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1. CJM Online first

Fahrner, Anne; Hausen, Jürgen
On intrinsic quadrics
An intrinsic quadric is a normal projective variety with a Cox ring defined by a single quadratic relation. We provide explicit descriptions of these varieties in the smooth case for small Picard numbers. As applications we figure out in this setting the Fano examples and (affirmatively) test Fujita's freeness conjecture.

Keywords:Fano variety, Cox ring, Mori dream space, intrinsic quadric
Categories:14C20, 14J10, 14J45

2. CJM Online first

Achar, Pramod N.; Riche, Simon; Vay, Cristian
Mixed perverse sheaves on flag varieties for Coxeter groups
In this paper we construct an abelian category of "mixed perverse sheaves" attached to any realization of a Coxeter group, in terms of the associated Elias--Williamson diagrammatic category. This construction extends previous work of the first two authors, where we worked with parity complexes instead of diagrams, and we extend most of the properties known in this case to the general setting. As an application we prove that the split Grothendieck group of the Elias-Williamson diagrammatic category is isomorphic to the corresponding Hecke algebra, for any choice of realization.

Keywords:perverse sheaf, flag variety, Coxeter group
Categories:14F05, 20C08, 20F55

3. CJM Online first

Iacono, Donatella; Manetti, Marco
On deformations of pairs (manifold, coherent sheaf)
We analyse infinitesimal deformations of pairs $(X,\mathcal F)$ with $\mathcal F$ a coherent sheaf on a smooth projective variety $X$ over an algebraically closed field of characteristic $0$. We describe a differential graded Lie algebra controlling the deformation problem, and we prove an analog of a Mukai-Artamkin Theorem about the trace map.

Keywords:deformation of manifold and coherent sheaf, differential graded Lie algebra
Categories:14D15, 13D10, 17B70, 18G50

4. CJM Online first

Bosser, Vincent; Gaudron, Éric
Logarithmes des points rationnels des variétés abéliennes
Nous démontrons une généralisation du théorème des périodes de Masser et Wüstholz où la période est remplacée par un logarithme non nul $u$ d'un point rationnel $p$ d'une variété abélienne définie sur un corps de nombres. Nous en déduisons des minorations explicites de la norme de $u$ et de la hauteur de Néron-Tate de $p$ qui dépendent des invariants classiques du problème dont la dimension et la hauteur de Faltings de la variété abélienne. Les démonstrations reposent sur une construction de transcendance du type Gel'fond-Baker de la théorie des formes linéaires de logarithmes dans laquelle se greffent des formules explicites provenant de la théorie des pentes d'Arakelov.

Keywords:periods theorem, abelian variety, logarithm, Gel'fond-Baker method, slope theory, Néron-Tate height, interpolation lemma
Categories:11J86, 11J95, 11G10, 11G50, 14G40

5. CJM Online first

Camere, Chiara; Garbagnati, Alice; Mongardi, Giovanni
Calabi-Yau quotients of hyperkähler four-folds
The aim of this paper is to construct Calabi-Yau 4-folds as crepant resolutions of the quotients of a hyperkähler 4-fold $X$ by a non symplectic involution $\alpha$. We first compute the Hodge numbers of a Calabi-Yau constructed in this way in a general setting and then we apply the results to several specific examples of non symplectic involutions, producing Calabi-Yau 4-folds with different Hodge diamonds. Then we restrict ourselves to the case where $X$ is the Hilbert scheme of two points on a K3 surface $S$ and the involution $\alpha$ is induced by a non symplectic involution on the K3 surface. In this case we compare the Calabi-Yau 4-fold $Y_S$, which is the crepant resolution of $X/\alpha$, with the Calabi-Yau 4-fold $Z_S$, constructed from $S$ through the Borcea-Voisin construction. We give several explicit geometrical examples of both these Calabi-Yau 4-folds describing maps related to interesting linear systems as well as a rational $2:1$ map from $Z_S$ to $Y_S$.

