DESIGNS AS MAXIMUM CODES IN POLYNOMIAL METRIC-SPACES

Finite and infinite metric spaces M that are polynomial with respect to a monotone substitution of variable t(d) are considered. A finite subset (code) W subset-or-equal-to M is characterized by the minimal distance d(W) between its distinct elements, by the number l(W) of distances between its distinct elements and by the maximal strength tau(W) of the design generated by the code W. A code W subset-or-equal-to M is called a maximum one if it has the greatest cardinality among subsets of M with minimal distance at least d(W), and diametrical if the diameter of W is equal to the diameter of the whole space M. Delsarte codes are codes W subset-or-equal-to M with tau(W) greater-than-or-equal-to 2l(W) - 1 or tau(W) = 2l(W) - 2 > 0 and W is a diametrical code. It is shown that all parameters of Delsarte codes W subset-or-equal-to M are uniquely determined by their cardinality absolute value of W or minimal distance d(W) and that the minimal polynomials of the Delsarte codes W subset-or-equal-to M are expansible with positive coefficients in an orthogonal system of polynomials {Q(i)(t)} corresponding to M. The main results of the present paper consist in a proof of maximality of all Delsarte codes provided that the system {Q(i)(t)} satisfies some condition and of a new proof confirming in this case the validity of all the results on the upper bounds for the maximum cardinality of codes W subset-or-equal-to M with a given minimal distance, announced by the author in 1978. Moreover, it appeared that this condition is satisfied for all infinite polynomial metric spaces as well as for distance-regular graphs, decomposable in a sense defined below. It is also proved that with one exception all classical distance-regular graphs are decomposable. In addition for decomposable distance-regular graphs an improvement of the absolute Delsarte bound for diametrical codes is obtained. For the Hamming and Johnson spaces, Euclidean sphere, real and complex projective spaces, tables containing parameters of known Delsarte codes are presented. Moreover, for each of the above-mentioned infinite spaces infinite sequences (of maximum) Delsarte codes not belonging to tight designs are indicated.