The isometries ofC p

Theorem.Let 1≦p≦∞,p ≠ 2, and let V be an isometry of C_p onto itself. Then there exist two unitary operators u and w on l₂ so that V acts on C_p in one of the following forms: \((i) Vx = u \cdot x \cdot w; (ii) Vx = u \cdot x^T \cdot w\) (where x^T is the transpose of x).     

A note on the sequence spacel (p v )

In this paper, the linear isometry of the sequence space l(p_v) into itself is specified as the automorphism of l(p_v) onto itself, when (p_v) satisfies the conditions, (i) 0 < p_v? 1, (ii) 1 +d ? p_v ? p < ∞,q < q_v < 1+d/d,d > o When (p_v) satisfies condition (ii),l (p_v) andl (q_v) are proved to be perfect spaces in the sense of Kothe and Toeplitz. A similar result connecting linear isometry and automorphism has been noted in the case of a non-normable complete linear metric space whose conjugate space is also determined.     

Complemented subspaces of (l 2⊕l 2⊕…) p (1<p<∞) with an unconditional basis

It is proved that every infinite dimensional complemented subspace of (l ₂⊕l ₂⊕…) _p (1<p<∞) with an unconditional basis is isomorphic to one of the following four spaces:l ₂,l _p,l ₂⊕l _p, (l ₂⊕l ₂⊕…) _p .     

A valuation criterion for normal basis generators in local fields of characteristic p

Let K be a complete local field of characteristic p with perfect residue field, and let L/K be a finite, totally ramified, Galois p-extension with G = Gal(L/K). Let v _L be the normalized valuation with ${v_L(L^{\times})=\mathbb{Z} }Let K be a complete local field of characteristic p with perfect residue field, and let L/K be a finite, totally ramified, Galois p-extension with G = Gal(L/K). Let v _L be the normalized valuation with v_L(L^×)=\mathbbZ {v_L(L^{\times})=\mathbb{Z} }. Let p_L ? L{\pi_L\in L} be a prime element, and let p′ (x) be the derivative of the minimal polynomial for π _L over K. We show that any element r ? L{\rho\in L} with v_L(r) o -v_L(p￠(p_L))-1 mod [L:K]{v_L(\rho)\equiv -v_L(p'(\pi_L))-1\bmod[L:K]} generates a normal basis: K[G]ρ = L. This criterion is tight: Given any integer i with i\not o -v_L(p￠(p_L))-1 mod [L:K]{i\not\equiv -v_L(p'(\pi_L))-1\bmod[L:K]}, there is a r_i ? L{\rho_i\in L} with v _L (ρ _i ) = i such that K[G]r_i\subsetneq L{K[G]\rho_i\subsetneq L}.     

A stability property of the unit vector basis ofl p

Let {e _n} be the unit vector basis ofl _p, l<p<∞, and letx _n=a_ne_n?b_nen+1. Necessary and sufficient conditions are given for the operatorT:l _p → span {x _n} defined byTe _i=x_i to be invertible.     

l q -subspaces of stablep-Banach spaces, 0<p≦1

Each infinite dimensional subspace ofL ^p (0<p≦1) is shown to contain a copy of somel ^q p≦q<∞, using arguments similar to the ones that appearin Krivine and Maurey's paper concerning stable Banach spaces. Generally speaking, ifX is a stable infinite dimensionalp-Banach space, with 0<p≦1, then, there exists aq(p≦q<∞), such that,X contains (1+ε)-isomorphic copies ofl ^q , for all ε>0. Moreover, it is possible to prove that if a stablep-Banach space, 0<p≦1, contains an isomorphic copy ofl ^q,p≦q<∞, then, it also contains (1+ε) -isomorphic copies ofl ^q , for all ε>0.     

Moduli of continuity defined by zero continuation of functions andK-functionals with restrictions

