Gen. Math. Notes, Vol. 19, No. 2, December, 2013, pp.37-58 ISSN 2219-7184; Copyright c ICSRS Publication, 2013 www.i-csrs.org Available free online at http://www.geman.in A Survey on Hahn Sequence Space Murat Kiri³ci Department of Mathematical Education Hasan Ali Yucel Education Faculty stanbul University, Vefa, 34470 Fatih, Istanbul, Turkey E-mail: [email protected]; [email protected] (Received: 5-4-13 / Accepted: 19-9-13) Abstract In this work, we investigate the studies related to the Hahn sequence space. Keywords: Sequence space, beta- and gamma-duals, BK−spaces, matrix transformations, geometric properties of sequence spaces. 1 Introduction By a sequence space, we understand a linear subspace of the space ! = CN of all complex sequences which contains φ, the set of all nitely non-zero sequences. We write `1, c and c0 for the classical spaces of all bounded, convergent and null sequences, respectively. Also by bs, cs, `1 and `p, we denote the space of all bounded, convergent, absolutely and p-absolutely convergent series, 38 Murat Kiri³ci R respectively. We dene bv; d`; σ1; σc; σs; dE and E as follows: 1 X bv = x = (xk) 2 ! : jxk − xk−1j < 1 ; k=1 1 X 1 d` = x = (x ) 2 ! : jx j < 1 ; 1 k k k k=1 n 1 X σ1 = x = (xk) 2 ! : sup xk < 1 ; n n k=1 n 1 X σc = x = (xk) 2 ! : lim xk exists ; n!1 n k=1 n X X k − 1 σs = x = (xk) 2 ! :(C − 1) − xk = lim 1 − xk exists ; n!1 n k=1 −1 dE = x = (xk) 2 ! :(k xk) 2 E ; Z E = x = (xk) 2 ! :(kxk) 2 E ; where dE and R E are called the dierentiated and integrated spaces of E, respectively. A coordinate space (or K−space) is a vector space of numerical sequences, where addition and scalar multiplication are dened pointwise. An FK−space is a locally convex Fréchet space which is made up of se- quences and has the property that coordinate projections are continuous. A BK− space is locally convex Banach space which is made up of sequences and has the property that coordinate projections are continuous. A BK−space X is said to have AK (or sectional convergence) if and only if kx[n] − xk ! 0 as n ! 1. X has (C; 1) − AK (Cesàro-sectional convergence of order one) if for all x 2 X and 8 < k−1 if 1 1 − n xk ; k = 1; 2; :::; n (Pn (x))k = : 0 ; if k = n + 1; n + 2; ::: 1 and 1 . Pn (x) 2 X kPn (x) − xkX ! 0 (n ! 1) Let X be an FK−space. A sequence (xk) in X is said to be weakly Cesàro 0 bounded, if [f(x1) + f(x2) + ::: + f(xk)]=k is bounded for each f 2 X , the A Survey on Hahn Sequence Space 39 dual space of X. Let Φ stand, for the set of all nite sequences. The space X is said to have AD (or) be an AD space if Φ is dense in X. We note that AK ) AD [3]. An FK−space X ⊃ Φ is said to have AB if (x[n]) is bounded set in X for each x 2 X. Let X be a BK−space. Then X is said to have monotone norm if kx[m]k ≥ [n] [n] kx k for m > n and kxk = sup[n]kx k. Let D = fx 2 Φ: kxk ≤ 1g be in a BK−space X, that is, D is the intersection of the closed unit sphere(disc) with Φ. A subset E of Φ is called a determining set for X if and only if its absolutely convex hull K is identical with D [18]. The normed space X is said to be rotund if and only if k(x + y)=2k < 1, whenever x 6= y and kxk = kyk ≤ 1 in X [17]. The set S(λ, µ) dened by S(λ, µ) = fz = (zk) 2 ! : xz = (xkzk) 2 µ for all x = (xk) 2 λg (1) is called the multiplier space of the sequence spaces λ and µ. One can eaisly observe for a sequence space υ with λ ⊃ υ ⊃ µ that the inclusions S(λ, µ) ⊂ S(υ; µ) and S(λ, µ) ⊂ S(λ, υ) hold. With the notation of (1), the alpha-, beta-, gamma- and sigma-duals of a sequence space λ, which are respectively denoted by λα, λβ, λγ and λσ are dened by α β γ σ λ = S(λ, `1); λ = S(λ, cs) λ = S(λ, bs) and λ = S(λ, σs): For