A note on local behavior of eigenfunctions of the Schr\"odinger operator

We show that a real eigenfunction of the Schr\"odinger operator changes sign near some point in $\mathbb{R}^n$ under a suitable assumption on the potential.Comment: To appear in J. Math. Phys., 6 page

From resolvent estimates to unique continuation for the Schr\"odinger equation

In this paper we develop an abstract method to handle the problem of unique continuation for the Schr\"odinger equation $(i\partial_t+\Delta)u=V(x)u$. In general the problem is to find a class of potentials $V$ which allows the unique continuation. The key point of our work is to make a direct link between the problem and the weighted $L^2$ resolvent estimates $\|(-\Delta-z)^{-1}f\|_{L^2(|V|)}\leq C\|f\|_{L^2(|V|^{-1})}$. We carry out it in an abstract way, and thereby we do not need to deal with each of the potential classes. To do so, we will make use of limiting absorption principle and Kato $H$-smoothing theorem in spectral theory, and employ some tools from harmonic analysis. Once the resolvent estimate is set up for a potential class, from our abstract theory the unique continuation would follow from the same potential class. Also, it turns out that there can be no dented surface on the boundary of the maximal open zero set of the solution $u$. In this regard, another main issue for us is to know which class of potentials allows the resolvent estimate. We establish such a new class which contains previously known ones, and will also apply it to the problem of well-posedness for the equation.Comment: To appear in Trans. Amer. Math. Soc., 31 page

Unique continuation for fractional Schr\"odinger operators in three and higher dimensions

We prove the unique continuation property for the differential inequality $|(-\Delta)^{\alpha/2}u|\leq|V(x)u|$, where $0<\alpha<n$ and $V\in L_{\textrm{loc}}^{n/\alpha,\infty}(\mathbb{R}^n)$, $n\geq3$.Comment: To appear in Proc. Amer. Math. Soc., 5 page

On absolute continuity of the spectrum of periodic Schr\"odinger operators

In this paper we find a new condition on a real periodic potential for which the self-adjoint Schr\"odinger operator may be defined by a quadratic form and the spectrum of the operator is purely absolutely continuous. This is based on resolvent estimates and spectral projection estimates in weighted $L^2$ spaces on the torus, and an oscillatory integral theorem.Comment: To appear in Monatsh. Math., 17 page

Global unique continuation from a half space for the Schr\"odinger equation

We obtain a global unique continuation result for the differential inequality $|(i\partial_t+\Delta)u|\leq|V(x)u|$ in $\mathbb{R}^{n+1}$. This is the first result on global unique continuation for the Schr\"odinger equation with time-independent potentials $V(x)$ in $\mathbb{R}^{n}$. Our method is based on a new type of Carleman estimates for the operator $i\partial_t+\Delta$ on $\mathbb{R}^{n+1}$. As a corollary of the result, we also obtain a new unique continuation result for some parabolic equations.Comment: Revised version, to appear in Journal of Functional Analysi

On minimal support properties of solutions of Schr\"odinger equations

In this paper we obtain minimal support properties of solutions of Schr\"odinger equations. We improve previously known conditions on the potential for which the measure of the support of solutions cannot be too small. We also use these properties to obtain some new results on unique continuation for the Schr\"odinger operator.Comment: Revised version, to appear in Journal of Mathematical Analysis and Application

On unique continuation for Schr\"odinger operators of fractional and higher orders

In this note we study the property of unique continuation for solutions of $|(-\Delta)^{\alpha/2}u|\leq|Vu|$, where $V$ is in a function class of potentials including $\bigcup_{p>n/\alpha}L^p(\mathbb{R}^n)$ for $n-1\leq\alpha<n$. In particular, when $n=2$, our result gives a unique continuation theorem for the fractional ($1<\alpha<2$) Schr\"odinger operator $(-\Delta)^{\alpha/2}+V(x)$ in the full range of $\alpha$ values.Comment: Revised version, to appear in Mathematische Nachrichte

A note on the Schr\"odinger smoothing effect

The Kato-Yajima smoothing estimate is a smoothing weighted $L^2$ estimate with a singular power weight for the Schr\"odinger propagator. The weight has been generalized relatively recently to Morrey-Campanato weights. In this paper we make this generalization more sharp in terms of the so-called Kerman-Sawyer weights. Our result is based on a more sharpened Fourier restriction estimate in a weighted $L^2$ space. Obtained results are also extended to the fractional Schr\"odinger propagator.Comment: To appear in Math. Nachr., 8 page

A remark on unique continuation for the Cauchy-Riemann operator

In this note we obtain a unique continuation result for the differential inequality $|\bar{\partial}u|\leq|Vu|$, where $\bar{\partial}=(i\partial_y+\partial_x)/2$ denotes the Cauchy-Riemann operator and $V(x,y)$ is a function in $L^2(\mathbb{R}^2)$.Comment: To appear in Bull. Korean Math. Soc., 5 page

On a reverse H\"older inequality for Schr\"odinger operators

We obtain a reverse H\"older inequality for the eigenfuctions of the Schr\"odinger operator with slowly decaying potentials. The class of potentials includes singular potentials which decay like $|x|^{-\alpha}$ with $0<\alpha<2$, especially the Coulomb potential.Comment: 7 pages, modified proo