We consider the followingK-functional: $$K(\delta ,f)_p : = \mathop {\sup }\limits_{g \in W_{p U}^r } \left\{ {\left\| {f - g} \right\|_{L_p } + \delta \sum\limits_{j = 0}^r {\left\| {g^{(j)} } \right\|_{L_p } } } \right\}, \delta \geqslant 0,$$ where ? ∈L _p :=L _p [0, 1] andW _p,U ^r is a subspace of the Sobolev spaceW _p ^r [0, 1], 1≤p≤∞, which consists of functionsg such that $\int_0^1 {g^{(l_j )} (\tau ) d\sigma _j (\tau ) = 0, j = 1, ... , n} $ . Assume that 0≤l _l ≤...≤l _n ≤r-1 and there is at least one point τ _j of jump for each function σ _j , and if τ _j =τ _s forj ≠s, thenl _j ≠l _s . Let $\hat f(t) = f(t)$ , 0≤t≤1, let $\hat f(t) = 0$ ,t<0, and let the modulus of continuity of the functionf be given by the equality $$\hat \omega _0^{[l]} (\delta ,f)_p : = \mathop {\sup }\limits_{0 \leqslant h \leqslant \delta } \left\| {\sum\limits_{j = 0}^l {( - 1)^j \left( \begin{gathered} l \hfill \\ j \hfill \\ \end{gathered} \right)\hat f( - hj)} } \right\|_{L_p } , \delta \geqslant 0.$$ We obtain the estimates $K(\delta ^r ,f)_p \leqslant c\hat \omega _0^{[l_1 ]} (\delta ,f)_p $ and $K(\delta ^r ,f)_p \leqslant c\hat \omega _0^{[l_1 + 1]} (\delta ^\beta ,f)_p $ , where β=(pl _l + 1)/p(l ₁ + 1), and the constantc>0 does not depend on δ>0 and ? ∈L _p . We also establish some other estimates for the consideredK-functional.     

Almost summing and multiple almost summing multilinear operators on l p spaces

We use the Maurey–Rosenthal factorization theorem to give a characterization of almost summing and multiple almost summing multilinear operators on a cartesian product of l _p spaces. As applications we give the necessary and sufficient conditions for the multiplication operator on l _p spaces be almost summing and multiple almost summing.     

Derived space of L p (G, H)

Let G denote a locally compact abelian group and H a separable Hilbert space. Let L _p (G, H), 1 ≤ p < ∞, be the space of H-valued measurable functions which are in the usual L _p space. Motivated by the work of Helgason [1], Figa-Talamanca [11] and Bachelis [2, 3], we have defined the derived space of the Banach space L _p (G, H) and have studied its properties. Similar to the scalar case, we prove that if G is a noncompact, locally compact abelian group, then L _p ⁰ (G, H) = {0} holds for 1 ≤ p < 2. Let G be a compact abelian group and Γ be its dual group. Let S _p (G, H) be the L ₁(G) Banach module of functions in L _p (G, H) having unconditionally convergent Fourier series in L _p -norm. We show that S _p (G, H) coincides with the derived space L _p ⁰ (G, H), as in the scalar valued case. We also show that if G is compact and abelian, then L _p ⁰ (G, H) = L ₂(G, H) holds for 1 ≤ p ≤ 2. Thus, if F ∈ L _p (G, H), 1 ≤ p < 2 and F has an unconditionally convergent Fourier series in L _p -norm, then F ∈ L ₂(G, H). Let Ω be the set of all functions on Γ taking only the values 1, ?1 and Ω* be the set of all complex-valued functions on Γ having absolute value 1. As an application of the derived space L _p ⁰ (G, H), we prove the following main result of this paper. Let G be a compact abelian group and F be an H-valued function on the dual group Γ such that $$ \sum \omega (\gamma )F(\gamma )\gamma $$ is a Fourier-Stieltjes series of some measure µ ∈ M(G, H) for every scalar function ω such that |ω(γ)| = 1. Then F ∈ l ²(Γ, H).     

On a family of topologies on the spaceL p

We study a family of topologies {_s}_{0 on the space lp, 0s} is the protective topology on l_p generated by the family of multipliers m_y:l_pl_s, m_y(x)=x · y, where y ranges over the space l_p and 1/p + 1/q=1/s. Here l_s is taken with its standard topology generated by the norm for s 1 or a pseudonorm if 0s}_{0sp is strictly increasing and that all the topologies s}, 0s are not locally convex when 0Matematicheskie Metody i Fiziko-Mekhanicheskie Polya, Issue 35, 1992, pp. 194–198.     

On the local structure of subspaces of Banach lattices

The conjecture that every Banach space contains uniformly complementedl _p ⁿ ’s for some 1≦p≦∞ is verified for subspaces of Banach lattices which do not containl _∞ ⁿ ’s uniformly.     

