Small point sets of PG(n, q 3) intersecting each k-subspace in 1 mod q points

The main result of this paper is that point sets of PG(n, q ³), q = p ^h , p ≥ 7 prime, of size less than 3(q ^3(n?k) + 1)/2 intersecting each k-space in 1 modulo q points (these are always small minimal blocking sets with respect to k-spaces) are linear blocking sets. As a consequence, we get that minimal blocking sets of PG(n, p ³), p ≥ 7 prime, of size less than 3(p ^3(n?k) + 1)/2 with respect to k-spaces are linear. We also give a classification of small linear blocking sets of PG(n, q ³) which meet every (n ? 2)-space in 1 modulo q points.     

Blocking sets in PG(2, q n ) from cones of PG(2n, q)

Let Ω and be a subset of Σ = PG(2n−1,q) and a subset of PG(2n,q) respectively, with Σ ⊂ PG(2n,q) and . Denote by K the cone of vertex Ω and base and consider the point set B defined by

Small Blocking Sets in PG(2, p3)

In this paper we show that a small minimalblocking set in PG(2,p³), with p 7,is of Rédei type.     

Tangency sets in PG(3, q)

A tangency set of PG (d,q) is a set Q of points with the property that every point P of Q lies on a hyperplane that meets Q only in P. It is known that a tangency set of PG (3,q) has at most points with equality only if it is an ovoid. We show that a tangency set of PG (3,q) with , or points is contained in an ovoid. This implies the non‐existence of minimal blocking sets of size , , and of with respect to planes in PG (3,q), and implies the extendability of partial 1‐systems of size , , or to 1‐systems on the hyperbolic quadric . © 2007 Wiley Periodicals, Inc. J Combin Designs 16: 462–476, 2008     

A class of ((p n−p m)(p n−1), p n−p m)-arcs in PG(2, p n)

A (k, d)-arc in PG(2, q) is a set of k points such that some d, but no d+1, of them are collinear. An outstanding problem is to find the maximum value of k for which a (k, d)-arc exists. A construction is given for a class of (k, p ⁿ–p ^m)-arcs in PG(2, p ⁿ). These arcs constitute a lower bound on the maximum possible value of k, and a subset of them is shown to be optimal.     

On the complement of the set of n-collinear points in PG(3, n)

Let ${S = (\mathcal{P}, \mathcal{L}, \mathcal{H})}$ be the finite planar space obtained from the 3-dimensional projective space PG(3, n) of order n by deleting a set of n-collinear points. Then, for every point ${p\in S}$ , the quotient geometry S/p is either a projective plane or a punctured projective plane, and every line of S has size n or n + 1. In this paper, we prove that a finite planar space with lines of size n + 1 ? s and n + 1, (s ≥ 1), and such that for every point ${p\in S}$ , the quotient geometry S/p is either a projective plane of order n or a punctured projective plane of order n, is obtained from PG(3, n) by deleting either a point, or a line or a set of n-collinear points.     

Cameron-Liebler line classes in PG (3,q)

A Cameron-Liebler line class is a set L of lines in PG(3, q) for which there exists a number x such that |LS|=x for all spreads S. There are many equivalent properties: Theorem 1 lists eight. This paper classifies Cameron-Liebler line classes with x4 (with two exceptions). It is also shown that the study of Cameron-Liebler line classes is equivalent to the study of weighted sets of points in PG(3, q) with two weights on lines.     

Classification of the (n, 3)-arcs in PG(2, 7)

In this paper the classification of the (n, 3)-arcs in PG(2, 7) is presented. It has been obtained using a computer-based exhaustive search that exploits projective equivalence and produces exactly one representative of each equivalence class. For each (n, 3)-arc, the automorphism group and the maximal size of a contained k-arc have been found.     

A characterization of exterior lines of certain sets of points in PG (2, q)

Let A and B be disjoint sets of points in PG(2, q) the Desarguesian projective plane of order q, with |A|q, |B|=q+1, such that each line through a point of A meets B (just once). Then B is a line.     

