A generalization of crossing families

For a set of points in the plane, a \emph{crossing family} is a set of line segments, each joining two of the points, such that any two line segments cross. We investigate the following generalization of crossing families: a \emph{spoke set} is a set of lines drawn through a point set such that each unbounded region of the induced line arrangement contains at least one point of the point set. We show that every point set has a spoke set of size $\sqrt{\frac{n}{8}}$. We also characterize the matchings obtained by selecting exactly one point in each unbounded region and connecting every such point to the point in the antipodal unbounded region.Comment: 14 pages, 10 figure

The untwisting number of a knot

The unknotting number of a knot is the minimum number of crossings one must change to turn that knot into the unknot. The algebraic unknotting number is the minimum number of crossing changes needed to transform a knot into an Alexander polynomial-one knot. We work with a generalization of unknotting number due to Mathieu-Domergue, which we call the untwisting number. The untwisting number is the minimum number (over all diagrams of a knot) of right- or left-handed twists on even numbers of strands of a knot, with half of the strands oriented in each direction, necessary to transform that knot into the unknot. We show that the algebraic untwisting number is equal to the algebraic unknotting number. However, we also exhibit several families of knots for which the difference between the unknotting and untwisting numbers is arbitrarily large, even when we only allow twists on a fixed number of strands or fewer.Comment: 18 pages, 6 figures; to appear in Pacific J. Mat

Positive scalar curvature and higher-dimensional families of Seiberg-Witten equations

We introduce an invariant of tuples of commutative diffeomorphisms on a 4-manifold using families of Seiberg-Witten equations. This is a generalization of Ruberman's invariant of diffeomorphisms defined using 1-parameter families of Seiberg-Witten equations. Our invariant yields an application to the homotopy groups of the space of positive scalar curvature metrics on a 4-manifold. We also study the extension problem for families of 4-manifolds using our invariant.Comment: 22 pages, accepted for publication by the Journal of Topolog

Infinite families of crossing-critical graphs with prescribed average degree and crossing number

Siran constructed infinite families of k-crossing-critical graphs for every k=>3 and Kochol constructed such families of simple graphs for every k=>2. Richter and Thomassen argued that, for any given k>=1 and r>=6, there are only finitely many simple k-crossing-critical graphs with minimum degree r. Salazar observed that the same argument implies such a conclusion for simple k-crossing-critical graphs of prescribed average degree r>6. He established existence of infinite families of simple k-crossing-critical graphs with any prescribed rational average degree r in [4,6) for infinitely many k and asked about their existence for r in (3,4). The question was partially settled by Pinontoan and Richter, who answered it positively for r in (7/2,4). The present contribution uses two new constructions of crossing critical simple graphs along with the one developed by Pinontoan and Richter to unify these results and to answer Salazar's question by the following statement: for every rational number r in (3,6) there exists an integer N_r, such that, for any k>N_r, there exists an infinite family of simple 3-connected crossing-critical graphs with average degree r and crossing number k. Moreover, a universal lower bound on k applies for rational numbers in any closed interval I in (3,6).Comment: 21 pages, 5 figure

Fifteen classes of solutions of the quantum two-state problem in terms of the confluent Heun function

We derive 15 classes of time-dependent two-state models solvable in terms of the confluent Heun functions. These classes extend over all the known families of three- and two-parametric models solvable in terms of the hypergeometric and the confluent hypergeometric functions to more general four-parametric classes involving three-parametric detuning modulation functions. In the case of constant detuning the field configurations describe excitations of two-state quantum systems by symmetric or asymmetric pulses of controllable width and edge-steepness. The classes that provide constant detuning pulses of finite area are identified and the factors controlling the corresponding pulse shapes are discussed. The positions and the heights of the peaks are mostly defined by two of the three parameters of the detuning modulation function, while the pulse width is mainly controlled by the third one, the constant term. The classes suggest numerous symmetric and asymmetric chirped pulses and a variety of models with two crossings of the frequency resonance. We discuss the excitation of a two-level atom by a pulse of Lorentzian shape with a detuning providing one or two crossings of the resonance. We derive closed form solutions for particular curves in the 3D space of the involved parameters which compose the complete return spectrum of the considered two-state quantum system

Logarithmic Kodaira-Akizuki-Nakano vanishing and Arakelov-Parshin boundedness for singular varieties

The article has two parts. The first part is devoted to proving a singular version of the logarithmic Kodaira-Akizuki-Nakano vanishing theorem of Esnault and Viehweg. This is then used to prove other vanishing theorems. In the second part these vanishing theorems are used to prove an Arakelov-Parshin type boundedness result for families of canonically polarized varieties with rational Gorenstein singularities.Comment: 23 pages, typos/errors corrected, definition of strong ampleness is made more general more typos/errors corrected, 6.4 made slightly more genera

Evolution for Khovanov polynomials for figure-eight-like family of knots

We look at how evolution method deforms, when one considers Khovanov polynomials instead of Jones polynomials. We do this for the figure-eight-like knots (also known as 'double braid' knots, see arXiv:1306.3197) -- a two-parametric family of knots which "grows" from the figure-eight knot and contains both two-strand torus knots and twist knots. We prove that parameter space splits into four chambers, each with its own evolution, and two isolated points. Remarkably, the evolution in the Khovanov case features an extra eigenvalue, which drops out in the Jones (t -> -1) limit.Comment: 9 pages, add some missing reference

Generalization of Neron models of Green, Griffiths and Kerr

We explain some recent developments in the theory of Neron models for families of Jacobians associated to variations of Hodge structures of weight -1.Comment: 8 page

A determinantal formula for the Hilbert series of one-sided ladder determinantal rings

We give a formula that expresses the Hilbert series of one-sided ladder determinantal rings, up to a trivial factor, in form of a determinant. This allows the convenient computation of these Hilbert series. The formula follows from a determinantal formula for a generating function for families of nonintersecting lattice paths that stay inside a one-sided ladder-shaped region, in which the paths are counted with respect to turns.Comment: 28 pages, AmS-LaTeX; an error in the definition of ladder determinantal ring was correcte

The isotriviality of families of canonically-polarized manifolds over a special quasi-projective base

In this paper we prove that a smooth family of canonically polarized manifolds parametrized by a special (in the sense of Campana) quasi-projective variety is isotrivial.Comment: Shorter arguments in Sect. 3 plus a correction in the equality of Cor. 3.2. To appear in Compositio Mathematic