arXiv subjects

K. Nyström

Publications and source records attributed to K. Nyström.

7 recordsLinked to original sources

arXiv2023

A structure theorem for elliptic and parabolic operators with applications to homogenization of operators of Kolmogorov type

We consider the operators \[ \nabla_X\cdot(A(X)\nabla_X),\ \nabla_X\cdot(A(X)\nabla_X)-\partial_t,\ \nabla_X\cdot(A(X)\nabla_X)+X\cdot\nabla_Y-\partial_t, \] where $X\in Ω$, $(X,t)\in Ω\times \mathbb R$ and $(X,Y,t)\in Ω\times \mathbb R^m\times \mathbb R$, respectively, and where $Ω\subset\mathbb R^m$ is a (unbounded) Lipschitz domain with defining function $ψ:\mathbb R^{m-1}\to\mathbb R$ being Lipschitz with constant bounded by $M$. Assume that the elliptic measure associated to the first of these operators is mutually absolutely continuous with respect to the surface measure $\mathrm{d} σ(X)$, and that the corresponding Radon-Nikodym derivative or Poisson kernel satisfies a scale invariant reverse Hölder inequality in $L^p$, for some fixed $p$, $1<p<\infty$, with constants depending only on the constants of $A$, $m$ and the Lipschitz constant of $ψ$, $M$. Under this assumption we prove that then the same conclusions are also true for the parabolic measures associated to the second and third operator with $\mathrm{d} σ(X)$ replaced by the surface measures $\mathrm{d} σ(X)\mathrm{d} t$ and $\mathrm{d} σ(X)\mathrm{d} Y\mathrm{d} t$, respectively. This structural theorem allows us to reprove several results previously established in the literature as well as to deduce new results in, for example, the context of homogenization for operators of Kolmogorov type. Our proof of the structural theorem is based on recent results established by the authors concerning boundary Harnack inequalities for operators of Kolmogorov type in divergence form with bounded, measurable and uniformly elliptic coefficients.

math.AP↗

arXiv2022

On local regularity estimates for fractional powers of parabolic operators with time-dependent measurable coefficients

We consider fractional operators of the form $$\mathcal{H}^s=(\partial_t -\mathrm{div}_{x} ( A(x,t)\nabla_{x}))^s,\ (x,t)\in\mathbb R^n\times\mathbb R,$$ where $s\in (0,1)$ and $A=A(x,t)=\{A_{i,j}(x,t)\}_{i,j=1}^{n}$ is an accretive, bounded, complex, measurable, $n\times n$-dimensional matrix valued function. We study the fractional operators ${\mathcal{H}}^s$ and their relation to the initial value problem $$(λ^{1-2s}\mathrm{u}')'(λ) =λ^{1-2s}\mathcal{H} \mathrm{u}(λ), \quad λ\in (0, \infty),$$ $$\mathrm{u}(0) = u,$$ in $\mathbb R_+\times \mathbb R^n\times\mathbb R$. Exploring this type of relation, and making the additional assumption that $A=A(x,t)=\{A_{i,j}(x,t)\}_{i,j=1}^{n}$ is real, we derive some local properties of solutions to the non-local Dirichlet problem $$\mathcal{H}^su=(\partial_t -\mathrm{div}_{x} ( A(x,t)\nabla_{x}))^s u=0\ \mbox{ for $(x,t)\in Ω\times J$},$$ $$ u=f\ \mbox{ for $(x,t)\in \mathbb R^{n+1}\setminus (Ω\times J)$}. $$ Our contribution is that we allow for non-symmetric and time-dependent coefficients.

math.AP↗

arXiv2021

The Dirichlet problem for Kolmogorov-Fokker-Planck type equations with rough coefficients

We establish the existence and uniqueness, in bounded as well as unbounded Lipschitz type cylinders of the forms $U_X\times V_{Y,t}$ and $Ω\times \mathbb R^{m}\times \mathbb R$, of weak solutions to Cauchy-Dirichlet problems for the strongly degenerate parabolic operator \[ \mathcal{L}:= \nabla_X\cdot(A(X,Y,t)\nabla_X)+X\cdot\nabla_Y-\partial_t, \] assuming that $A=A(X,Y,t)=\{a_{i,j}(X,Y,t)\}$ is a real $m\times m$-matrix valued, measurable function such that $A(X,Y,t)$ is symmetric, bounded and uniformly elliptic. Subsequently we solve the continuous Dirichlet problem and establish the representation of the solution using associated parabolic measures. The paper is motivated, through our recent studies, arXiv:2012.03654, arXiv:2012.04278, arXiv:2012.07446, by a growing need and interest to gain a deeper understanding of the Dirichlet problem for the operator $\mathcal{L}$ in Lipschitz type domains. The key idea underlying our results is to prove, along the lines of Brezis and Ekeland, and in particular following the recent work of S. Armstrong and J-C. Mourrat, arXiv:1902.04037, concerning variational methods for the kinetic Fokker-Planck equation, that the solution can be obtained as the minimizer of a uniformly convex functional.

math.AP↗

arXiv2020

Potential theory for a class of strongly degenerate parabolic operators of Kolmogorov type with rough coefficients

