We describe a compactification by KSBA stable pairs of the five-dimensional moduli space of K3 surfaces with a purely non-symplectic automorphism of order four and $U(2)\oplus D_4^{\oplus 2}$ lattice polarization. These K3 surfaces can be realized as the minimal resolution of the double cover of $\mathbb {P}^{1}\times \mathbb {P}^{1}$ branched along a specific $(4,\,4)$ curve. We show that, up to a finite group action, this stable pairs compactification is isomorphic to Kirwan's partial desingularization of the GIT quotient $(\mathbb {P}^{1})^{8}{/\!/}\mathrm {SL}_2$ with the symmetric linearization.

We construct a family of compactifications of the affine cone of the Grassmannian variety of $2$ -planes. We show that both the tropical variety of the Plücker ideal and familiar valuations associated to the construction of Newton–Okounkov bodies for the Grassmannian variety can be recovered from these compactifications. In this way, we unite various perspectives for constructing toric degenerations of flag varieties.

We show that every coarse moduli space, parametrizing complex special linear rank-2 local systems with fixed boundary traces on a surface with nonempty boundary, is log Calabi–Yau in that it has a normal projective compactification with trivial log canonical divisor. We connect this to a novel symmetry of generating series for counts of essential multicurves on the surface.

Culler and Vogtmann defined a simplicial space $O\left( g \right)$, called outer space, to study the outer automorphism group of the free group ${{F}_{g}}$. Using representation theoretic methods, we give an embedding of $O\left( g \right)$ into the analytification of $x\left( {{F}_{g}}\,,\,S{{L}_{2}}\left( \mathbb{C} \right) \right)$, the $S{{L}_{2}}\left( \mathbb{C} \right)$ character variety of ${{F}_{g}}$, reproving a result of Morgan and Shalen. Then we show that every point $v$ contained in a maximal cell of $O\left( g \right)$ defines a flat degeneration of $x\left( {{F}_{g}}\,,\,S{{L}_{2}}\left( \mathbb{C} \right) \right)$ to a toric variety $X\left( {{P}_{\Gamma }} \right)$. We relate $x\left( {{F}_{g}}\,,\,S{{L}_{2}}\left( \mathbb{C} \right) \right)$ and $X\left( v \right)$ topologically by showing that there is a surjective, continuous, proper map ${{\Xi }_{v}}\,:\,x\left( {{F}_{g}}\,,\,S{{L}_{2}}\,\left( \mathbb{C} \right) \right)\,\to \,X\left( v \right)$. We then show that this map is a symplectomorphism on a dense open subset of $x\left( {{F}_{g}}\,,\,S{{L}_{2}}\left( \mathbb{C} \right) \right)$ with respect to natural symplectic structures on $x\left( {{F}_{g}}\,,\,S{{L}_{2}}\left( \mathbb{C} \right) \right)$ and $X\left( v \right)$. In this way, we construct an integrable Hamiltonian system in $x\left( {{F}_{g}}\,,\,S{{L}_{2}}\left( \mathbb{C} \right) \right)$ for each point in a maximal cell of $O\left( g \right)$, and we show that each $v$ defines a topological decomposition of $x\left( {{F}_{g}}\,,\,S{{L}_{2}}\left( \mathbb{C} \right) \right)$ derived from the decomposition of $X\left( {{P}_{\Gamma }} \right)$ by its torus orbits. Finally, we show that the valuations coming from the closure of a maximal cell in $O\left( g \right)$ all arise as divisorial valuations built from an associated projective compactification of $x\left( {{F}_{g}}\,,\,S{{L}_{2}}\left( \mathbb{C} \right) \right)$.

Let G be a countably infinite discrete group, let βG be the Stone–Čech compactification of G, and let ${G^{\rm{*}}} = \beta G \setminus G$. An idempotent $p \in {G^{\rm{*}}}$ is left (right) maximal if for every idempotent $q \in {G^{\rm{*}}}$, pq = p (qp = P) implies qp = q (qp = q). An idempotent $p \in {G^{\rm{*}}}$ is strongly right maximal if the equation xp = p has the unique solution x = p in G*. We show that there is an idempotent $p \in {G^{\rm{*}}}$ which is both left maximal and strongly right maximal.

The space of shapes of quadrilaterals can be identified with $\mathbb{CP}^{2}$. We deal with the subset of $\mathbb{CP}^{2}$ corresponding to convex quadrilaterals and the subset which corresponds to simple (that is, without self-intersections) quadrilaterals. We provide a complete description of the topological closures in $\mathbb{CP}^{2}$ of both spaces. Although the interior of each space is homeomorphic to a disjoint union $\mathbb{R}^{4}\sqcup \mathbb{R}^{4}$, their closures are topologically different. In particular, the boundary of the space corresponding to convex quadrilaterals is homeomorphic to a pair of three-dimensional spheres glued along a Möbius strip while the boundary of the space corresponding to simple quadrilaterals is more complicated.

In the paper ‘Bruhat–Tits theory from Berkovich's point of view. I. Realizations and compactifications of buildings’, we investigated various realizations of the Bruhat–Tits building of a connected and reductive linear algebraic group G over a non-Archimedean field k in the framework of Berkovich's non-Archimedean analytic geometry. We studied in detail the compactifications of the building which naturally arise from this point of view. In the present paper, we give a representation theoretic flavour to these compactifications, following Satake's original constructions for Riemannian symmetric spaces.

We first prove that Berkovich compactifications of a building coincide with the compactifications, previously introduced by the third named author and obtained by a gluing procedure. Then we show how to recover them from an absolutely irreducible linear representation of G by embedding in the building of the general linear group of the representation space, compactified in a suitable way. Existence of such an embedding is a special case of Landvogt's general results on functoriality of buildings, but we also give another natural construction of an equivariant embedding, which relies decisively on Berkovich geometry.

This article treats compactifications of the space of generalized spin curves. Generalized spin curves, or r-spin curves, are pairs (X,L) with X a smooth curve and L a line bundle whose rth tensor power is isomorphic to the canonical bundle of X. These are a natural generalization of 2-spin curves (algebraic curves with a theta-characteristic), which have been of interest recently, in part because of their applications to fermionic string theory. Three different compactifications over Z[1/r], all using torsion-free sheaves, are constructed. All three yield algebraic stacks, one of which is shown to have Gorenstein singularities that can be described explicitly, and one of which is smooth. All three compactifications generalize constructions of Deligne and Cornalba done for the case when r=2.

In [3], Reed establishes a bijection between the (equivalence classes of) principal T1-extensions of a topological space X and the compatible, cluster-generated, Lodato nearnesses on X. We extend Reed's result to the T0 case by obtaining a one-to-one correspondence between the principal T0-extensions of a space X and the collections of sets (called “t-grill sets”) which generate a certain class of nearnesses which we call “t-bunch generated” nearnesses. This correspondence specializes to principal T0-compactifications. Finally, we show that there is a bijection between these t-grill sets and the filter systems of Thron [5], and that the corresponding extensions are equivalent.