arXiv subjects

Eduard Curcă

Publications and source records attributed to Eduard Curcă.

3 recordsLinked to original sources

arXiv2026

Bourgain-Brezis spaces obtained by real interpolation

In 2002, Bourgain and Brezis proved that for the space $X=W^{1,d}$ (on $\mathbb{T}^{d}$, with $d\geq2$) we have the equality of images \begin{equation} \operatorname{div} (L^{\infty}\cap X)=\operatorname{div} X, \tag{$\ast$} \end{equation} i.e., given a vector field $v\in X$ there exists a vector field $u\in L^{\infty }\cap X$ such that $\operatorname{div} u=\operatorname{div] v $. In this paper we show that if $X$ is a function space satisfying ($\ast$) then, any real interpolation space $X_{θ,q}=(L^{\infty},X)_{θ,q}$ (where $θ\in (0,1)$ and $q\in [1,\infty)$) also satisfies ($\ast$). The proof is based on a general method that allows us to interpolate solutions of linear equations.

math.FA↗

arXiv2026

Nonexistence of Henkin type projections via a Wiener theorem for multipliers

Let $d\geq 2$, $l\geq 0$ and suppose $X$ is one of the function spaces $W^{l,1}(\mathbb{T}^{d})$, $W^{l,\infty }(\mathbb{T}^{d})$ or $C^{l}(\mathbb{T}^{d})$. We extend a result of Henkin (1967), showing that, for appropriate $N\times N$ matrix operators $A(D)$, the subspace of $X^{N}$ consisting of $A(D)-$free elements is noncomplemented. In order to prove this we establish a new property of the Fourier multipliers that are bounded on $X$: the kernel $k$ of any such multiplier obeys a weaker version of Wiener's theorem for the singularities of measures.

math.FA↗

arXiv2026

Comparison of polynomial matrix differential operators

We characterize matrix polynomials $P,Q$ such that the inequality $$ \left\Vert Q(D)u\right\Vert _{L^{2}}\leq C\left\Vert P(D)u\right\Vert _{L^{2}}\quad\text{for all }u\in C_c^\infty(Ω), $$ holds on bounded open sets $Ω$. We also characterize the operators $P,Q$ for which the linear continuous embedding above is compact, i.e., if $u_n\in C_c^\infty(Ω)$ are such that $(P(D)u_n)_{n\geq 1}$ is bounded in $L^2$, then $(Q(D)u_n)_{n\geq 1}$ is strongly compact in $L^2$.

math.FA↗