Let a and b be fixed real numbers such that 0 < mina,b < 1 < a + b. We prove that every function f:(0,∞) → ℝ satisfying f(as + bt) ≤ af(s) + bf(t), s,t > 0, and such that $limsu{p}_{t\to 0+}f\left(t\right)\le 0$ must be of the form f(t) = f(1)t, t > 0. This improves an earlier result in [5] where, in particular, f is assumed to be nonnegative. Some generalizations for functions defined on cones in linear spaces are given. We apply these results to give a new characterization of the $L^p$-norm.

Let X be a rearrangement-invariant space of Lebesgue-measurable functions on ${ℝ}^n$, such as the classical Lebesgue, Lorentz or Orlicz spaces. Given a nonnegative, measurable (weight) function on ${ℝ}^n$, define $X\left(w\right)=F:{ℝ}^n\to ℂ:\infty >\parallel F{\parallel }_{X\left(w\right)}:=\parallel Fw{\parallel }_X$. We investigate conditions on such a weight w that guarantee X(w) is an algebra under the convolution product F∗G defined at $x\in {ℝ}^n$ by $\left(F\ast G\right)\left(x\right)={ʃ}_{{ℝ}^n}F(x-y)G\left(y\right)dy$; more precisely, when $\parallel F\ast G{\parallel }_{X\left(w\right)}\le \parallel F{\parallel }_{X\left(w\right)}\parallel G{\parallel }_{X\left(w\right)}$ for all F,G ∈ X(w).

We generalize the classical coorbit space theory developed by Feichtinger and Gröchenig to quasi-Banach spaces. As a main result we provide atomic decompositions for coorbit spaces defined with respect to quasi-Banach spaces. These atomic decompositions are used to prove fast convergence rates of best n-term approximation schemes. We apply the abstract theory to time-frequency analysis of modulation spaces $M_m^{p,q}$, 0 < p,q ≤ ∞.

Criteria in order that a Musielak-Orlicz sequence space $l^{\Phi }$ contains an isomorphic as well as an isomorphically isometric copy of $l^1$ are given. Moreover, it is proved that if $\Phi =\left({\Phi }_i\right)$, where ${\Phi }_i$ are defined on a Banach space, $X$ does not satisfy the ${\delta }_2^o$-condition, then the Musielak-Orlicz sequence space $l^{\Phi }\left(X\right)$ of $X$-valued sequences contains an almost isometric copy of $c_o$. In the case of $X=IR$ it is proved also that if $l^{\Phi }$ contains an isomorphic copy of $c_o$, then $\Phi$ does not satisfy the ${\delta }_2^o$-condition. These results extend some...

In this paper the author proved the boundedness of the multidimensional Hardy type operator in weighted Lebesgue spaces with variable exponent. As an application he proved the boundedness of certain sublinear operators on the weighted variable Lebesgue space. The proof of the boundedness of the multidimensional Hardy type operator in weighted Lebesgue spaces with a variable exponent does not contain any mistakes. But in the proof of the boundedness of certain sublinear operators on the weighted...

In this paper, some necessary and sufficient conditions for $sup\{k_x:\parallel x{\parallel }^0=1\}<\infty$ in Musielak-Orlicz function spaces as well as in Musielak-Orlicz sequence spaces are given.

Criteria are given for determining the weak compactness, or otherwise, of the integration map associated with a vector measure. For instance, the space of integrable functions of a weakly compact integration map is necessarily normable for the mean convergence topology. Results are presented which relate weak compactness of the integration map with the property of being a bicontinuous isomorphism onto its range. Finally, a detailed description is given of the compactness properties for the integration...

We are concerned with imbeddings of general spaces of Besov and Lizorkin-Triebel type with dominating mixed derivatives in the first critical case. We employ multivariate exponential Orlicz and Lorentz-Orlicz spaces as targets. We study basic properties of the target spaces, in particular, we compare them with usual exponential spaces, showing that in this case the multivariate clones are in fact better adapted to the character of smoothness of the imbedded spaces. Then we prove sharp limiting imbedding...