In this paper we study generalized weights as an algebraic invariant of a code. We first describe anticodes in the Hamming and in the rank metric, proving in particular that optimal anticodes in the rank metric coincide with Frobenius-closed spaces. Then we characterize both generalized Hamming and rank weights of a code in terms of the intersection of the code with optimal anticodes in the respective metrics. Inspired by this description, we propose a new algebraic invariant, which we call "Delsarte generalized weights", for Delsarte rank-metric codes based on optimal anticodes of matrices. We show that our invariant refines the generalized rank weights for Gabidulin codes proposed by Kurihara, Matsumoto and Uyematsu, and establish a series of properties of Delsarte generalized weights. In particular, we characterize Delsarte optimal codes and anticodes in terms of their generalized weights. We also present a duality theory for the new algebraic invariant, proving that the Delsarte generalized weights of a code completely determine the Delsarte generalized weights of the dual code. Our results extend the theory of generalized rank weights for Gabidulin codes. Finally, we prove the analogue for Gabidulin codes of a theorem of Wei, proving that their generalized rank weights characterize the worst-case security drops of a Gabidulin rank-metric code.

We compare the two duality theories of rank-metric codes proposed by Delsarte and Gabidulin, proving that the former generalizes the latter. We also give an elementary proof of MacWilliams identities for the general case of Delsarte rank-metric codes. The identities which we derive are very easy to handle, and allow us to re-establish in a very concise way the main results of the theory of rank-metric codes first proved by Delsarte employing the theory of association schemes and regular semilattices. We also show that our identities imply as a corollary the original MacWilliams identities established by Delsarte. We describe how the minimum and maximum rank of a rank-metric code relate to the minimum and maximum rank of the dual code, giving some bounds and characterizing the codes attaining them. Then we study optimal anticodes in the rank metric, describing them in terms of optimal codes (namely, MRD codes). In particular, we prove that the dual of an optimal anticode is an optimal anticode. Finally, as an application of our results to a classical problem in enumerative combinatorics, we derive both a recursive and an explicit formula for the number of k × m matrices over a finite field with given rank and h-trace. * 1 special case of Delsarte codes. It is however not clear in general how the duality theories of these two families of codes relate to each other. This is one of the questions that we address in this work.Both linear Delsarte and Gabidulin codes have interesting applications in information theory. Recently it was shown how to employ them for error correction in non-coherent linear network coding and in coherent linear network coding under an adversarial channel model (see e.g. [21] and the references within). Rank-metric codes were also proposed to secure a network coding communication system against an eavesdropper in a universal way (see [22] for details).Motivated by these applications, in this paper we study the duality theories of linear Delsarte and Gabidulin codes, mainly focusing on their MacWilliams identities. In coding theory, a MacWilliams identity establishes a relation between metric properties of a code and metric properties of the dual code. MacWilliams identities exist for several types of codes and metrics. As Gluesing-Luerssen observed in [12], association schemes provide the most general approach to MacWilliams identities, and apply to both linear and non-linear codes (see [4], [3] and [7]). On the other hand, the machinery of association schemes and of the related Bose-Mesner algebras is a very elaborated mathematical tool. Several authors proved independently the MacWilliams identities for the various types of codes in less sophisticated ways.A different viewpoint on MacWilliams identities for general additive codes was recently proposed by Gluesing-Luerssen in [12]. The approach is based on character theory and partitions of groups. See also [14] for a character-theoretic approach to MacWilliams identities for the rank and the Hamming metric.Both the theory of association sch...

We introduce the class of partition-balanced families of codes, and show how to exploit their combinatorial invariants to obtain upper and lower bounds on the number of codes that have a prescribed property. In particular, we derive precise asymptotic estimates on the density functions of several classes of codes that are extremal with respect to minimum distance, covering radius, and maximality. The techniques developed in this paper apply to various distance functions, including the Hamming and the rank metric distances. Applications of our results show that, unlike the Fqm -linear MRD codes, the Fq-linear MRD codes are not dense in the family of codes of the same dimension. More precisely, we show that the density of Fq-linear MRD codes in F n×m q in the set of all matrix codes of the same dimension is asymptotically at most 1/2, both as q → +∞ and as m → +∞. We also prove that MDS and Fqm -linear MRD codes are dense in the family of maximal codes. Although there does not exist a direct analogue of the redundancy bound for the covering radius of Fq-linear rank metric codes, we show that a similar bound is satisfied by a uniformly random matrix code with high probability. In particular, we prove that codes meeting this bound are dense. Finally, we compute the average weight distribution of linear codes in the rank metric, and other parameters that generalize the total weight of a linear code.2010 Mathematics Subject Classification. 05A16, 11T71.

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This paper contributes to the study of rank-metric codes from an algebraic and combinatorial point of view. We introduce q-polymatroids, the q-analogue of polymatroids, and develop their basic properties. We associate a pair of q-polymatroids to a rank-metric codes and show that several invariants and structural properties of the code, such as generalized weights, the property of being MRD or an optimal anticode, and duality, are captured by the associated combinatorial object.We start by establishing the notation and the definitions used throughout the paper.Notation 1.1. In the sequel, we fix integers n, m ≥ 2 and a prime power q. For an integer t, we let [t] := {1, ..., t}. We denote by F q the finite field with q elements. The space of n × m matrices with entries in F q is denoted by Mat. Up to transposition, we assume without loss of generality that n ≤ m. We let Mat(J, c) = {M ∈ Mat | colsp(M ) ⊆ J} and Mat(J, r) = {M ∈ Mat | rowsp(M ) ⊆ J}.Throughout the paper, we only consider linear codes. All dimensions are computed over F q , unless otherwise stated.