The conorm code of an AG-code

Abstract

Given a suitable extension F′/F of algebraic function fields over a finite field Fq, we introduce the conorm code ConF′/F(C) defined over F′ which is constructed from an algebraic geometry code C defined over F. We study the parameters of ConF′/F(C) in terms of the parameters of C, the ramification behavior of the places used to define C and the genus of F. In the case of unramified extensions of function fields we prove that ConF′/F(C)⊥=ConF′/F(C⊥) when the degree of the extension is coprime to the characteristic of Fq. We also study the conorm of cyclic algebraic-geometry codes and we show that some repetition codes, Hermitian codes and all Reed-Solomon codes can be represented as conorm codes.