Using the notation of the matrix domain, the sequence spacesp(F) and∞(F) may be

redefined by

p(F) = (p)F (≤p<∞) and ∞(F) = (∞)F, (.)

where the matrixF= (fnk) is defined by

fnk= ⎧ ⎪ ⎪ ⎨ ⎪ ⎪ ⎩


fn (k=n– ),


fn+ (k=n),

 (≤k<n– ) or (k>n)

(n,k∈N). (.)

Further, it is clear that the spacesp(F) and∞(F) areBKspaces with the norms given by


n yn(x)


; (≤p<∞) andx∞(F)=sup n∈N

yn(x), (.)

where the sequencey= (yn) = (Fn(x)) is theF-transform of a sequencex= (xn),i.e.,

yn=Fn(x) = ⎧ ⎨ ⎩ f

fx=x (n= ),




fn xn– (n≥)

(n∈N). (.)

2 Methods of measure of noncompactness

The first measure of noncompactness, the functionα, was defined and studied by Kura-towski [] in . Darbo [] used this measure to generalize both the classical Schauder fixed point principle and (a special variant of ) the Banach contraction mapping principle for so-called condensing operators. The Hausdorff or ball measure of noncompactnessχ

was introduced by Goldenšteinet al.[] in , and later studied by Goldenštein and Markus [].

LetXandY be infinite dimensional Banach spaces. We recall that a linear operatorL

fromXintoYis calledcompactif its domain is all ofXand, for every bounded sequence (xn) inX, the sequence the sequence (L(xn)) has a convergent subsequence. We denote the

class of all compact operators inB(X,Y) byC(X,Y).

Let (X,d) be a metric space,x∈X andr> . Then we write, as usual,B(x,r) ={x

X:d(x,x) <r}for the open ball of radiusrand centerx. LetMXdenote the class of all

bounded subsets ofX. IfQMX, then theHausdorff measure of noncompactness of the

set Q, denoted byχ(Q), is defined by

χ(Q) =inf

>  :Q



B(xk,rk),xkX,rk<(k= , , . . .),n∈N


The functionχ:MX→[,∞) is called theHausdorff measure of noncompactness.

The basic properties of the Hausdorff measure of noncompactness can be found in [, –].


measures of noncompactness onXandY, respectively. An operatorL:XYis said to be (χ,χ)-boundedifL(Q)∈MYfor allQMXand there exists a constantC≥ such that

χ(L(Q))≤(Q) for allQMX. If an operatorLis (χ,χ)-bounded then the number L(χ,χ)=inf

C≥ :χ

L(Q)≤(Q) for allQMX

is called the (χ,χ)-measure of noncompactness of L. If χ =χ =χ, then we write L(χ,χ)=.

Now we outline the applications of the Hausdorff measure of noncompactness to the characterization of compact operators between Banach spaces. LetXandY be Banach spaces andLB(X,Y). Then the Hausdorff measure of noncompactness ofLis given by ([], Theorem .)




andLis compact if and only if ([], Corollary . (.))

= . (.)

The identities in (.) and (.) reduce the characterization of compact operators LB(X,Y) to the determination of the Hausdorff measure of noncompactness χ(Q) of bounded setsQin a Banach spaceX. IfXhas a Schauder basis, then there exist estimates or even identities forχ(Q).

Theorem .([] or [], Theorem .) Let X be a Banach space with a Schauder basis

(bk)∞k=,QMX,Pn:XX be the projectors onto the linear span of{b,b, . . . ,bn}and

Rn=IPnfor n= , , . . . ,where I denotes the identity map on X.Then we have

a·lim supn→∞

sup x∈Q

Rn(x)≤χ(Q)≤lim sup n→∞

sup x∈Q


where a=lim supn→∞Rn.

In particular, the following result shows how to compute the Hausdorff measure of non-compactness in the spacescandp(≤p<∞), which areBK-spaces withAK.

Theorem .([], Theorem .) Let Q be a bounded subset of the normed space X,

where X ispfor≤p<∞or c.If Pn:XX is the operator defined byPn(x) =x[n]for

all x= (xk)∞k=X andRn=IPnfor n= , , . . . ,then we have

χ(Q) = lim n→∞

sup x∈Q

Rn(x). (.)


technique has recently been used by several authors in many research papers (see for in-stance [–]). In this paper, we characterize the matrix classes (,p(F)) (≤p<∞)

and obtain an identity for the norms of the bounded linear operatorsLAdefined by these

matrix transformations. We also find conditions to obtain the corresponding subclasses of compact matrix operators by using the Hausdorff measure of noncompactness.

