In this paper we initiate the study of almost complex surfaces in the nearly Kähler $S^3\times S^3$. We show that on such a surface it is possible to define a global holomorphic differential, which is induced by an almost product structure on the nearly Kähler $S^3\times S^3$. We also find a local correspondence between almost complex surfaces in the nearly Kähler $S^3\times S^3$ and solutions of the general $H$-system equation introduced by Wente ([13]), thus obtaining a geometric interpretation of solutions of the general $H$-system equation. From this we deduce a correspondence between constant mean curvature surfaces in $\mathbb{R}^3$ and almost complex surfaces in the nearly Kähler $S^3\times S^3$ with vanishing holomorphic differential. This correspondence allows us to obtain a classification of the totally geodesic almost complex surfaces. Moreover, we prove that almost complex topological 2-spheres in $S^3\times S^3$ are totally geodesic. Finally, we also show that every almost complex surface with parallel second fundamental form is totally geodesic.

We study the existence of strong Kähler with torsion (SKT) metrics and of symplectic forms taming invariant complex structures $J$ on solvmanifolds $G/\varGamma$ providing some negative results for some classes of solvmanifolds. In particular, we show that if either $J$ is invariant under the action of a nilpotent complement of the nilradical of $G$ or $J$ is abelian or $G$ is almost abelian (not of type (I)), then the solvmanifold $G/\varGamma$ cannot admit any symplectic form taming the complex structure $J$, unless $G/\varGamma$ is Kähler. As a consequence, we show that the family of non-Kähler complex manifolds constructed by Oeljeklaus and Toma cannot admit any symplectic form taming the complex structure.

Let $\sigma$ be an operator mean and $f$ a non-constant operator monotone function on $(0, \infty)$ associated with $\sigma$. If operators $A, B$ satisfy $0 \leq A \leq B$, then it holds that $Y \sigma (tA+X) \leq Y \sigma (tB+X)$ for any non-negative real number $t$ and any positive, invertible operators $X,Y$. We show that the condition $ Y \sigma (tA+X) \leq Y \sigma (tB+X)$ for a sufficiently small $t > 0$ implies $A \leq B$ if and only if $X$ is a positive scalar multiple of $Y$ or the associated operator monotone function $f$ with $\sigma$ has the form $f(t) = (at+b)/(ct+d)$, where $a,b,c,d$ are real numbers satisfying $ad-bc > 0$.

We introduce a Möbius invariant energy associated to planar domains, as well as a generalization to space curves. This generalization is a Möbius version of Banchoff-Pohl's notion of area enclosed by a space curve. A relation with Gauss-Bonnet theorems for complete surfaces in hyperbolic space is also described.

The Néron–Ogg–Šafarevič criterion for abelian varieties tells that the Galois action on the $l$-adic étale cohomology of an abelian variety over a local field determines whether the variety has good reduction or not. We prove an analogue of this criterion for a certain type of K3 surfaces closely related to abelian surfaces. We also prove its $p$-adic analogue. This paper includes T. Ito's unpublished result on Kummer surfaces.

We study the geometry of surfaces in $\mathbb{R}^3$ with corank 1 singularities. At a singular point we define the curvature parabola using the first and second fundamental forms of the surface, which contains all the local second order geometrical information about the surface. The curvature parabola is used to introduce the concepts of asymptotic directions and umbilic curvature, which are related to contact properties of the surface with planes and spheres.

In this paper we will exhibit a class of kernels generating $r$-nuclear operators. The class includes the Fox-Li and related operators. Estimates for the corresponding asymptotic behaviour of the eigenvalues are also derived.

Let $(\varSigma_1, g_1)$ and $(\varSigma_2, g_2)$ be connected, complete and orientable 2-dimensional Riemannian manifolds. Consider the two canonical Kähler structures $(G^{\varepsilon}, J, \varOmega^{\varepsilon})$ on the product 4-manifold $\varSigma_1\times\varSigma_2$ given by $G^{\varepsilon}=g_1\oplus \varepsilon g_2$, $\varepsilon=\pm 1$ and $J$ is the canonical product complex structure. Thus for $\varepsilon=1$ the Kähler metric $G^+$ is Riemannian while for $\varepsilon=-1$, $G^-$ is of neutral signature. We show that the metric $G^{\varepsilon}$ is locally conformally flat if and only if the Gauss curvatures $\kappa(g_1)$ and $\kappa(g_2)$ are both constants satisfying $\kappa(g_1)=-\varepsilon\kappa(g_2)$. We also give conditions on the Gauss curvatures for which every $G^{\varepsilon}$-minimal Lagrangian surface is the product $\gamma_1\times\gamma_2\subset\varSigma_1\times\varSigma_2$, where $\gamma_1$ and $\gamma_2$ are geodesics of $(\varSigma_1, g_1)$ and $(\varSigma_2, g_2)$, respectively. Finally, we explore the Hamiltonian stability of projected rank one Hamiltonian $G^{\varepsilon}$-minimal surfaces.