Determining when a truncated generalised Reed-Solomon code is Hermitian self-orthogonal

Abstract

We prove that there is a Hermitian self-orthogonal k -dimensional truncated generalised Reed-Solomon code of length n¿q2 over Fq2 if and only if there is a polynomial g¿Fq2 of degree at most (q-k)q-1 such that g+gq has q2-n distinct zeros. This allows us to determine the smallest n for which there is a Hermitian self-orthogonal k -dimensional truncated generalised Reed-Solomon code of length n over Fq2 , verifying a conjecture of Grassl and Rötteler. We also provide examples of Hermitian self-orthogonal k -dimensional generalised Reed-Solomon codes of length q2+1 over Fq2 , for k=q-1 and q an odd power of two.