Keywords:irreducible holomorphic symplectic manifold, Hyperkähler manifold, Calabi-Yau 4-fold, Borcea-Voisin construction, automorphism, quotient map, non symplectic involution
Categories:14J32, 14J35, 14J50, 14C05

6. CJM Online first

Shimada, Ichiro
On an Enriques surface associated with a quartic Hessian surface
Let $Y$ be a complex Enriques surface whose universal cover $X$ is birational to a general quartic Hessian surface. Using the result on the automorphism group of $X$ due to Dolgachev and Keum, we obtain a finite presentation of the automorphism group of $Y$. The list of elliptic fibrations on $Y$ and the list of combinations of rational double points that can appear on a surface birational to $Y$ are presented. As an application, a set of generators of the automorphism group of the generic Enriques surface is calculated explicitly.

Keywords:Enriques surface, K3 surface, automorphism, lattice
Categories:14J28, 14Q10

7. CJM Online first

Scaduto, Christopher W.; Stoffregen, Matthew
The mod two cohomology of the moduli space of rank two stable bundles on a surface and skew Schur polynomials
We compute cup product pairings in the integral cohomology ring of the moduli space of rank two stable bundles with odd determinant over a Riemann surface using methods of Zagier. The resulting formula is related to a generating function for certain skew Schur polynomials. As an application, we compute the nilpotency degree of a distinguished degree two generator in the mod two cohomology ring. We then give descriptions of the mod two cohomology rings in low genus, and describe the subrings invariant under the mapping class group action.

Keywords:stable bundle, mod two cohomology, skew schur polynomial
Categories:14D20, 57R58

8. CJM Online first

Hartl, Urs; Singh, Rajneesh Kumar
Local Shtukas and Divisible Local Anderson Modules
We develop the analog of crystalline Dieudonné theory for $p$-divisible groups in the arithmetic of function fields. In our theory $p$-divisible groups are replaced by divisible local Anderson modules, and Dieudonné modules are replaced by local shtukas. We show that the categories of divisible local Anderson modules and of effective local shtukas are anti-equivalent over arbitrary base schemes. We also clarify their relation with formal Lie groups and with global objects like Drinfeld modules, Anderson's abelian $t$-modules and $t$-motives, and Drinfeld shtukas. Moreover, we discuss the existence of a Verschiebung map and apply it to deformations of local shtukas and divisible local Anderson modules. As a tool we use Faltings's and Abrashkin's theory of strict modules, which we review to some extent.

Keywords:local shtuka, formal Drinfeld module, formal t-module
Categories:11G09, 13A35, 14L05

9. CJM Online first

Wang, Zhenjian
On algebraic surfaces associated with line arrangements
For a line arrangement $\mathcal{A}$ in the complex projective plane $\mathbb{P}^2$, we investigate the compactification $\overline{F}$ in $\mathbb{P}^3$ of the affine Milnor fiber $F$ and its minimal resolution $\widetilde{F}$. We compute the Chern numbers of $\widetilde{F}$ in terms of the combinatorics of the line arrangement $\mathcal{A}$. As applications of the computation of the Chern numbers, we show that the minimal resolution is never a quotient of a ball; in addition, we also prove that $\widetilde{F}$ is of general type when the arrangement has only nodes or triple points as singularities; finally, we compute all the Hodge numbers of some $\widetilde{F}$ by using some knowledge about the Milnor fiber monodromy of the arrangement.