each xed positive integer k, we write δk = f0; 0; :::; 1; 0; :::g, 1 in the k − th place and zeros elsewhere. Given an FK−space X containing Φ, its conjugate is denoted by X0 and its f−dual or sequential dual is denoted by Xf and is given by Xf = f all sequences (f(δk)) : f 2 X0g. An FK−space X f containing Φ is said to be semi replete if X ⊂ σ(`1). The space bv is semi replete, because bv = bs [13]. Let λ and µ be two sequence spaces, and A = (ank) be an innite matrix of complex numbers , where . Then, we say that denes a matrix ank k; n 2 N A mapping from λ into µ, and we denote it by writing A : λ ! µ if for every sequence x = (xk) 2 λ. The sequence Ax = f(Ax)ng, the A-transform of x, is 40 Murat Kiri³ci in µ; where X for each (2) (Ax)n = ankxk n 2 N: k Throughout the text, for short we suppose that the summation without limits runs from 1 to 1. By (λ : µ), we denote the class of all matrices A such that A : λ ! µ. Thus, A 2 (λ : µ) if and only if the series on the right side of (2) converges for each and each and we have n 2 N x 2 λ Ax = f(Ax)ngn2N 2 µ for all x 2 λ. A sequence x is said to be A-summable to l if Ax converges to l which is called the A-limit of x. Lemma 1.1. ([18], Theorem 7.2.7) Let X be an FK−space with X ⊃ Φ. Then, (i) Xβ ⊂ Xγ ⊂ Xf . (ii) If X has AK, Xβ = Xf . (iii) If X has AD, Xβ = Xγ. 2 Hahn Sequence Space Hahn [7] introduced the BK−space h of all sequences x = (xk) such that ( 1 ) X h = x : kj∆xkj < 1 and lim xk = 0 ; k!1 k=1 where , for all . The following norm ∆xk = xk − xk+1 k 2 N X kxkh = kj∆xkj + sup jxkj k k was dened on the space h by Hahn [7] (and also [6]). Rao ([12], Proposition 2.1) dened a new norm on as P h kxk = k kj∆xkj: Hahn proved following properties of the space h: Lemma 2.1. (i) h is a Banach space. R (ii) h ⊂ `1 \ c0: β (iii) h = σ1: A Survey on Hahn Sequence Space 41 Clearly, h is a BK−space [6]. In [6], Goes and Goes studied functional analytic properties of the BK−space bv0 \ d`1. Additionally, Goes and Goes considered the arithmetic means of se- quences in bv0 and bv0 \ d`1, and used an important fact which the sequence of arithmetic means −1 Pn of an is a quasiconvex null sequence. (n k=1 xk) x 2 bv0 Rao [12] studied some geometric properties of Hahn sequence space and gave the characterizations of some classes of matrix transformations. Now we give some additional properties of h which proved by Goes and Goes [6]. R R Theorem 2.2. ([6], Theorem 3.2) h = `1 \ bv = `1 \ bv0. Proof. For k = 1; 2; ::: k∆xk = xk+1 + ∆(kxk): (3) Hence x 2 h implies 1 1 1 X X X 1 > kj∆xkj ≥ j∆(kxk)j − jxk+1j: k=1 k=1 k=1 The last series is convergent since h ⊂ `1 by Part of (ii) of Lemma 2.1. Hence also P1 and therefore R . k=1 j∆(kxk)j < 1 h ⊂ `1 \ bv R Conversely, (3) implies for x 2 `1 \ bv that 1 1 1 X X X 1 > jxk+1j + j∆(kxk)j ≥ kj∆xkj and lim xk = 0: k!1 k=1 k=1 k=1 R R Thus, `1 \ bv ⊂ h. Hence, we have shown that h = `1 \ bv. The second equality in the theorem follows now from Lemma 2.1(ii). Lemma 2.3. ([6], Lemma 3.3) If X and Y are β−dual (σ−dual) Köthe spaces, then X \ Y is also a β−dual (σ−dual) Köthe space. Proof. We use the known fact that if ζ = β or ζ = σ, then E is a ζ−dual Köthe space if and only if E = (Eζ )ζ ≡ Eζζ ([5], p.139, Theorem 3). Hence if X and Y are ζ−dual Köthe spaces, then (X \ Y )ζζ = (Xζ + Y ζ )ζζζ = (Xζ + Y ζ )ζ = Xζζ \ Y ζζ = X \ Y: 42 Murat Kiri³ci β Theorem 2.4. ([6], Theorem 3.4) h = (σ1) . β Proof. By Part (iii) of Lemma 2.1, h = σ1. Hence by the remark in the beginning of the last proof it is enough to show that h is a β−dual Köthe R β space. In fact: By Theorem 2.2, h = `1 \ bv and as is well known `1 = (c0) and R bv = (d(cs))β since bv = (cs)β.