Configurations of 2 n - 2 Quadrics in R n with 3 · 2 n-1 Common Tangent Lines

   Abstract. We construct 2n-2 smooth quadrics in R ⁿ whose equations have the same degree 2 homogeneous parts such that these quadrics have 3⋅ 2 ^n-1 isolated common real tangent lines. Special cases of the construction give examples of 2n-2 spheres with affinely dependent centres such that all but one of the radii are equal, and of 2n-2 quadrics which are translated images of each other.     

The common curve of quadrics sharing a self-polar simplex

Summary When n-1 quadrics in projective space [n] of n dimensions have a common self-polar simplex their common curve Γ admits a group of2 ⁿ self-projectivities. The consequent properties of Γ are investigated, and further specialisations are imposed which amplify the the group and endow Γ with further properties. There is some reference to the osculating spaces and principal chords of Γ, and some properties of particular curves in four and five dimensions are described. Entrata in Redazione il 23 giugno 1976.     

Maximal sets of non-polar points of quadrics and symplectic polarities over GF(2)

A cap on a non-singular quadric over GF(2) is a set of points that are pairwise non-polar; equivalently the join of any two of the points is a chord. A non-secant set of the quadric is a set of points off the quadric that are pairwise non-polar; equivalently the join of any two of the points is skew to the quadric. We determine all the maximal caps and all the maximal non-secant sets of all non-singular quadrics over GF(2); and also all the maximal sets of non-polar points for symplectic polarities over GF(2). The classification is in terms of caps of greatest size on elliptic quadrics Q _– ^8k+3 (2), hyperbolic quadrics Q ₊ ^8k+7 (2) and on quadrics Q _4k+2(2), and of non-secant sets of greatest size of Q _– ^8k+1 (2), Q ₊ ^8k+5 (2) and Q _4k (2), for all quadrics of these types that occur as sections of the parent quadric or belong to the symplectic polarity. The sets of greatest size for these types of quadrics are larger than for other types. The results have implications about the non-existence of ovoids and the exterior sets of Thas. Only one part of the simple geometric inductive argument extends to larger ground fields.     

Gröbner Flags and Gorenstein Algebras

The goal of this paper is to study the Koszul property and the property of having a Gröbner basis of quadrics for classical varieties and algebras as canonical curves, finite sets of points and Artinian Gorenstein algebras with socle in low degree. Our approach is based on the notion of Gröbner flags and Koszul filtrations. The main results are the existence of a Gröbner basis of quadrics for the ideal of the canonical curve whenever it is defined by quadrics, the existence of a Gröbner basis of quadrics for the defining ideal of s 2n points in general linear position in P ⁿ , and the Koszul property of the generic Artinian Gorenstein algebra of socle degree 3.     

The Number of Geometric Bistellar Neighbors of a Triangulation

The theory of secondary and fiber polytopes implies that regular (also called convex or coherent) triangulations of configurations with n points in R ^d have at least n-d-1 geometric bistellar neighbors. Here we prove that, in fact, all triangulations of n points in R ² have at least n-3 geometric bistellar neighbors. In a similar way, we show that for three-dimensional point configurations, in convex position and with no three points collinear, all triangulations have at least n-4 geometric bistellar flips. In contrast, we exhibit three-dimensional point configurations, with a single interior point, having deficiency on the number of geometric bistellar flips. A lifting technique allows us to obtain a triangulation of a simplicial convex 4-polytope with less than n-5 neighbors. We also construct a family of point configurations in R ³ with arbitrarily large flip deficiency. Received November 25, 1996, and in revised form March 10, 1997.     

A Nonlinear Version of Noether's Type Theorem

Abstract

Let L be a line bundle on a smooth curve C, which defines a birational morphism onto Φ(C) ? P ^r . We prove that, under suitable assumptions on L, which are satisfied by Castelnuovo's curves, a generic section in H ⁰(C, L ²) can be written as α² + β² + γ², with α, β, γ ∈ H ⁰(C, L). If there are no quadrics of rank 3 containing Φ(C), this is true for any section. For canonical curves, this gives a non linear version of Noether's Theorem.     

Skew loops and quadric surfaces

A skew loop is a closed curve without parallel tangent lines. We prove: The only complete surfaces in R ³ with a point of positive curvature and no skew loops are the quadrics. In particular: Ellipsoids are the only closed surfaces without skew loops. Our efforts also yield results about skew loops on cylinders and positively curved surfaces. Received: January 7, 2002 RID="*" ID="*"The first author was partially supported by the NSF grant DMS-0204190.     

Real Hypersurfaces Having Many Pseudo-hyperplanes

Abstract

Let nand dbe natural integers satisfying n ≥ 3 and d ≥ 10. Let Xbe an irreducible real hypersurface Xin ? ⁿ of degree dhaving many pseudo-hyperplanes. Suppose that Xis not a projective cone. We show that the arrangement ? of all d ? 2 pseudo-hyperplanes of Xis trivial, i.e., there is a real projective linear subspace Lof ? ⁿ (?) of dimension n ? 2 such that L ? Hfor all H ∈ ?. As a consequence, the normalization of Xis fibered over ?¹in quadrics. Both statements are in sharp contrast with the case n = 2; the first statement also shows that there is no Brusotti-type result for hypersurfaces in ? ⁿ , for n ≥ 3.     

Pseudo unitary conformal groups and Clifford algebras for standard pseudo hermitian spaces

This paper, self-contained, deals with pseudo-unitary spin geometry. First, we present pseudo-unitary conformal structures over a 2n-dimensional complex manifold V and the corresponding projective quadrics for standard pseudo-hermitian spaces H_p,q. Then we develop a geometrical presentation of a compactification for pseudo-hermitian standard spaces in order to construct the pseudo-unitary conformal group of H_p,q. We study the topology of the projective quadrics and the “generators” of such projective quadrics. Then we define the space S of spinors canonically associated with the pseudo-hermitian scalar product of signature (2ⁿ⁻¹, 2ⁿ⁻¹). The spinorial group Spin U(p,q) is imbedded into SU(2ⁿ⁻¹, 2ⁿ⁻¹). At last, we study the natural imbeddings of the projective quadrics      

The Number of Directions Determined by Points in the Three-Dimensional Euclidean Space

Abstract. Let X be a set of n points in the three-dimensional Euclidean space such that no three points in X are on the same line and there is no plane containing all points in X . An old conjecture states that pairs of points in X determine at least 2n-3 directions. We prove the weaker result that X determines at least 1.75n-2 directions.