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We investigate fat projective linear codes over ${\mathbb Z}_{p^{m}}$ , $m\geqslant 2$ , with two nonzero homogeneous weights (“two-weight codes”), building on the graph theory approach developed by Delsarte for codes over fields. Our main result is the classification of such codes under the additional assumption that the columns of a generator matrix of the code determine a cap in the projective Hjelmslev geometry $\mathop {\mathrm {PHG}}\nolimits (k-1, {\mathbb Z}_{p^{m}})$. This generalizes a result on projective two weight codes with dual distance at least four (Calderbank, 1982). The proof relies on a careful analysis of a certain strongly regular graph built on the cosets of the dual code, and on an interpretation of its parameters in terms of projective Hjelmslev geometry. [ABSTRACT FROM AUTHOR]

We study the codes of the title by the CRT method, that decomposes such codes into constituent codes, which are shorter codes over larger alphabets. Criteria on these constituent codes for self-duality and linear complementary duality of the decomposed codes are derived. The special class of the one-generator codes is given a polynomial representation and exactly enumerated. In particular, we present some illustrative examples of binary optimal linear codes with respect to the Griesmer bound derived from the $\mathbb {Z}_{2} \mathbb {Z}_{4}$ -additive quasi-cyclic codes. [ABSTRACT FROM AUTHOR]

We investigate the largest number of nonzero weights of quasi-cyclic codes. In particular, we focus on the function $\Gamma _{Q}(n,\ell,k,q)$ , that is defined to be the largest number of nonzero weights a quasi-cyclic code of index $\gcd (\ell,n)$ , length $n$ and dimension $k$ over $\mathbb F_{q}$ can have, and connect it to similar functions related to linear and cyclic codes. We provide several upper and lower bounds on this function, using different techniques and studying its asymptotic behavior. Moreover, we determine the smallest index for which a $q$ -ary Reed-Muller code is quasi-cyclic, a result of independent interest. [ABSTRACT FROM AUTHOR]

The dual of the Kasami code of length $q^{2}-1$ , with $q$ a power of 2, is constructed by concatenating a cyclic MDS code of length $q+1$ over $F_{q}$ with a Simplex code of length $q-1$. This yields a new derivation of the weight distribution of the Kasami code, a new description of its coset graph, and a new proof that the Kasami code is completely regular. The automorphism groups of the Kasami code and the related $q$ -ary MDS code are determined. New cyclic completely regular codes over finite fields a power of 2, generalized Kasami codes, are constructed; they have coset graphs isomorphic to that of the Kasami codes. Another wide class of completely regular codes, including additive codes, as well as unrestricted codes, is obtained by combining cosets of the Kasami or generalized Kasami code. [ABSTRACT FROM AUTHOR]

Upper and lower bounds on the largest number of weights in a cyclic code of given length, dimension and alphabet are given. An application to irreducible cyclic codes is considered. Sharper upper bounds are given for the special cyclic codes (called here strongly cyclic), whose nonzero codewords have period equal to the length of the code. Asymptotics are derived on the function $\Gamma (k,q)$ , that is defined as the largest number of nonzero weights a cyclic code of dimension $k$ over $\mathbb {F}_{q}$ can have, and an algorithm to compute it is sketched. The nonzero weights in some infinite families of Reed-Muller codes, either binary or $q$ -ary, as well as in the $q$ -ary Hamming code are determined, two difficult results of independent interest. [ABSTRACT FROM AUTHOR]

Sidel’nikov proved in 1971 that the weight distribution of long binary codes is asymptotically Gaussian. Delsarte sketched in 1975 an extension of this result to $Q$ -ary codes when $Q > 2$. In this note, we complete Delsarte’s proof. [ABSTRACT FROM AUTHOR]

Quasi-twisted codes of fixed index >1 have been shown recently to be asymptotically good (A. Alahmadi, C. Güneri, H. Shoaib, P. Solé, 2017). We use this result to construct asymptotically good additive constacyclic codes on any extension of a fixed degree of the base field. A similar result is proved for additive cyclic codes under Artin conjecture on primitive roots. [ABSTRACT FROM AUTHOR]

We study linear complementary pairs (LCP) of codes $(C, D)$ , where both codes belong to the same algebraic code family. We especially investigate constacyclic and quasi-cyclic LCP of codes. We obtain characterizations for LCP of constacyclic codes and LCP of quasi-cyclic codes. Our result for the constacyclic complementary pairs extends the characterization of linear complementary dual (LCD) cyclic codes given by Yang and Massey. We observe that when $C$ and $D$ are complementary and constacyclic, the codes $C$ and $D^\bot $ are equivalent to each other. Hence, the security parameter $\min (d(C),d(D^\bot))$ for LCP of codes is simply determined by one of the codes in this case. The same holds for a special class of quasi-cyclic codes, namely 2D cyclic codes, but not in general for all quasi-cyclic codes, since we have examples of LCP of double circulant codes not satisfying this conclusion for the security parameter. We present examples of binary LCP of quasi-cyclic codes and obtain several codes with better parameters than known binary LCD codes. Finally, a linear programming bound is obtained for binary LCP of codes and a table of values from this bound is presented in the case $d(C)=d(D^\bot)$. This extends the linear programming bound for LCD codes. [ABSTRACT FROM AUTHOR]