Keywords:line arrangement, Milnor fiber, algebraic surface, Chern number
Categories:32S22, 32S25, 14J17, 14J29, 14J70

10. CJM 2018 (vol 70 pp. 1173)

Viada, Evelina
An Explicit Manin-Dem'janenko Theorem in Elliptic Curves
Let $\mathcal{C}$ be a curve of genus at least $2$ embedded in $E_1 \times \cdots \times E_N$ where the $E_i$ are elliptic curves for $i=1,\dots, N$. In this article we give an explicit sharp bound for the Néron-Tate height of the points of $\mathcal{C}$ contained in the union of all algebraic subgroups of dimension $\lt \max(r_\mathcal{C}-t_\mathcal{C},t_\mathcal{C})$ where $t_\mathcal{C}$, respectively $r_\mathcal{C}$, is the minimal dimension of a translate, respectively of a torsion variety, containing $\mathcal{C}$. As a corollary, we give an explicit bound for the height of the rational points of special curves, proving new cases of the explicit Mordell Conjecture and in particular making explicit (and slightly more general in the CM case) the Manin-Dem'janenko method in products of elliptic curves.

Keywords:height, elliptic curve, explicit Mordell conjecture, explicit Manin-Demjanenko theorem, rational points on a curve
Categories:11G50, 14G40

11. CJM 2018 (vol 70 pp. 1008)

Elazar, Boaz; Shaviv, Ary
Schwartz Functions on Real Algebraic Varieties
We define Schwartz functions, tempered functions and tempered distributions on (possibly singular) real algebraic varieties. We prove that all classical properties of these spaces, defined previously on affine spaces and on Nash manifolds, also hold in the case of affine real algebraic varieties, and give partial results for the non-affine case.

Keywords:real algebraic geometry, Schwartz function, tempered distribution
Categories:14P99, 14P05, 22E45, 46A11, 46F05

12. CJM 2018 (vol 70 pp. 868)

Ivorra, Florian; Yamazaki, Takao
Nori Motives of Curves with Modulus and Laumon $1$-motives
Let $k$ be a number field. We describe the category of Laumon $1$-isomotives over $k$ as the universal category in the sense of Nori associated with a quiver representation built out of smooth proper $k$-curves with two disjoint effective divisors and a notion of $H^1_\mathrm{dR}$ for such "curves with modulus". This result extends and relies on the theorem of J. Ayoub and L. Barbieri-Viale that describes Deligne's category of $1$-isomotives in terms of Nori's Abelian category of motives.

Keywords:motive, curve with modulus, quiver representation
Categories:19E15, 16G20, 14F42

13. CJM 2018 (vol 70 pp. 628)

Luo, Ye; Manjunath, Madhusudan
Smoothing of Limit Linear Series of Rank One on Saturated Metrized Complexes of Algebraic Curves
We investigate the smoothing problem of limit linear series of rank one on an enrichment of the notions of nodal curves and metrized complexes called saturated metrized complexes. We give a finitely verifiable full criterion for smoothability of a limit linear series of rank one on saturated metrized complexes, characterize the space of all such smoothings, and extend the criterion to metrized complexes. As applications, we prove that all limit linear series of rank one are smoothable on saturated metrized complexes corresponding to curves of compact-type, and prove an analogue for saturated metrized complexes of a theorem of Harris and Mumford on the characterization of nodal curves contained in a given gonality stratum. In addition, we give a full combinatorial criterion for smoothable limit linear series of rank one on saturated metrized complexes corresponding to nodal curves whose dual graphs are made of separate loops.

Keywords:limit linear series, metrized complex

14. CJM 2017 (vol 70 pp. 1038)

Elduque, Alberto
Order $3$ Elements in $G_2$ and Idempotents in Symmetric Composition Algebras
Order three elements in the exceptional groups of type $G_2$ are classified up to conjugation over arbitrary fields. Their centralizers are computed, and the associated classification of idempotents in symmetric composition algebras is obtained. Idempotents have played a key role in the study and classification of these algebras. Over an algebraically closed field, there are two conjugacy classes of order three elements in $G_2$ in characteristic not $3$ and four of them in characteristic $3$. The centralizers in characteristic $3$ fail to be smooth for one of these classes.