3 Main results

In [], the classes (p(F),Y), (∞(F),Y), ((F),Y),Y=∞,c,c, and (p(F),), ((F),p)

of compact operators were characterized. In this paper, we characterize the classes

B(,λp) for (≤p<∞) and compute the norm of operators inB(,λp). We also apply the

results of the previous section to determine the Hausdorff measure of noncompactness of operators inB(,λp), and to characterize the classesC(,p) for ≤p<∞.

Here we characterize the classesB(,p(F)) for (≤p<∞) and compute the norm of

operators inB(,p(F)). We also apply the results of the previous section to determine the

Hausdorff measure of noncompactness of operators inB(,p(F)) and to characterize the

classesC(,p(F)) for ≤p<∞.

The following result is useful.

Lemma .([], Theorem .) Let T be a triangle and X and Y be arbitrary subsets ofω.

(a) Then we haveA∈(X,YT)if and only ifC=T·A∈(X,Y),whereCdenotes the

matrix product ofTandA.

(b) IfXandYareBspaces andA∈(X,YT)then

LA=LC. (.)

First we establish the characterizations of the classesB(,p(F)) for (≤p<∞) and an

identity for the operator norm.

Theorem . Let≤p<∞.

(a) We haveLB(,p(F))if and only if there exists an infinite matrixA∈(,p(F))

such that

A=sup k

n fn






p/p<∞ (.)


L(x) =Ax for allx. (.)

(b) IfLB(,p(F))then

L=A. (.)

Proof Since is aBK space withAKit follows from Lemma . thatLB(,p(F)) for


Also we have by Lemma .(a) thatA∈(,p(F)) if and only ifC=F·A∈(,p), where

the matrixC= (cnk) is defined by








Furthermore, we have by [], Example ..D,C∈(,p) if and only if

C=sup k

n= |cnk|p



This completes the proof of part (a).

(b) IfLB(,λp) then it follows from (.) thatL=LC, whereLCB(,p) is given

byLC(x) =Cxfor allx. It follows by the Minkowski inequality that LC(x)p=




p/p ≤

k= |xk|

n= |cnk|p


C · x=A · x,

and so

LA. (.)

We also obtain fore(k)∈S(k∈N)



n= |cnk|p



and soLA. This and (.) yield (.).

This completes the proof. Now we are going to establish a formula for the Hausdorff measure of noncompactness of operators inB(,p(F)). We need the following result.

Lemma .([], Theorem .) Let X be a linear metric space with a translation invariant metric,T be a triangle andχ,andχTdenote the Hausdorff measures of noncompactness

onMXandMXT,respectively.ThenχT(Q) =χ(TQ)for all QMXT.

Theorem . Let LB(,p(F)) (≤p<∞)and A denote the matrix which represents L.

Then we have




n=m fn





aj–,k p


. (.)

Proof We writeS=S, for short, andC

[m](mN) for the matrix with the rowsC[m] n = 

for ≤nmandC[m]


and (.) that




= lim m→∞




= lim m→∞



C[m]xp= lim m→∞C


= lim m→∞



n=m+ fn





aj–,k p



This completes the proof. Finally, the characterization ofC(,p(F)) is an immediate consequence of Theorem .

and (.).

Theorem . Let LB(,p(F)) (≤p<∞)and A denote the matrix which represents L.

Then L is compact if and only if

lim m→∞



n=m fn





aj–,k p

= .

4 Conclusions

The Hausdorff measure of noncompactness can be most effectively used to characterize compact operators between Banach spaces. We have shown how the Hausdorff measure of noncompactness could be applied in the characterization of compact matrix operators between BK spaces. Here we characterized the classesB(,p(F)) for (≤p<∞) and

computed the norm of operators inB(,p(F)). We also determined the Hausdorff

mea-sure of noncompactness of operators inB(,p(F)) and then characterized the classes

C(,p(F)) for ≤p<∞.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

The authors contributed equally and significantly in writing this paper. All authors read and approved the final manuscript.


This project was funded by the Deanship of Scientific Research (DSR) at King Abdulaziz University, Jeddah, under grant no. (69-130-35-RG). The authors, therefore, acknowledge with thanks DSR technical and financial support.

Received: 19 March 2015 Accepted: 22 May 2015 References

1. Basar, F: Summability Theory and Its Applications. Bentham Science Publishers, Istanbul (2011)

2. Wilansky, A: Summability Through Functional Analysis. North-Holland Mathematics Studies, vol. 85. Elsevier, Amsterdam (1984)

3. Bana´s, J, Mursaleen, M: Sequence Spaces and Measures of Noncompactness with Applications to Differential and Integral Equations. Springer, Berlin (2014)

4. Koshy, T: Fibonacci and Lucas Numbers with Applications. Wiley, New York (2001)

5. Ba¸sarır, M, Ba¸sar, F, Kara, EE: On the spaces of Fibonacci difference null and convergent sequences (2013). arXiv:1309.0150v1 [math.FA]

