## Abstract

Consider a complete flag { 0 } = C< C_{1}< ⋯ < C_{n}= F^{n} of one-point AG codes of length n over the finite field F. The codes are defined by evaluating functions with poles at a given point Q in points P_{1}, ⋯ , P_{n} distinct from Q. A flag has the isometry-dual property if the given flag and the corresponding dual flag are the same up to isometry. For several curves, including the projective line, Hermitian curves, Suzuki curves, Ree curves, and the Klein curve over the field of eight elements, the maximal flag, obtained by evaluation in all rational points different from the point Q, is self-dual. More generally, we ask whether a flag obtained by evaluation in a proper subset of rational points is isometry-dual. In Geil et al. (2011) it is shown, for a curve of genus g, that a flag of one-point AG codes defined with a subset of n> 2 g+ 2 rational points is isometry-dual if and only if the last code C_{n} in the flag is defined with functions of pole order at most n+ 2 g- 1. Using a different approach, we extend this characterization to all subsets of size n≥ 2 g+ 2. Moreover we show that this is best possible by giving examples of isometry-dual flags with n= 2 g+ 1 such that C_{n} is generated by functions of pole order at most n+ 2 g- 2. We also prove a necessary condition, formulated in terms of maximum sparse ideals of the Weierstrass semigroup of Q, under which a flag of punctured one-point AG codes inherits the isometry-dual property from the original unpunctured flag.

Original language | English (US) |
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Pages (from-to) | 1617-1638 |

Number of pages | 22 |

Journal | Designs, Codes, and Cryptography |

Volume | 88 |

Issue number | 8 |

DOIs | |

State | Published - Aug 1 2020 |

## Keywords

- 11T71
- 94B27
- AG code
- Dual code
- Punctured code

## ASJC Scopus subject areas

- Theoretical Computer Science
- Computer Science Applications
- Discrete Mathematics and Combinatorics
- Applied Mathematics