Akihiro Munemasa

Type IV self-dual codes over rings

We study Type IV self-dual codes over the commutative rings of order 4. Gleason-type theorems of Type IV codes and their shadow codes are investigated. A mass formula of Type IV codes over these rings are given. We give a classification of Type TV codes over Z/sub 4/ and F2+uF/sub 2/ for reasonable lengths. We also construct a number of optimal Type TV codes.

The nonexistence of certain tight spherical designs

In this paper, the nonexistence of tight spherical designs is shown in some cases left open to date. Tight spherical 5-designs may exist in dimension n = (2m + 1)2 − 2, and the existence is known only for m = 1, 2. In the paper, the existence is ruled out under a certain arithmetic condition on the integer m, satisfied by infinitely many values of m, including m = 4. Also, nonexistence is shown for m = 3. Tight spherical 7-designs may exist in dimension n = 3d2 − 4, and the existence is known only for d = 2, 3. In the paper, the existence is ruled out under a certain arithmetic condition on d, satisfied by infinitely many values of d, including d = 4. Also, nonexistence is shown for d = 5. The fact that the arithmetic conditions on m for 5-designs and on d for 7-designs are satisfied by infinitely many values of m and d, respectively, is shown in the Appendix written by Y.-F. S. Pétermann. §

GENERALIZED SPIN MODELS

We introduce a generalization of spin models by dropping the symmetry condition. The partition function of a generalized spin model on a connected oriented link diagram is invariant under Reidemeister moves of type II and III, giving an invariant for oriented links.

Amorphous association schemes over the Galois rings of characteristic 4

We classify certain amorphous association schemes over the Galois rings of characteristic 4. The result contains a new family of amorphous association schemes, which are closely related to the distance-regular digraphs constructed by Liebler and Mena.

Hyperplane partitions and difference systems of sets

Difference Systems of Sets (DSS) are combinatorial configurations that arise in connection with code synchronization. This paper gives new constructions of DSS obtained from partitions of hyperplanes in a finite projective space, as well as DSS obtained from balanced generalized weighing matrices and partitions of the complement of a hyperplane in a finite projective space.

Equiangular lines in Euclidean spaces

We obtain several new results contributing to the theory of real equiangular line systems. Among other things, we present a new general lower bound on the maximum number of equiangular lines in d dimensional Euclidean space; we describe the two-graphs on 12 vertices; and we investigate Seidel matrices with exactly three distinct eigenvalues. As a result, we improve on two long-standing upper bounds regarding the maximum number of equiangular lines in dimensions d = 14 and d = 16 . Additionally, we prove the nonexistence of certain regular graphs with four eigenvalues, and correct some tables from the literature.

On Type II codes over F/sub 4/

Previously, Type II codes over F/sub 4/ have been introduced as Euclidean self-dual codes with the property that all Lee weights are divisible by four. In this paper, a number of properties of Type II codes are presented. We construct several extremal Type II codes and a number of extremal Type I codes. It is also shown that there are seven Type II codes of length 12, up to permutation equivalence.

Some restrictions on weight enumerators of singly even self-dual codes

In this correspondence, we give some restrictions on weight enumerators of singly even self-dual [n,n/2,d] codes whose shadows have minimum weight d/2. As a consequence, we determine the weight enumerators for which there is an extremal singly even self-dual [40,20,8] code and an optimal singly even self-dual [50,25,10] code

Classification of Ternary Extremal Self-Dual Codes of Length 28

All 28-dimensional unimodular lattices with minimum norm 3 are known. Using this classification, we give a classification of ternary extremal self-dual codes of length 28. Up to equivalence, there are 6,931 such codes.

On triply even binary codes

A triply even code is a binary linear code in which the weight of every codeword is divisible by 8. We show how two doubly even codes of lengths m_1 and m_2 can be combined to make a triply even code of length m_1+m_2, and then prove that every maximal triply even code of length 48 can be obtained by combining two doubly even codes of length 24 in a certain way. Using this result, we show that there are exactly 10 maximal triply even codes of length 48 up to equivalence.