We consider continuous ${\mathrm {SL}}(2,{\mathbb R})$ valued cocycles over general dynamical systems and discuss a variety of uniformity notions. In particular, we provide a description of uniform one-parameter families of continuous ${\mathrm {SL}}(2,{\mathbb R})$ cocycles as $G_\delta $-sets. These results are then applied to Schrödinger operators with dynamically defined potentials. In the case where the base dynamics is given by a subshift satisfying the Boshernitzan condition, we show that for a generic continuous sampling function, the associated Schrödinger cocycles are uniform for all energies and, in the aperiodic case, the spectrum is a Cantor set of zero Lebesgue measure.

We study a class of delta-like perturbations of the Laplacian on the half-line, characterized by Robin boundary conditions at the origin. Using the formalism of nonstandard analysis, we derive a simple connection with a suitable family of Schrödinger operators with potentials of very large (infinite) magnitude and very short (infinitesimal) range. As a consequence, we also derive a similar result for point interactions in the Euclidean space $\mathbb {R}^3$ , in the case of radial potentials. Moreover, we discuss explicitly our results in the case of potentials that are linear in a neighborhood of the origin.

We consider a class of Schrödinger operators on ${\open R}^N$ with radial potentials. Viewing them as self-adjoint operators on the space of radially symmetric functions in $L^2({\open R}^N)$, we show that the following properties are generic with respect to the potential:

1. (P1) the eigenvalues below the essential spectrum are nonresonant (i.e., rationally independent) and so are the square roots of the moduli of these eigenvalues;

2. (P2) the eigenfunctions corresponding to the eigenvalues below the essential spectrum are algebraically independent on any nonempty open set.

Spectral and dynamical properties of some one-dimensional continuous Schrödinger and Dirac operators with a class of sparse potentials (which take non-zero values only at some sparse and suitably randomly distributed positions) are studied. By adapting and extending to the continuous setting some of the techniques developed for the corresponding discrete operator cases, the Hausdorff dimension of their spectral measures and lower dynamical bounds for transport exponents are determined. Furthermore, it is found that the condition for the spectral Hausdorff dimension to be positive is the same for the existence of a singular continuous spectrum.

Assuming the negative part of the potential is uniformly locallyL1, we prove a pointwiseLp estimate on derivatives ofeigenfunctions of one-dimensional Schrödinger operators. In particular, if aneigenfunction is in Lp, then so is itsderivative, for 1 ≤ p ≤ ∞.

In this note, I wish to describe the first order semiclassical approximation to thespectrum of one frequency quasi-periodic operators. In the case of a sampling functionwith two critical points, the spectrum exhibits two gaps in the leading orderapproximation. Furthermore, I will give an example of a two frequency quasi-periodicoperator, which has no gaps in the leading order of the semiclassical approximation.

In this paper, we obtain some results on the existence of solutions for the system

$$ (-\Delta+q_i)u_i=\mu_im_iu_i+f_i(x,u_1,\dots,u_n)\text{ in }\mathbb{R}^{N},\quad i=1,\dots,n, $$

where each of the $q_i$ are positive potentials satisfying $\lim_{|x|\rightarrow+\infty}q_i(x)=+\infty$, each of the $m_i$ are bounded positive weights and each of the $\mu_i$ are real parameters. Depending upon the hypotheses on $f_i$, we use either the method of sub- and supersolutions or a bifurcation method.