New extremal Type II ${\mathbb{Z}}_4$-codes of length 64 by the doubling method

Abstract

Extremal Type II ${\mathbb{Z}}_4$-codes are a class of self-dual ${\mathbb{Z}}_4$-codes with Euclidean weights divisible by eight and the largest possible minimum Euclidean weight for a given length. A small number of such codes is known for lengths greater than or equal to $48.$ The doubling method is a method for constructing Type II ${\mathbb{Z}}_4$-codes from a given Type II ${\mathbb{Z}}_4$-code. Based on the doubling method, in this paper we develop a method to construct new extremal Type II ${\mathbb{Z}}_4$-codes starting from an extremal Type II ${\mathbb{Z}}_4$-code of type $4^k$ with an extremal residue code and length $48,56$ or $64$. Using this method, we construct three new extremal Type II ${\mathbb{Z}}_4$-codes of length $64$ and type $4^{31}{2}^2$. Extremal Type II ${\mathbb{Z}}_4$-codes of length $64$ of this type were not known before. Moreover, the residue codes of the constructed extremal ${\mathbb{Z}}_4$-codes are new best known $[64,31]$ binary codes and the supports of the minimum weight codewords of the residue code and the torsion code of one of these codes form self-orthogonal $1$-designs.