In this paper we develop a potential theory for strongly degenerate parabolic operators of the form \[ \mathcal{L}:=\nabla_X\cdot(A(X,Y,t)\nabla_X)+X\cdot\nabla_{Y}-\partial_t, \] in unbounded domains of the form \[ Ω=\{(X,Y,t)=(x,x_{m},y,y_{m},t)\in\mathbb R^{m-1}\times\mathbb R\times\mathbb R^{m-1}\times\mathbb R\times\mathbb R\mid x_m>ψ(x,y,y_m,t)\}, \] where $ψ$ is assumed to satisfy a uniform Lipschitz condition adapted to the dilation structure and the (non-Euclidean) Lie group underlying the operator $\mathcal{L}$. Concerning $A=A(X,Y,t)$ we assume that $A$ is bounded, measurable, symmetric and uniformly elliptic (as a matrix in $\mathbb R^{m}$). Beyond the solvability of the Dirichlet problem and other fundamental properties our results include scale and translation invariant boundary comparison principles, boundary Harnack inequalities and doubling properties of associated parabolic measures. All of our estimates are translation- and scale-invariant with constants only depending on the constants defining the boundedness and ellipticity of $A$ and the Lipschitz constant of $ψ$. Our results represent a version, for operators of Kolmogorov type with bounded, measurable coefficients, of the by now classical results of Fabes and Safonov, any several others, concerning boundary estimates for uniformly parabolic equations in (time-dependent) Lipschitz type domains.

math.AP↗

arXiv2020

On the fine properties of parabolic measures associated to strongly degenerate parabolic operators of Kolmogorov type

We consider strongly degenerate parabolic operators of the form \[ \mathcal{L}:=\nabla_X\cdot(A(X,Y,t)\nabla_X)+X\cdot\nabla_Y-\partial_t \] in unbounded domains \[ Ω=\{(X,Y,t)=(x,x_{m},y,y_{m},t)\in\mathbb R^{m-1}\times\mathbb R\times\mathbb R^{m-1}\times\mathbb R\times\mathbb R\mid x_m>ψ(x,y,t)\}. \] We assume that $A=A(X,Y,t)$ is bounded, measurable and uniformly elliptic (as a matrix in $\mathbb R^{m}$) and concerning $ψ$ and $Ω$ we assume that $Ω$ is what we call an (unbounded) Lipschitz domain: $ψ$ satisfies a uniform Lipschitz condition adapted to the dilation structure and the (non-Euclidean) Lie group underlying the operator $\mathcal{L}$. We prove, assuming in addition that $ψ$ is independent of the variable $y_m$, that $ψ$ satisfies an additional regularity condition formulated in terms of a Carleson measure, and additional conditions on $A$, that the associated parabolic measure is absolutely continuous with respect to a surface measure and that the associated Radon-Nikodym derivative defines an $A_\infty$-weight with respect to the surface measure.

math.AP↗

arXiv2015

Extension Properties and Boundary Estimates for a Fractional Heat Operator

The square root of the heat operator $\sqrt{\partial_t-Δ}$, can be realized as the Dirichlet to Neumann map of the heat extension of data on $\mathbb R^{n+1}$ to $\mathbb R^{n+2}_+$. In this note we obtain similar characterizations for general fractional powers of the heat operator, $(\partial_t-Δ)^s$, $s\in (0,1)$. Using the characterizations we derive properties and boundary estimates for parabolic integro-differential equations from purely local arguments in the extension problem.

math.AP↗

arXiv2010

Non-divergence form parabolic equations associated with non-commuting vector fields: Boundary behavior of nonnegative solutions

In a cylinder $Ω_T=Ω\times (0,T)\subset \R^{n+1}_+$ we study the boundary behavior of nonnegative solutions of second order parabolic equations of the form \[ Hu =\sum_{i,j=1}^ma_{ij}(x,t) X_iX_ju - \p_tu = 0, \ (x,t)\in\R^{n+1}_+, \] where $X=\{X_1,...,X_m\}$ is a system of $C^\infty$ vector fields in $\Rn$ satisfying Hörmander's finite rank condition \eqref{frc}, and $Ω$ is a non-tangentially accessible domain with respect to the Carnot-Carathéodory distance $d$ induced by $X$. Concerning the matrix-valued function $A=\{a_{ij}\}$, we assume that it be real, symmetric and uniformly positive definite. Furthermore, we suppose that its entries $a_{ij}$ be Hölder continuous with respect to the parabolic distance associated with $d$. Our main results are: 1) a backward Harnack inequality for nonnegative solutions vanishing on the lateral boundary (Theorem \ref{T:back}); 2) the Hölder continuity up to the boundary of the quotient of two nonnegative solutions which vanish continuously on a portion of the lateral boundary (Theorem \ref{T:quotients}); 3) the doubling property for the parabolic measure associated with the operator $H$ (Theorem \ref{T:doubling}). These results generalize to the subelliptic setting of the present paper, those in Lipschitz cylinders by Fabes, Safonov and Yuan in [FSY] and [SY]. With one proviso: in those papers the authors assume that the coefficients $a_{ij}$ be only bounded and measurable, whereas we assume Hölder continuity with respect to the intrinsic parabolic distance.

math.AP↗