A common characterization of the projective spaces PG(4, n) and PG(5, n)

A common characterization of the projective spaces PG(4, n) and PG(5, n) in terms of finite irreducible planar spaces is given.     

Existence of Z-cyclic 3PTWh (p) for any Prime p≡ 1 (mod 4)

We show that a Z-cyclic triplewhist tournament on p players with three-person property, briefly 3PTWh(p), exists for any prime p≡ 1 (mod 4) with the only exceptions of p=5,13,17.     

On the upper chromatic number and multiple blocking sets of PG(n,q)

We investigate the upper chromatic number of the hypergraph formed by the points and the $k$‐dimensional subspaces of $\text{PG}\left(n,q\right)$; that is, the most number of colors that can be used to color the points so that every $k$‐subspace contains at least two points of the same color. Clearly, if one colors the points of a double blocking set with the same color, the rest of the points may get mutually distinct colors. This gives a trivial lower bound, and we prove that it is sharp in many cases. Due to this relation with double blocking sets, we also prove that for $t\le \frac{3}{8}p+1$, a small $t$‐fold (weighted) $\left(n?k\right)$‐blocking set of $\text{PG}\left(n,p\right),p$ prime, must contain the weighted sum of $t$ not necessarily distinct $\left(n?k\right)$‐spaces.     

Embedding 1-Factorizations of K n in PG(2, 32)

1-Factorizations of the complete graph K _n embedded in a finite Desarguesian projective plane PG(2, q), q even, are hyperfocused arcs of size n. The classification of hyperfocused arcs is motivated by applications to 2-level secret sharing schemes. So far it has been done for q  ≤ 16, and for special types of hyperfocused arcs. In this paper the case q = 32 is investigated and the following two results are proven. (i) Uniqueness of hyperfocused 12-arcs, up to projectivities. (ii) Non-existence of hyperfocused 14-arcs.     

Small values ofn 2α (mod 1)

Dedicated to Professor Wolfgang Schmidt on the occasion of his sixtieth birthday     

Subsets of PG(n,2) and Maximal Partial Spreads in PG(4,2)

Put θ _n = # {points in PG(n,2)} and φ _n = #{lines in PG(n,2)}. Let ψ be anypoint-subset of PG(n,2). It is shown thatthe sum of L = #{internal lines of ψ} and L′= #{external lines of ψ} is the same for all ψ having the same cardinality:[6pt] Theorem A If k is defined by k = |ψ| ? θ _{n ? 1}, then $$L + L' = \phi _{n - 1} + k(k - 1)/2.$$ (The generalization of this to subsets of PG(n,3) is also obtained.) Let $\mathcal{S}$ be a partial spreadof lines in PG(4,2) and let N denote the number of reguli contained in $\mathcal{S}$ .Use of Theorem A gives rise to a simple proof of:[6pt] Theorem B If $\mathcal{S}$ is maximal then one of the followingholds: (i) $\left| \mathcal{S} \right| = 5,{\text{ }}N = 10;{\text{ }}$ (ii) $\left| \mathcal{S} \right| = 7,{\text{ }}N = 4;{\text{ }}$ (iii) $\left| \mathcal{S} \right| = 9,{\text{ }}N = 4.$ If (i) holds then $\mathcal{S}$ is spread in a hyperplane.It is shown that possibility (ii) is realized by precisely threeprojectively distinct types of partial spread. Explicit examplesare also given of four projectively distinct types of partialspreads which realize possibility (iii). For one of these types,type X, the four reguli have a common line. It isshown that those partial spreads in PG(4,2) of size 9 which arise, by a simple construction, from a spreadin PG(5,2), are all of type X.     

Caps in PG(n,q),q even,n⩾3

Letm′₂(3,q) be the largest value ofk(k<q ²+1) for which there exists a completek-cap in PG(3,q),q even. In this paper, the known upper bound onm′₂(3,q) is improved. We also describe a number of intervals, fork, for which there does not exist a completek-cap in PG(3,q),q even. These results are then used to improve the known upper bounds on the number of points of a cap in PG(n, q),q even,n?4.