Keywords:symmetric composition algebra, Okubo algebra, automorphism group, centralizer, idempotent
Categories:17A75, 14L15, 17B25, 20G15

15. CJM 2017 (vol 70 pp. 451)

Zhang, Chao
Ekedahl-Oort strata for good reductions of Shimura varieties of Hodge type
For a Shimura variety of Hodge type with hyperspecial level structure at a prime~$p$, Vasiu and Kisin constructed a smooth integral model (namely the integral canonical model) uniquely determined by a certain extension property. We define and study the Ekedahl-Oort stratifications on the special fibers of those integral canonical models when $p\gt 2$. This generalizes Ekedahl-Oort stratifications defined and studied by Oort on moduli spaces of principally polarized abelian varieties and those defined and studied by Moonen, Wedhorn and Viehmann on good reductions of Shimura varieties of PEL type. We show that the Ekedahl-Oort strata are parameterized by certain elements $w$ in the Weyl group of the reductive group in the Shimura datum. We prove that the stratum corresponding to $w$ is smooth of dimension $l(w)$ (i.e. the length of $w$) if it is non-empty. We also determine the closure of each stratum.

Keywords:Shimura variety, F-zip
Categories:14G35, 11G18

16. CJM 2017 (vol 70 pp. 481)

Asakura, Masanori; Otsubo, Noriyuki
CM Periods, CM Regulators and Hypergeometric Functions, I
We prove the Gross-Deligne conjecture on CM periods for motives associated with $H^2$ of certain surfaces fibered over the projective line. Then we prove for the same motives a formula which expresses the $K_1$-regulators in terms of hypergeometric functions ${}_3F_2$, and obtain a new example of non-trivial regulators.

Keywords:period, regulator, complex multiplication, hypergeometric function
Categories:14D07, 19F27, 33C20, 11G15, 14K22

17. CJM 2017 (vol 70 pp. 702)

Xia, Eugene Z.
The Algebraic de Rham Cohomology of Representation Varieties
The $\operatorname{SL}(2,\mathbb C)$-representation varieties of punctured surfaces form natural families parameterized by monodromies at the punctures. In this paper, we compute the loci where these varieties are singular for the cases of one-holed and two-holed tori and the four-holed sphere. We then compute the de Rham cohomologies of these varieties of the one-holed torus and the four-holed sphere when the varieties are smooth via the Grothendieck theorem. Furthermore, we produce the explicit Gauss-Manin connection on the natural family of the smooth $\operatorname{SL}(2,\mathbb C)$-representation varieties of the one-holed torus.

Keywords:surface, algebraic group, representation variety, de Rham cohomology
Categories:14H10, 13D03, 14F40, 14H24, 14Q10, 14R20

18. CJM 2017 (vol 70 pp. 354)

Manon, Christopher
Toric geometry of $SL_2(\mathbb{C})$ free group character varieties from outer space
Culler and Vogtmann defined a simplicial space $O(g)$ called outer space to study the outer automorphism group of the free group $F_g$. Using representation theoretic methods, we give an embedding of $O(g)$ into the analytification of $\mathcal{X}(F_g, SL_2(\mathbb{C})),$ the $SL_2(\mathbb{C})$ 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 $O(g)$ defines a flat degeneration of $\mathcal{X}(F_g, SL_2(\mathbb{C}))$ to a toric variety $X(P_{\Gamma})$. We relate $\mathcal{X}(F_g, SL_2(\mathbb{C}))$ and $X(v)$ topologically by showing that there is a surjective, continuous, proper map $\Xi_v: \mathcal{X}(F_g, SL_2(\mathbb{C})) \to X(v)$. We then show that this map is a symplectomorphism on a dense, open subset of $\mathcal{X}(F_g, SL_2(\mathbb{C}))$ with respect to natural symplectic structures on $\mathcal{X}(F_g, SL_2(\mathbb{C}))$ and $X(v)$. In this way, we construct an integrable Hamiltonian system in $\mathcal{X}(F_g, SL_2(\mathbb{C}))$ for each point in a maximal cell of $O(g)$, and we show that each $v$ defines a topological decomposition of $\mathcal{X}(F_g, SL_2(\mathbb{C}))$ derived from the decomposition of $X(P_{\Gamma})$ by its torus orbits. Finally, we show that the valuations coming from the closure of a maximal cell in $O(g)$ all arise as divisorial valuations built from an associated projective compactification of $\mathcal{X}(F_g, SL_2(\mathbb{C})).$

