Adrian I. Nachman | Searcharxiv

arXiv2019

A Nonlinear Plancherel Theorem with Applications to Global Well-Posedness for the Defocusing Davey-Stewartson Equation and to the Inverse Boundary Value Problem of Calderón

We prove a Plancherel theorem for a nonlinear Fourier transform in two dimensions arising in the Inverse Scattering method for the defocusing Davey-Stewartson II equation. We then use it to prove global well-posedness and scattering in $L^2$ for defocusing DSII. This Plancherel theorem also implies global uniqueness in the inverse boundary value problem of Calderón in dimension $2$, for conductivities $σ>0$ with $\log σ\in \dot H^1$. The proof of the nonlinear Plancherel theorem includes new estimates on classical fractional integrals, as well as a new result on $L^2$-boundedness of pseudo-differential operators with non-smooth symbols, valid in all dimensions.

math.AP↗

arXiv2013

Existence and uniqueness of minimizers of general least gradient problems

Motivated by problems arising in conductivity imaging, we prove existence, uniqueness, and comparison theorems - under certain sharp conditions - for minimizers of the general least gradient problem \[\inf_{u\in BV_f(Ω)} \int_Ωφ(x,Du),\] where $f:\partial Ω\to \R$ is continuous, \[ BV_f(Ω):=\{v\in BV(Ω): \ \ \forall x\in \partial Ω, \ \ \lim_{r\to 0} \ \esssup_{y\in Ω, |x-y|<r} |f(x) - v(y)| = 0 \ \} %BV_f(Ω)=\{u\in BV(Ω): {0.1cm} u|_{\partial Ω}=f {0.1cm} \hbox{and} {0.1cm} {0.1cm} u {0.1cm} \hbox{is continuous at} {0.1cm} \partial Ω\}. \] and $φ(x,ξ)$ is a function that, among other properties, is convex and homogeneous of degree 1 with respect to the $ξ$ variable. In particular we prove that if $a\in C^{1,1}(Ω)$ is bounded away from zero, then minimizers of the weighted least gradient problem $\inf_{u \in BV_f}\int_Ω a|Du|$ are unique in $BV_f(Ω)$. We construct counterexamples to show that the regularity assumption $a\in C^{1,1}$ is sharp, in the sense that it can not be replaced by $a\in C^{1,α}(Ω)$ with any $α<1$.

math.FA↗