Construction of self-dual binary [2²ⁿ,2²ⁿ⁻¹,2ⁿ]-codes

The binary Reed-Muller code RM(m−n,m) corresponds to the n-th power of the radical of GF(2)[G], where G is an elementary abelian group of order 2m. Self-dual RM-codes (i.e. some powers of the radical of the previously mentioned group algebra) exist only for odd m. The group algebra approach enables...

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Date:2016
Main Authors: Hannusch, C., Lakatos, P.
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Cite this:Construction of self-dual binary [2²ⁿ,2²ⁿ⁻¹,2ⁿ]-codes / C. Hannusch, P. Lakatos // Algebra and Discrete Mathematics. — 2016. — Vol. 21, № 1. — С. 59-68. — Бібліогр.: 15 назв. — англ.

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spelling irk-123456789-1552032019-06-17T01:26:53Z Construction of self-dual binary [2²ⁿ,2²ⁿ⁻¹,2ⁿ]-codes Hannusch, C. Lakatos, P. The binary Reed-Muller code RM(m−n,m) corresponds to the n-th power of the radical of GF(2)[G], where G is an elementary abelian group of order 2m. Self-dual RM-codes (i.e. some powers of the radical of the previously mentioned group algebra) exist only for odd m. The group algebra approach enables us to find a self-dual code for even m=2n in the radical of the previously mentioned group algebra with similarly good parameters as the self-dual RM codes. 2016 Article Construction of self-dual binary [2²ⁿ,2²ⁿ⁻¹,2ⁿ]-codes / C. Hannusch, P. Lakatos // Algebra and Discrete Mathematics. — 2016. — Vol. 21, № 1. — С. 59-68. — Бібліогр.: 15 назв. — англ. 1726-3255 2010 MSC:94B05, 11T71, 20C05. http://dspace.nbuv.gov.ua/handle/123456789/155203 en Algebra and Discrete Mathematics Інститут прикладної математики і механіки НАН України
institution Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection DSpace DC
language English
description The binary Reed-Muller code RM(m−n,m) corresponds to the n-th power of the radical of GF(2)[G], where G is an elementary abelian group of order 2m. Self-dual RM-codes (i.e. some powers of the radical of the previously mentioned group algebra) exist only for odd m. The group algebra approach enables us to find a self-dual code for even m=2n in the radical of the previously mentioned group algebra with similarly good parameters as the self-dual RM codes.
format Article
author Hannusch, C.
Lakatos, P.
spellingShingle Hannusch, C.
Lakatos, P.
Construction of self-dual binary [2²ⁿ,2²ⁿ⁻¹,2ⁿ]-codes
Algebra and Discrete Mathematics
author_facet Hannusch, C.
Lakatos, P.
author_sort Hannusch, C.
title Construction of self-dual binary [2²ⁿ,2²ⁿ⁻¹,2ⁿ]-codes
title_short Construction of self-dual binary [2²ⁿ,2²ⁿ⁻¹,2ⁿ]-codes
title_full Construction of self-dual binary [2²ⁿ,2²ⁿ⁻¹,2ⁿ]-codes
title_fullStr Construction of self-dual binary [2²ⁿ,2²ⁿ⁻¹,2ⁿ]-codes
title_full_unstemmed Construction of self-dual binary [2²ⁿ,2²ⁿ⁻¹,2ⁿ]-codes
title_sort construction of self-dual binary [2²ⁿ,2²ⁿ⁻¹,2ⁿ]-codes
publisher Інститут прикладної математики і механіки НАН України
publishDate 2016
url http://dspace.nbuv.gov.ua/handle/123456789/155203
citation_txt Construction of self-dual binary [2²ⁿ,2²ⁿ⁻¹,2ⁿ]-codes / C. Hannusch, P. Lakatos // Algebra and Discrete Mathematics. — 2016. — Vol. 21, № 1. — С. 59-68. — Бібліогр.: 15 назв. — англ.
series Algebra and Discrete Mathematics
work_keys_str_mv AT hannuschc constructionofselfdualbinary22n22n12ncodes
AT lakatosp constructionofselfdualbinary22n22n12ncodes
first_indexed 2025-07-14T07:16:38Z
last_indexed 2025-07-14T07:16:38Z