Keywords:character variety, outer space, analytification, compactification, integrable system
Categories:14M25, 14T05, 14D20

19. CJM 2016 (vol 69 pp. 1274)

Favacchio, Giuseppe; Guardo, Elena
The Minimal Free Resolution of Fat Almost Complete Intersections in $\mathbb{P}^1\times \mathbb{P}^1$
A current research theme is to compare symbolic powers of an ideal $I$ with the regular powers of $I$. In this paper, we focus on the case that $I=I_X$ is an ideal defining an almost complete intersection (ACI) set of points $X$ in $\mathbb{P}^1 \times \mathbb{P}^1$. In particular, we describe a minimal free bigraded resolution of a non arithmetically Cohen-Macaulay (also non homogeneous) set $\mathcal Z$ of fat points whose support is an ACI, generalizing a result of S. Cooper et al. for homogeneous sets of triple points. We call $\mathcal Z$ a fat ACI. We also show that its symbolic and ordinary powers are equal, i.e, $I_{\mathcal Z}^{(m)}=I_{\mathcal Z}^{m}$ for any $m\geq 1.$

Keywords:points in $\mathbb{P}^1\times \mathbb{P}^1$, symbolic powers, resolution, arithmetically Cohen-Macaulay
Categories:13C40, 13F20, 13A15, 14C20, 14M05

20. CJM 2016 (vol 69 pp. 767)

Choi, Suyoung; Park, Hanchul
Wedge Operations and Torus Symmetries II
A fundamental idea in toric topology is that classes of manifolds with well-behaved torus actions (simply, toric spaces) are classified by pairs of simplicial complexes and (non-singular) characteristic maps. The authors in their previous paper provided a new way to find all characteristic maps on a simplicial complex $K(J)$ obtainable by a sequence of wedgings from $K$. The main idea was that characteristic maps on $K$ theoretically determine all possible characteristic maps on a wedge of $K$. In this work, we further develop our previous work for classification of toric spaces. For a star-shaped simplicial sphere $K$ of dimension $n-1$ with $m$ vertices, the Picard number $\operatorname{Pic}(K)$ of $K$ is $m-n$. We refer to $K$ as a seed if $K$ cannot be obtained by wedgings. First, we show that, for a fixed positive integer $\ell$, there are at most finitely many seeds of Picard number $\ell$ supporting characteristic maps. As a corollary, the conjecture proposed by V.V. Batyrev in 1991 is solved affirmatively. Second, we investigate a systematic method to find all characteristic maps on $K(J)$ using combinatorial objects called (realizable) puzzles that only depend on a seed $K$. These two facts lead to a practical way to classify the toric spaces of fixed Picard number.

Keywords:puzzle, toric variety, simplicial wedge, characteristic map
Categories:57S25, 14M25, 52B11, 13F55, 18A10

21. CJM 2016 (vol 68 pp. 1362)

Papikian, Mihran; Rabinoff, Joseph
Optimal Quotients of Jacobians with Toric Reduction and Component Groups
Let $J$ be a Jacobian variety with toric reduction over a local field $K$. Let $J \to E$ be an optimal quotient defined over $K$, where $E$ is an elliptic curve. We give examples in which the functorially induced map $\Phi_J \to \Phi_E$ on component groups of the Néron models is not surjective. This answers a question of Ribet and Takahashi. We also give various criteria under which $\Phi_J \to \Phi_E$ is surjective, and discuss when these criteria hold for the Jacobians of modular curves.

