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Bogdan Raiţă

Publications and source records attributed to Bogdan Raiţă.

26 records · Page 2Linked to original sources
arXiv2020

Quasiconvexity, null Lagrangians, and Hardy space integrability under constant rank constraints

We present a systematic treatment of the theory of Compensated Compactness under Murat's constant rank assumption. We give a short proof of a sharp weak lower semicontinuity result for signed integrands, extending the results of Fonseca--Müller. The null Lagrangians are an important class of signed integrands, since they are the weakly continuous functions. We show that they are precisely the compensated compactness quantities with Hardy space integrability, thus proposing an answer to a question raised by Coifman-Lions-Meyer-Semmes. Finally we provide an effective way of computing the null Lagrangians associated with a given operator.

math.AP↗
arXiv2020

On the necessity of the constant rank condition for $L^p$ estimates

We consider a generalization of the elliptic $L^p$-estimate suited for linear operators with non-trivial kernels. A classical result of Schulenberger and Wilcox (Ann. Mat. Pura Appl. (4) 88: 229-305, 1971) shows that if the operator has constant rank then the estimate holds. We prove necessity of the constant rank condition for such an estimate.

math.CA↗
arXiv2019

Embeddings for $\mathbb{A}$-weakly differentiable functions on domains

We prove that the critical embedding $\mathrm{W}^{\mathbb{A},1}(B)\hookrightarrow \mathrm{W}^{k-1,\frac{n}{n-1}}$ holds if and only if the $k$-homogeneous, linear differential operator $\mathbb{A}$ on $\mathbb{R}^n$ from $\mathbb{R}^N$ to $\mathbb{R}^m$ has finite dimensional null-space. Here $B$ is a ball in $\mathbb{R}^n$ and $\mathrm{W}^{\mathbb{A},1}(B)$ denotes the space of maps $u\in \mathrm{L}^1(B,\mathbb{R}^N)$ such that the vector valued distribution $\mathbb{A}u$ is an integrable map. The result was previously known only for several examples of $\mathbb{A}$. Our result contrasts the homogeneous embedding in full-space. Namely, Van Schaftingen proved that $\dot{\mathrm{W}}{^{\mathbb{A},1}}(\mathbb{R}^n)\hookrightarrow \dot{\mathrm{W}}{^{k-1,\frac{n}{n-1}}}$ if and only if $\mathbb{A}$ is elliptic and cancelling. We show that this condition is (strictly) implied by $\mathbb{A}$ having finite dimensional null-space.

math.AP↗
arXiv2019

A Note on Estimates for Elliptic Systems with $L^1$ Data

In this paper we give necessary and sufficient conditions on the compatibility of a $k$th order homogeneous linear elliptic differential operator $\mathbb{A}$ and differential constraint $\mathcal{C}$ for solutions of \begin{align*} \mathbb{A} u=f\quad\text{subject to}\quad \mathcal{C} f=0\quad\text{ in }\mathbb{R}^n \end{align*} to satisfy the estimates \begin{align*} \|D^{k-j}u\|_{L^{\frac{n}{n-j}}(\mathbb{R}^n)}\leq c\|f\|_{L^1(\mathbb{R}^n)} \end{align*} for $j\in \{1,\ldots,\min\{k,n-1\}\}$ and \begin{align*} \|D^{k-n}u\|_{L^{\infty}(\mathbb{R}^n)}\leq c\|f\|_{L^1(\mathbb{R}^n)} \end{align*} when $k\geq n$.

math.AP↗
arXiv2019

Critical $\mathrm{L}^p$-differentiability of $\mathrm{BV}^{\mathbb{A}}$-maps and canceling operators

We give a generalization of Dorronsoro's Theorem on critical $\mathrm{L}^p$-Taylor expansions for $\mathrm{BV}^k$-maps on $\mathbb{R}^n$, i.e., we characterize homogeneous linear differential operators $\mathbb{A}$ of $k$-th order such that $D^{k-j}u$ has $j$-th order $\mathrm{L}^{n/(n-j)}$-Taylor expansion a.e. for all $u\in\mathrm{BV}^\mathbb{A}_{\text{loc}}$ (here $j=1,\ldots, k$, with an appropriate convention if $j\geq n$). The space $\mathrm{BV}^\mathbb{A}_{\text{loc}}$ consists of those locally integrable maps $u$ such that $\mathbb{A} u$ is a Radon measure on $\mathbb{R}^n$. A new $\mathrm{L}^\infty$-Sobolev inequality is established to cover higher order expansions. Lorentz refinements are also considered. The main results can be seen as pointwise regularity statements for linear elliptic systems with measure-data.

math.AP↗
arXiv2019

Continuity and canceling operators of order $n$ on $\mathbb{R}^n$

We prove that for elliptic and canceling linear differential operators $\mathbb{B}$ of order $n$ on $\mathbb{R}^n$, continuity of a map $u$ can be inferred from the fact that $\mathbb{B} u$ is a measure. We also prove strict continuity of the embedding of the space $\mathrm{BV}^{\mathbb{B}}(\mathbb{R}^n)$ of functions of bounded $\mathbb{B}$-variation into the space of continuous functions vanishing at infinity.

math.AP↗
arXiv2018

$\mathrm{L}^1$-estimates for constant rank operators

We show that the inequality $$ \|D^{k-1}(u-πu)\|_{\mathrm{L}^{n/(n-1)}(\mathbb{R}^n)}\leq c\|\mathbb{B}(D) u\|_{\mathrm{L}^1(\mathbb{R}^n)} $$ holds for vector fields $u\in\mathrm{C}^\infty_c$ if and only if $\mathbb{B}$ is canceling. Here $π$ denotes the $\mathrm{L}^2$-orthogonal projection onto the kernel of the $k$-homogeneous differential operator $\mathbb{B}(D)$ of \emph{constant rank} on $\mathbb{R}^n$. Other critical embeddings are established.

math.AP↗