6. Kara, EE: Some topological and geometrical properties of new Banach sequence spaces. J. Inequal. Appl.2013, 38 (2013)

7. Kuratowski, K: Sur les espaces complets. Fundam. Math.15, 301-309 (1930)


9. Goldenštein, LS, Gohberg, IT, Markus, AS: Investigations of some properties of bounded linear operators with their

q-norms. Uˇcen. Zap. Kishinevsk. Univ.29, 29-36 (1957)

10. Goldenštein, LS, Markus, AS: On a measure of noncompactness of bounded sets and linear operators. In: Studies in Algebra and Mathematical Analysis, pp. 45-54. Karta Moldovenjaske, Kishinev (1965)

11. Akhmerov, RR, Kamenskij, MI, Potapov, AS, Rodkina, AE, Sadovskii, BN: Measures of Noncompactness and Condensing Operators. Operator Theory: Advances and Applications, vol. 55. Birkhäuser, Basel (1992)

12. Ayerbe Toledano, JM, Domínguez Benavides, T, López Azedo, G: Measures of Noncompactness in Metric Fixed Point Theory. Birkhäuser, Basel (1997)

13. Bana´s, J, Goebel, K: Measures of Noncompactness in Banach Spaces. Lecture Notes in Pure and Applied Mathematics, vol. 60. Dekker, New York (1980)

14. Malkowsky, E, Rakoˇcevi´c, V: An introduction into the theory of sequence spaces and measures of noncompactness. Zb. Rad. (Beogr.)9(17), 143-234 (2000)

15. Alotaibi, A, Malkowsky, E, Mursaleen, M: Measure of noncompactness for compact matrix operators on someBK

spaces. Filomat28, 1081-1086 (2014)

16. Ba¸sarır, M, Kara, EE: On compact operators on the RieszB(m)-difference sequence spaces. Iran. J. Sci. Technol., Trans. A, Sci.35(A4), 279-285 (2011)

17. Ba¸sarır, M, Kara, EE: On some difference sequence spaces of weighted means and compact operators. Ann. Funct. Anal.2, 114-129 (2011)

18. Ba¸sarır, M, Kara, EE: On theB-difference sequence space derived by generalized weighted mean and compact operators. J. Math. Anal. Appl.391, 67-81 (2012)

19. Kara, EE, Ba¸sarır, M: On compact operators and some EulerB(m)-difference sequence spaces. J. Math. Anal. Appl.379, 499-511 (2011)

20. de Malafosse, B, Malkowsky, E, Rakoˇcevi´c, V: Measure of noncompactness of operators and matrices on the spacesc

andc0. Int. J. Math. Math. Sci.2006, 1-5 (2006)

21. de Malafosse, B, Rakoˇcevi´c, V: Applications of measure of noncompactness in operators on the spaces,s0 α,s(αc),



J. Math. Anal. Appl.323(1), 131-145 (2006)

22. Mursaleen, M, Karakaya, V, Polat, H, Simsek, N: Measure of noncompactness of matrix operators on some difference sequence spaces of weighted means. Comput. Math. Appl.62, 814-820 (2011)

23. Mursaleen, M, Mohiuddine, SA: Applications of measures of noncompactness to the infinite system of differential equations inpspaces. Nonlinear Anal.75, 2111-2115 (2012)

24. Mursaleen, M, Noman, AK: Compactness by the Hausdorff measure of noncompactness. Nonlinear Anal.73, 2541-2557 (2010)

25. Mursaleen, M, Noman, AK: Compactness of matrix operators on some new difference sequence spaces. Linear Algebra Appl.436(1), 41-52 (2012)


Related documents

As the engine power is delivered to the flywheel, the jerk applied will set the flywheel into motion, so also the jerk is applied to the masses, and as explained in the two spring

Table 2 and Figure 2 shows that age wise comparison of PPIUCD Acceptors and it reveals that women who accepted PPIUCD with &lt; 20 years was increased gradually from

In the present study we carried out a meta-analysis of the data based on the ED used per tooth; it was observed that ED levels between 50 and 75 J/cm 2 per tooth were effective

A security attack scenario involves possible attacks to an information system, a possible attacker, the resources that are attacked, and the actors of the system related to the

0 Report of a conference on the Role of Pediatric Services in the Prevention of Juvenile Delinquency, sponsored by the American Academy of Pediatrics and the Community Council

By literature review and expert methods, 53 indicators were established, including 22 variables reflecting exposure level (divided into 07 groups related to

Analysis of response methods for a 100,000 gallon Bakken crude oil spill with respect to costs, effectiveness, and environmental impact.

The herbarium numbers, latin name of the plants, local names, families, village number, parts used and form of usage were listed alphabetically in the table.. In this study, 45