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Researchers studying the theory of error-correcting codes have discovered, in recent years, that finite geometries and designs can provide the basis for excellent communications schemes. The basic idea is to take the linear span (over some finite field) of the rows of the incidence matrix of such a structure as the allowable messages. Mariner 9, for example, transmitted data to Earth by using a code derived from the structure of the hyperplanes in a five-dimensional vector space over F2 the field with two elements.
The purpose of this monograph is to allow coding theory to repay some of its debt to the combinatorial theory of designs. Specifically, I have tried to show herein how the objects introduced by coding theorists can offer great insight into the study of symmetric designs.
The vector spaces and modules (over appropriate rings) generated by the incidence matrices of symmetric designs provide a natural setting for invoking much algebraic machinery– – most notably, the theory of group representations– –which has hitherto not found much application in this combinatorial subject. In doing so, they provide a point of view which unifies a number of diverse results as well as makes possible many new theorems. My own investigation into this subject is surely not definitive, and if anyone is stimulated to further develop this point of view, I will have accomplished something.
Two goals have informed my choice of organization. First, since my object is to expose a particular approach to the study of symmetric designs, I have chosen to develop the subject from scratch.
This appendix presents the theory of bilinear forms and quadratic forms which we shall require in Chapters 2 and 3. While the reader has undoubtedly met these topics in a course on linear algebra, the first two sections review them from, perhaps, a more abstract point of view. The abstraction facilitates proofs, in the third section, of Witt's Extension Theorem and Cancellation Theorem for quadratic forms over an arbitrary field (including the case of fields of characteristic 2.) The last section classifies nondegenerate forms over Fp (p odd).
BILINEAR FORMS
Let V be a vector space of finite dimension n over a field F. A bilinear form on V is a map B: V x V→F which is F-linear in each coordinate.
(1) B is said to be a symmetric bilinear form if B(u,v) = B(v,u) for all u, vεV. (B is also said to be an orthogonal bilinear form or a scalar product.)
(2) B is said to be an alternating bilinear form if B(u,u) = 0 for all u V or, equivalently, if B(u,v) = -B(v,u) for all u, vεV. (B is also said to be symplectic.)
In this monograph we are only concerned with symmetric bilinear forms. (Note that an alternating form is symmetric if char F=2.) A pair (V,B), where B is a symmetric bilinear form on the vector space V, is called a scalar product space.
Barnes and Sloane recently described a “general construction” for lattice packings of equal spheres in Euclidean space. In the present paper we simplify and further generalize their construction, and make it suitable for iteration. As a result we obtain lattice packings in ℝm with density Δ satisfying , as m → ∞ where is the smallest value of k for which the k-th iterated logarithm of m is less than 1. These appear to be the densest lattices that have been explicitly constructed in high-dimensional space. New records are also established in a number of lower dimensions, beginning in dimension 96.
Let q = pn, p a rational prime, and let be the finite field with q elements. The polynomial ring is considered as an analogue of the ring of rational integers ℤ. Completing the quotient field with respect to the normalized valuation at ∞, and then taking algebraic closure, we obtained the field k∞ whose elements will be called “numbers”.
Before turning to the questions to be considered in this paper, we recall two other problems. Let C(a, p) be the class of all convex discs of area not less than a given constant a and perimeter not greater than a given constant p. What is the densest packing and what is the most economical covering of the Euclidean plane with discs from C(a, p)?
Both problems are interesting only if p2/a < 8√3, i.e. if p is less than the perimeter of a regular hexagon of area a. In this case, the densest packing arises from a regular hexagonal tiling by rounding off the corners of the tiles by equal circular arcs so as to obtain smooth hexagons of area a and perimeter p.
If C and Co are two convex bodies in Ed we say that C slides (rolls) freely inside Co if the following condition is satisfied: for each x ∈ ∂C0 (and each rotation R) there is a translation t such that, if gC = C + t (= RC + t), then gC ⊂ Co and x ∈ ∂gC. This work establishes certain topological conditions which ensure the free rolling and sliding of C inside Co. One consequence of these conditions is that, if ∂K ∩ int gK is a topological ball for all rigid motions g, then K is a ball in the geometrical sense.
The classical mean value theorem for Dirichlet's polynomials states that
see H. L. Montgomery [7]. This formula is very useful in the theory of the Riemann zeta-function ζ(s). From the approximate functional equation
where | χ(½ + it)| = 1, u, v ≥ 1, 2πuv = t (see E. C. Titchmarsh [8]) it follows that χ(½ + it) can be well approximated by Dirichlet's polynomials of length N< t½.