We construct two new infinite families of trace codes of dimension 2m , over the ring \mathbb {F}_{p}+u\mathbb {F}_{p} , with u^{2}=u , when p is an odd prime. They have the algebraic structure of abelian codes. Their Lee weight distribution is computed by using Gauss sums. By Gray mapping, we obtain two infinite families of linear p -ary codes of respective lengths and 2(p^m-1)^2 . When m is singly even, the first family gives five-weight codes. When m is odd and p\equiv 3 \pmod {4} , the first family yields p$ -ary two-weight codes, which are shown to be optimal by application of the Griesmer bound. The second family consists of two-weight codes that are shown to be optimal, by the Griesmer bound, whenever $p=3$ and $m \ge 3$ , or $p\ge 5$ and $m\ge 4$ . Applications to secret sharing schemes are given. [ABSTRACT FROM PUBLISHER]

We construct an infinite family of two-Lee-weight codes over the ring \mathbb F2+u\mathbb F2 . These codes are defined as trace codes. They have the algebraic structure of abelian codes. Their Lee weight distribution is computed by using character sums. By Gray mapping, we obtain an infinite family of abelian binary two-weight codes. They are shown to be optimal by application of the Griesmer bound. An application to secret sharing schemes is given. [ABSTRACT FROM PUBLISHER]

We introduce complementary information set codes of higher order. A binary linear code of length \(tk\) and dimension \(k\) is called a complementary information set code of order \(t\) ( \(t\) -CIS code for short) if it has \(t\) pairwise disjoint information sets. The duals of such codes permit to reduce the cost of masking cryptographic algorithms against side-channel attacks. As in the case of codes for error correction, given the length and the dimension of a \(t\) -CIS code, we look for the highest possible minimum distance. In this paper, this new class of codes is investigated. The existence of good long CIS codes of order 3 is derived by a counting argument. General constructions based on cyclic and quasi-cyclic codes and on the building up construction are given. A formula similar to a mass formula is given. A classification of 3-CIS codes of length \(\le 12\) is given. Nonlinear codes better than linear codes are derived by taking binary images of \( {\mathbb Z}_{4}\) -codes. A general algorithm based on Edmonds’ basis packing algorithm from matroid theory is developed with the following property: given a binary linear code of rate \(1/t\) , it either provides \(t\) disjoint information sets or proves that the code is not \(t\) -CIS. Using this algorithm, all optimal or best known \([tk, k]\) codes, where \(t=3, 4, {\dots }, 256\) and \(1 \le k \le \lfloor 256/t \rfloor \) are shown to be \(t\) -CIS for all such \(k\) and \(t\) , except for \(t=3\) with \(k=44\) and \(t=4\) with \(k=37\) . [ABSTRACT FROM AUTHOR]

Hermitian self-dual abelian codes in a group ring \BBFq^{2}[G], where \BBFq^{2} is a finite field of order q^2 and G is a finite abelian group, are studied. Using the well-known discrete Fourier transform decomposition for a semisimple group ring, a characterization of Hermitian self-dual abelian codes in \BBFq^{2}[G] is given, together with an alternative proof of necessary and sufficient conditions for the existence of such a code in \BBFq^{2}[G], i.e., there exists a Hermitian self-dual abelian code in \BBFq^{2}[G] if and only if the order of G is even and q=2^l for some positive integer l. Later on, the study is further restricted to the case where \BBF2^{2l}[G] is a principal ideal group ring, or equivalently, G\cong A\oplus\BBZ2^{k} with 2\nmid\vert A\vert. Based on the characterization obtained, the number of Hermitian self-dual abelian codes in \BBF{2^{2l}}[A\oplus\BBZ2^{k}] can be determined easily. When A is cyclic, this result answers an open problem of Jia concerning Hermitian self-dual cyclic codes. In many cases, \BBF{2^{2l}}[A\oplus\BBZ2^{k}] contains a unique Hermitian self-dual abelian code. The criteria for such cases are determined in terms of l and the order of A. Finally, the distribution of finite abelian groups A such that a unique Hermitian self-dual abelian code exists in \BBF{2^{2l}}[A\oplus\BBZ2] is established, together with the distribution of odd integers m such that a unique Hermitian self-dual cyclic code of length 2 m over \BBF{2^{2l}} exists. [ABSTRACT FROM AUTHOR]