Integral points on quadrics with prime coordinates

Let f (x ₁, . . . , x _s ) be a regular indefinite integral quadratic form, and t an integer. Denote by V the affine quadric {x : f (x) = t}, and by \({V(\mathbb {P})}\) the set of \({{\bf x}\in V}\) whose coordinates are simultaneously prime. It is proved that, under suitable conditions, \({V(\mathbb{P})}\) is Zariski dense in V as long as s ≥ 10.     

Integral points on quadrics in three variables whose coordinates have few prime factors

The main theorem states that if f(x ₁, x ₂, x ₃) is an indefinite anisotropic integral quadratic form with determinant d(f), and t a non-zero integer such that d(f)t is square-free, then as long as there is one integer solution to f(x ₁, x ₂, x ₃) = t there are infinitely many such solutions for which the product x ₁ x ₂ x ₃ has at most 26 prime factors. The proof relies on the affine linear sieve and in particular automorphic spectral methods to obtain a sharp level of distribution in the associated counting problem. The 26 comes from applying the sharpest known bounds towards Selberg’s eigenvalue conjecture. Assuming the latter the number 26 may be reduced to 22.     

Tropical quadrics through three points

We tropicalize the rational map that takes triples of points in the projective plane to the plane of quadrics passing through these points. The image of its tropicalization is contained in the tropicalization of its image. We identify these objects inside the tropical Grassmannian of planes in projective 5-space, and we explore a small tropical Hilbert scheme.     

Generic points of quadrics and Chow groups

In this article we study the sufficient conditions for the k̅-defined element of the Chow group of a smooth variety to be k-rational (defined over k). For 0-cycles this question was addressed earlier. Our methods work for cycles of arbitrary dimension. We show that it is sufficient to check this property over the generic point of a quadric of sufficiently large dimension. Among the applications one should mention the uniform construction of fields with all known u-invariants.     

Integral points on convex curves

The Ramanujan Journal - We estimate the maximal number of integral points which can be on a convex arc in $${mathbb {R}}^2$$ with given length, minimal radius of curvature and initial slope.     

Natural neighbor coordinates of points on a surface

Natural neighbor coordinates and natural neighbor interpolation have been introduced by Sibson for interpolating multivariate scattered data. In this paper, we consider the case where the data points belong to a smooth surface , i.e., a (d−1)-manifold of . We show that the natural neighbor coordinates of a point X belonging to tends to behave as a local system of coordinates on the surface when the density of points increases. Our result does not assume any knowledge about the ordering, connectivity or topology of the data points or of the surface. An important ingredient in our proof is the fact that a subset of the vertices of the Voronoi diagram of the data points converges towards the medial axis of when the sampling density increases.     

Primitive symmetric designs with prime power number of points

In this paper we either prove the non‐existence or give explicit construction of primitive symmetric (v, k, λ) designs with v=p^m<2500, p prime and m>1. The method of design construction is based on an automorphism group action; non‐existence results additionally include the theory of difference sets, multiplier theorems in particular. The research involves programming and wide‐range computations. We make use of software package GAP and the library of primitive groups which it contains. © 2009 Wiley Periodicals, Inc. J Combin Designs 18: 141–154, 2010     

Integral circulant graphs of prime power order with maximal energy

The energy of a graph is the sum of the moduli of the eigenvalues of its adjacency matrix. We study the energy of integral circulant graphs, also called gcd graphs, which can be characterized by their vertex count n and a set D of divisors of n in such a way that they have vertex set Z_n and edge set {{a,b}:a,b∈Z_n,gcd(a-b,n)∈D}. Using tools from convex optimization, we analyze the maximal energy among all integral circulant graphs of prime power order p^s and varying divisor sets D. Our main result states that this maximal energy approximately lies between s(p-1)p^s-1 and twice this value. We construct suitable divisor sets for which the energy lies in this interval. We also characterize hyperenergetic integral circulant graphs of prime power order and exhibit an interesting topological property of their divisor sets.     

Integral Apollonian circle packings and prime curvatures

It is shown that any primitive integral Apollonian circle packing captures a fraction of the prime numbers. Basically, the method consists of applying the circle method and considering the curvatures produced by a well-chosen family of binary quadratic forms.     

Light rays having extreme points with the same spatial coordinates

In this paper we deal with lightlike geodesics in Lorentzian manifolds which are closed with respect to the spatial coordinates. We consider the case where the initial point is fixed or moves on a level of a given time function. The case where the initial point is free is also considered, under a periodicity condition on the metric.     

Integral points on strictly convex closed curves

A negative answer is given to Swinnerton-Dyer's question: Is it true that for any > 0 there exists a positive integer n such that for any planar closed strictly convex n-times differentiable curve , when it is blown up a sufficiently large number of times, the number of integral points on the resultant curve will be less than . An example has been constructed when this number for an infinite number is not less than ^1/2, while is infinitely differentiable.Translated from Matematicheskie Zametki, Vol. 21, No. 6, pp. 799–805, June, 1977.The author thanks S. B. Stechkin for attention to the work.     

Fixed points of projectivities of prime order

This work begins with a review of the classical results for fixed points of projectivities in a projective plane over a general commutative field. The second section of this work features all the material necessary to prove the main result, which is presented in Theorem 2.8. It is shown that, in a finite projective plane of order q, there exists a projectivity g? of prime order p?>?3 if and only if p divides exactly one of the integers q ? 1, q, q?+?1, q ² + q + 1. Theorem 2.8 establishes a correspondence between the possible structures of points fixed by g?, as presented in Theorem 1.3, and the integer that is divisible by p. The special case of p = 2 is handled in Sect. 2.1, where it is shown that every involution is a harmonic homology for q odd and an elation for q even. The special case of p?=?3 is handled in Sect. 2.2, and Theorem 2.8 is adapted for p?=?3 and presented as Theorem 2.15. An application of Theorems 2.8 and 2.15 is determining the sizes of (n, r)-arcs that are stabilized by projectivities of prime order p in the finite projective plane of order q; in Sect. 3, this application is presented in Propositions 3.2 and 3.3.     

Threshold for the volume spanned by random points with independent coordinates

Let μ be an even compactly supported Borel probability measure on the real line. For every N > n consider N independent random vectors X ₁, ..., X _N in ℝ ⁿ , with independent coordinates having distribution μ. We establish a sharp threshold for the volume of the random polytope K _N ≔ conv{X ₁, ..., X _N }, provided that the Legendre transform λ of the cumulant generating function of μ satisfies the condition