Keywords:Jacobians with toric reduction, component groups, modular curves
Categories:11G18, 14G22, 14G20

22. CJM 2016 (vol 69 pp. 143)

Levinson, Jake
One-dimensional Schubert Problems with Respect to Osculating Flags
We consider Schubert problems with respect to flags osculating the rational normal curve. These problems are of special interest when the osculation points are all real -- in this case, for zero-dimensional Schubert problems, the solutions are "as real as possible". Recent work by Speyer has extended the theory to the moduli space $ \overline{\mathcal{M}_{0,r}} $, allowing the points to collide. These give rise to smooth covers of $ \overline{\mathcal{M}_{0,r}} (\mathbb{R}) $, with structure and monodromy described by Young tableaux and jeu de taquin. In this paper, we give analogous results on one-dimensional Schubert problems over $ \overline{\mathcal{M}_{0,r}} $. Their (real) geometry turns out to be described by orbits of Schützenberger promotion and a related operation involving tableau evacuation. Over $\mathcal{M}_{0,r}$, our results show that the real points of the solution curves are smooth. We also find a new identity involving "first-order" K-theoretic Littlewood-Richardson coefficients, for which there does not appear to be a known combinatorial proof.

Keywords:Schubert calculus, stable curves, Shapiro-Shapiro Conjecture, jeu de taquin, growth diagram, promotion
Categories:14N15, 05E99

23. CJM 2016 (vol 68 pp. 1096)

Smith, Benjamin H.
Singular $G$-Monopoles on $S^1\times \Sigma$
This article provides an account of the functorial correspondence between irreducible singular $G$-monopoles on $S^1\times \Sigma$ and $\vec{t}$-stable meromorphic pairs on $\Sigma$. A theorem of B. Charbonneau and J. Hurtubise is thus generalized here from unitary to arbitrary compact, connected gauge groups. The required distinctions and similarities for unitary versus arbitrary gauge are clearly outlined and many parallels are drawn for easy transition. Once the correspondence theorem is complete, the spectral decomposition is addressed.

Keywords:connection, curvature, instanton, monopole, stability, Bogomolny equation, Sasakian geometry, cameral covers
Categories:53C07, 14D20

24. CJM 2016 (vol 68 pp. 784)

Doran, Charles F.; Harder, Andrew
Toric Degenerations and Laurent Polynomials Related to Givental's Landau-Ginzburg Models
For an appropriate class of Fano complete intersections in toric varieties, we prove that there is a concrete relationship between degenerations to specific toric subvarieties and expressions for Givental's Landau-Ginzburg models as Laurent polynomials. As a result, we show that Fano varieties presented as complete intersections in partial flag manifolds admit degenerations to Gorenstein toric weak Fano varieties, and their Givental Landau-Ginzburg models can be expressed as corresponding Laurent polynomials. We also use this to show that all of the Laurent polynomials obtained by Coates, Kasprzyk and Prince by the so called Przyjalkowski method correspond to toric degenerations of the corresponding Fano variety. We discuss applications to geometric transitions of Calabi-Yau varieties.

Keywords:Fano varieties, Landau-Ginzburg models, Calabi-Yau varieties, toric varieties
Categories:14M25, 14J32, 14J33, 14J45

25. CJM 2016 (vol 69 pp. 613)

Moon, Han-Bom
Mori's Program for $\overline{M}_{0,7}$ with Symmetric Divisors
We complete Mori's program with symmetric divisors for the moduli space of stable seven-pointed rational curves. We describe all birational models in terms of explicit blow-ups and blow-downs. We also give a moduli theoretic description of the first flip, which has not appeared in the literature.

Keywords:moduli of curves, minimal model program, Mori dream space
Categories:14H10, 14E30
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