Hermitian codes and complete intersections

Abstract

In this paper we present a geometrical characterization for the minimum-weight codewords of the Hermitian codes over the fields $\mathbb{F}_{q^2}$ in the third and fourth phase, namely with distance $d \geq q^2-q$. We consider the unique writing $μq + λ(q+1)$ of the distance $d$ with $μ, λ$ non negative integers, and $μ\leq q$, and prove that the minimum-weight codewords correspond to complete intersection divisors cut on the Hermitian curve $\mathcal{H}$ by curves $\mathcal X$ of degree $μ+λ$ having $x^μy^λ$ as leading term w.r.t. the $\texttt{DegRevLex}$ term ordering (with $y>x$). Moreover, we show that any such curve $\mathcal X$ corresponds to minimum-weight codewords provided that the complete intersection divisor $\mathcal{H}\cap \mathcal X$ is made of simple $\mathbb{F}_{q^2}$-points. Finally, using this geometric characterization, we propose an algorithm to compute the number of minimum weight codewords and we present comparison tables between our algorithm and MAGMA command $\mathtt{MinimumWords}$.