Bounds on data limits for all-to-all comparison from combinatorial designs

In situations where every item in a data set must be compared with every other item in the set, it may be desirable to store the data across a number of machines in such a way that any two data items are stored together on at least one machine. One way to evaluate the efficiency of such a distribution is by the largest fraction of the data it requires to be allocated to any one machine. The all-to-all comparison (ATAC) data limit for $m$ machines is a measure of the minimum of this value across all possible such distributions. In this paper we further the study of ATAC data limits. We observe relationships between them and the previously studied combinatorial parameters of fractional matching numbers and covering numbers. We also prove a lower bound on the ATAC data limit that improves on one of Hall, Kelly and Tian, and examine the special cases where equality in this bound is possible. Finally, we investigate the data limits achievable using various classes of combinatorial designs. In particular, we examine the cases of transversal designs and projective Hjelmslev planes.Comment: 16 pages, 1 figur

Mutually unbiased bases and related structures

A set of bases of a d dimensional complex vector space, each pair of which is unbiased, is a set of mutually unbiased bases (MUBs). MUBs have applications in quantum physics and quantum information theory. Although the motivation to study MUBs comes from physical properties, MUBs are a mathematical structure. This is a mathematical investigation. There are many open problems in the theory of MUBS, some with conjectured solutions. For example: What is the maximum number of MUBs in a d dimensional vector space? Do complete sets of MUBs exist in all dimensions? One such conjectured solution states that a complete set of MUBs exists in a d dimensional complex vector space if and only if a complete set of mutually orthogonal Latin squares (MOLS) of order d exists (Saniga et. al., Journal of Optics B, 6: L19-20, 2004). The aim of this research was to find evidence for or against this conjecture. Inspired by constructions of MUBs that use sets of MOLS, complete sets of MOLS were constructed from two complete sets of MUBs. It is interesting to note that the MOLS structure emerges not from the vectors, but from the inner products of the vectors. Analogous properties between Hjelmslev planes and MUBs, and gaps in this knowledge motivated investigation of Hjelmslev planes. The substructures of a Hjelmslev plane over a Galois ring, and a combinatorial algorithm for generating Hjelmslev planes were developed. It was shown that the analogous properties of Hjelmslev planes and MUBs occur only for odd prime powers, making a strong connection between MUBs and Hjelmslev planes unlikely. A construction of MUBs that uses planar functions was generalised by using an automorphism on the additive group of a Galois field. It is still unclear whether this generalisation is equivalent to the original construction. Relation algebras were constructed from the structure of MUBs which do not share any similarities with relation algebras constructed from MOLS. It is possible that further investigation may yield relation algebras that are similar. It was shown that a set of Wooters and Fields type MUBs, when represented as elements of a group ring, forms a commutative monoid, whereas a set of Alltop type MUBs when similarly represented does not form a closed algebraic structure. It is known that WF and Alltop MUBs are equivalent, thus the lack of a closed structure in the Alltop MUBs suggests that the monoid is not a property of MUBs in general. Complete sets of MOLS and complete sets of MUBs are `similar in spirit', but perhaps this is not an inherent feature of MUBs and MOLS. Since all the known constructions of MUBs rely on algebraic structures which exist only in prime power dimensions, the connection may not be with MOLS, but with algebraic structures which generate both MOLS and MUBs

Characterisations and classifications in the theory of parapolar spaces

This thesis in incidence geometry is divided into two parts, which can both be linked to the geometries of the Freudenthal-Tits magic square. The first and main part consists of an axiomatic characterisation of certain plane geometries, defined via the Veronese mapping using degenerate quadratic alternative algebras (over any field) with a radical that is (as a ring) generated by a single element. This extends and complements earlier results of Schillewaert and Van Maldeghem, who considered such geometries over non-degenerate quadratic alternative algebras. The second and smaller part deals with a classification of parapolar spaces exhibiting the feature that the dimensions of intersections of pairs of symplecta cannot take all possible sensible values, with the only further requirement that, if the parapolar spaces have symplecta of rank 2, then they are strong. This part is based on a joint work with Schillewaert, Van Maldeghem and Victoor

Split buildings of type F₄ in buildings of type E₆

A symplectic polarity of a building of type is a polarity whose fixed point structure is a building of type containing residues isomorphic to symplectic polar spaces (i.e., so-called split buildings of type ). In this paper, we show in a geometric way that every building of type contains, up to conjugacy, a unique class of symplectic polarities. We also show that the natural point-line geometry of each split building of type fully embedded in the natural point-line geometry of arises from a symplectic polarity