Decomposition of Complete Graphs into Cycles and Stars

Let C _k denote a cycle of length k and let S _k denote a star with k edges. As usual K _n denotes the complete graph on n vertices. In this paper we investigate decomposition of K _n into C _l ’s and S _k ’s, and give some necessary or sufficient conditions for such a decomposition to exist. In particular, we give a complete solution to the problem in the case l = k = 4 as follows: For any nonnegative integers p and q and any positive integer n, there exists a decomposition of K _n into p copies of C ₄ and q copies of S ₄ if and only if ${4(p + q)={n \choose 2}, q\ne 1}$ if n is odd, and ${q\geq max\{3, \lceil{\frac{n}{4}\rceil}\}}$ if n is even.     

Approximation methods for common fixed points for a countable family of nonexpansive mappings

Let E=L_p or l_p space, 1<p<∞. Let K be a closed, convex and nonempty subset of E. Let be a family of nonexpansive self-mappings of K. For arbitrary fixed δ∈(0,1), define a family of nonexpansive maps by S_i?(1−δ)I+δT_i where I is the identity map of K. Let . It is proved that the iterative sequence {x_n} defined by: x₀∈K,x_n+1=α_nu+∑_i≥1σ_i,tnS_ix_n,n≥0 converges strongly to a common fixed point of the family where {α_n} and {σ_i,tn} are sequences in (0,1) satisfying appropriate conditions, in each of the following cases: (a) E=l_p,1<p<∞, and (b) E=L_p,1<p<∞ and at least one of the maps T_i’s is demicompact. Our theorems extend the results of [P. Maingé, Approximation methods for common fixed points of nonexpansive mappings in Hilbert space, J. Math. Anal. Appl. 325 (2007) 469-479] from Hilbert spaces to l_p spaces, 1<p<∞.     

Strong capacity-estimates for “fractional” norms

It is proved that for all fractionall the integral \(\int\limits_0^\infty {(p,\ell ) - cap(M_t )} dt^p\) is majorized by the P-th power norm of the functionu in the space ? _p ^l (Rⁿ) (here M_t={x∶¦u(x)¦?t} and (p,l)-cap(e) is the (p,l)-capacity of the compactum e?Rⁿ). Similar results are obtained for the spaces W _p ^l (Rⁿ) and the spaces of M. Riesz and Bessel potentials. One considers consequences regarding imbedding theorems of “fractional” spaces in ?_q(dμ), whereμ is a nonnegative measure in Rⁿ. One considers specially the case p=1.     

On positive entire solutions of indefinite semilinear elliptic equations

We establish that the elliptic equation Δu+K(x)up+μf(x)=0 in Rn has a continuum of positive entire solutions for small μ?0 under suitable conditions on K, p and f. In particular, K behaves like l|x| at ∞ for some l?−2, but may change sign in a compact region. For given l>−2, there is a critical exponent pc=pc(n,l)>1 in the sense that the result holds for p?pc and involves partial separation of entire solutions. The partial separation means that the set of entire solutions possesses a non-trivial subset in which any two solutions do not intersect. The observation is well known when K is non-negative. The point of the paper is to remove the sign condition on compact region. When l=−2, the result holds for any p>1 while pc is decreasing to 1 as l decreases to −2.     

Strong and A-statistical comparisons for sequences

Let T and A be two nonnegative regular summability matrices and W(T,p)∩l^∞ and c_A(b) denote the spaces of all bounded strongly T-summable sequences with index p>0, and bounded summability domain of A, respectively. In this paper we show, among other things, that is a multiplier from W(T,p)∩l^∞ into c_A(b) if and only if any subset K of positive integers that has T-density zero implies that K has A-density zero. These results are used to characterize the A-statistical comparisons for both bounded as well as arbitrary sequences. Using the concept of A-statistical Tauberian rate, we also show that is not a multiplier from W(T,p)∩l^∞ into c_A(b) that leads to a Steinhaus type result.     

Linear best approximation using a class ofk-majorl p norms

Consideration is given to problems of linear best approximation using a variant of the usuall _p norms referred to ask-majorl _p norms, for the cases when 1<p<. The underlying problem is the minimization of a piecewise smooth function. Best approximations are characterized, and a descent algorithm is developed.     

Metrics of L p -type and distributional equivalence principle

Given a frequency table ${F=\{f_{jk},(j,k)\in\,J\times K\}}$ crossing two categorical variables J and K, we consider a family of metrics of L _p -type on J defined by ${d_J^p (j,j^{\prime}) = \Sigma_k g(f_{.k})|f_{jk}/f_{j.} - f_{j^{\prime}k}/f_{j^{\prime}.}|^p}$ , where g is a positive function, and a symmetrical one on K. We investigate under which conditions on g, the famous principle of distributional equivalence is fulfilled by these metrics for every rational or every real F.