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We formulate an {\it effective cone conjecture} for klt Calabi--Yau pairs $(X,\Delta)$, pertaining to the structure of the cone of effective divisors $\mathrm{Eff}(X)$ modulo the action of the subgroup of pseudo-automorphisms $\mathrm{PsAut}(X,\Delta)$. Assuming the existence of good minimal models in dimension $\dim(X)$, known to hold in dimension up to $3$, we prove that the effective cone conjecture for $(X,\Delta)$ is equivalent to the Kawamata--Morrison--Totaro movable cone conjecture for $(X,\Delta)$. As an application, we show that the movable cone conjecture unconditionally holds for the smooth Calabi--Yau threefolds introduced by Schoen and studied by Namikawa, Grassi and Morrison. We also show that for such a Calabi--Yau threefold $X$, all of its minimal models, apart from $X$ itself, have rational polyhedral nef cones., Comment: 31 pages

In this article, we study the orbifold fundamental group $\pi_1^{\rm reg}(X,\Delta)$ of a log Calabi--Yau pair $(X,\Delta)$. We conjecture that the orbifold fundamental group $\pi_1^{\rm orb}(X,\Delta)$ of a $n$-dimensional log Calabi--Yau pair admits a normal solvable subgroup of rank at most $2n$ and index at most $c(n)$. We prove this conjecture in the case that $n=2$. More precisely, for a log Calabi--Yau surface pair $(X,\Delta)$ we show that $\pi_1^{\rm orb}(X,\Delta)$ is the extension of a nilpotent group of length $2$ and rank at most $4$ by a finite group of order at most $7200$. In the previous setting, we show that the group $\pi_1^{\rm orb}(X,\Delta)$ may not be virtually abelian. Further, the rank and the order stated above are optimal. Finally, we show some geometric features of log Calabi--Yau surfaces $(X,\Delta)$ for which $\pi_1^{\rm orb}(X,\Delta)$ is infinite., Comment: 50 pages, 3 figures

The aim of this paper is twofold. First of all, we confirm a few basic criteria of the finiteness of real forms of a given smooth complex projective variety, in terms of the Galois cohomology set of the discrete part of the automorphism group, the cone conjecture and the topological entropy. We then apply them to show that a smooth complex projective surface has at most finitely many non-isomorphic real forms unless it is either rational or a non-minimal surface birational to either a K3 surface or an Enriques surface. In the second part of the paper, we construct an Enriques surface whose blow-up at one point admits infinitely many non-isomorphic real forms. This answers a question of Kondo to us and also shows the three exceptional cases really occur., Comment: v2: the structure of the paper has been changed to improve the exposition; the blow-up at one point of an Enriques surface with infinitely many real forms that we construct is now proved to have non-finitely generated automorphism group

We prove a decomposition theorem for the nef cone of smooth fiber products over curves, subject to the necessary condition that their N\'eron--Severi space decomposes. We apply it to describe the nef cone of so-called Schoen varieties, which are the higher dimensional analogues of the Calabi--Yau threefolds constructed by Schoen. Schoen varieties give rise to Calabi--Yau pairs, and in each dimension at least three, there exist Schoen varieties with non-polyhedral nef cone. We prove the Kawamata--Morrison--Totaro Cone Conjecture for the nef cones of Schoen varieties, which generalizes the work by Grassi and Morrison., Comment: v2. Accepted by Forum Math. Sigma. Besides minor changes, the assumption of Theorem 1.4 and the assumption in Subsection 4.2 were weakened, and Example 3.6 and Lemma 4.6 were newly added

We prove that a smooth projective variety $X$ of dimension $n$ with strictly nef third, fourth or $(n-1)$-th exterior power of the tangent bundle is a Fano variety. Moreover, in the first two cases, we provide a classification for $X$ under the assumption that $\rho(X)\ne 1$., Comment: [v4]: final version, updated after referee report

Let $A$ be an abelian variety, and $G \subset Aut(A)$ a finite group acting freely in codimension two. We discuss whether the singular quotient $A/G$ admits a resolution that is a Calabi-Yau manifold. While Oguiso constructed two examples in dimension $3$, we show that there are none in dimension $4$. We also classify up to isogeny the possible abelian varieties $A$ in arbitrary dimension., Comment: [v1]: 54 pages, including the programs in the 6-page-long Appendix. [v2]: final version, updated after referee report. Theorem 1.5 is slightly more general now

We prove that the tangent and the reflexivized cotangent sheaves of any normal projective klt Calabi-Yau or irreducible holomorphic symplectic variety are not pseudoeffective, generalizing results of A. H\"oring and T. Peternell arXiv:1710.06183v2 [math.AG]. We provide examples of Calabi-Yau varieties of small dimension with singularities in codimension 2., Comment: [v2]: final version, updated after referee report

Polyhedral K\"ahler surfaces are a class of complex surfaces, which are flat everywhere except on a two-dimensional skeleton. They are defined as a generalisation of the "gluing a polygon side by side" construction of flat Riemann surfaces. In this article, we introduce two classes of polyhedral K\"ahler surfaces with trivial holonomy: products of polyhedral K\"ahler curves with zero holonomy and ramified coverings of tori, and prove that none of these classes is contained in the other. Existence of other types of polyhedral K\"ahler surfaces is still open., Comment: 13 pages, 4 figures