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fulltext Algebra and Discrete Mathematics RESEARCH ARTICLE Volume 21 (2016). Number 1, pp. 59–68 © Journal “Algebra and Discrete Mathematics” Construction of self-dual binary [22k, 22k−1, 2k]-codes Carolin Hannusch∗ and Piroska Lakatos Communicated by V. I. Sushchansky Abstract. The binary Reed-Muller code RM(m − k, m) corresponds to the k-th power of the radical of GF (2)[G], where G is an elementary abelian group of order 2m (see [2]). Self-dual RM- codes (i.e. some powers of the radical of the previously mentioned group algebra) exist only for odd m. The group algebra approach enables us to find a self-dual code for even m = 2k in the radical of the previously mentioned group algebra with similarly good parameters as the self-dual RM codes. In the group algebra GF (2)[G] ∼= GF (2)[x1, x2, . . . , xm]/(x2 1 − 1, x2 2 − 1, . . . x2 m − 1) we construct self-dual binary C = [22k, 22k−1, 2k] codes with prop- erty RM(k − 1, 2k) ⊂ C ⊂ RM(k, 2k) for an arbitrary integer k. In some cases these codes can be obtained as the direct product of two copies of RM(k −1, k)-codes. For k > 2 the codes constructed are doubly even and for k = 2 we get two non-isomorphic [16, 8, 4]- codes. If k > 2 we have some self-dual codes with good parameters which have not been described yet. ∗Research of the first author was partially supported by funding of EU’s FP7/2007- 2013 grant No. 318202. 2010 MSC: 94B05, 11T71, 20C05. Key words and phrases: Reed–Muller code, Generalized Reed–Muller code, radical, self-dual code, group algebra, Jacobson radical. 60 Construction of self-dual binary [22k, 22k−1, 2k]-codes Introduction and Notation Let K be a finite field of characteristic p and let V be a vector space over K, and C be a subspace of V. Then C is called a linear code. Let x, y ∈ C, then the Hamming weight of x is the number of its non-zero coordinates and the Hamming distance of x and y is the weight of x − y. The Hamming distance (or weight) of a linear code C is the minimum of all Hamming distances of its codewords. In the study of binary codes C ⊆ V it is convenient that the space V has an additional algebraic structure. For example, if V is a group algebra K[G], where G is a finite abelian p-group and C is an ideal of such a group algebra, then C is called an abelian group code. The Hamming distance of a linear code determines the ability of error-correcting property of the code. The authors in [6] proved that for any 1 6 d 6 [ m+1 2 ] there exists an Abelian 2-group Gd that a power of the radical is a self-dual code with parameters (2m, 2m−1, 2d). These codes are ideals in the group algebra GF (2)[Gd] and they are “monomial codes” in the sense of [5] as defined below. Throughout, p will denote a prime and K a field of p elements. Let G = 〈g1〉 × · · · × 〈gm〉 ∼= Cm p be an elementary abelian p-group of order pm i.e. K[G] is a modular group algebra, then the group algebra K[G] and Kn are isomorphic as vector spaces by the mapping ϕ : K[G] 7→ Kn, where ϕ ( n ∑ i=1 aigi ) 7→ (a1, a2, . . . , an) := c ∈ C. Reed-Muller (RM) binary codes were introduced in [12] as binary functions. These codes are frequently used in applications and have good error correcting properties. Now we are looking for self-dual codes in the radical of K[G] with similarly good parameters as the RM codes. If K is a field of characteristic 2 Berman [2] and in the general case Charpin [3] proved that all Generalized Reed-Muller (GRM) codes coincide with powers of the radical of the modular group algebra of K[G], where G is an elementary abelian p-group. This group algebra is clearly isomorphic with the quotient algebra GF (p)[x1, x2, . . . xm]/(xp 1 − 1, . . . xp m − 1). Self-dual RM-codes (i.e. some power of the radical of the group algebra GF (2)[G]) exist only for odd m. They are (2m, 2m−1, 2 m+1 2 )-codes. C. Hannusch, P. Lakatos 61 For any basis {g1, g2, . . . gm} of such a group G consider the algebra isomorphism µ mapping gj 7→ xj (1 6 j 6 m), and therefore we have the algebra isomorphism Ap,m ∼= GF (p)[x1, x2, . . . , xm]/(xp 1 − 1, xp 2 − 1, . . . xp m − 1), where GF (p)[x1, x2, . . . , xm] denotes the algebra of polynomials in m variables with coefficients in GF (p). It is known ([7]) that the set of monomial functions (ki ∈ N ∪ 0) { m ∏ i=1 (xi − 1)ki where 0 6 ki < p } form a linear basis of the radical Jp,m. Clearly the nilpotency index of Jp,m (i.e. the smallest positive integer t, such that J t p,m = 0) is equal to t = m(p − 1) + 1. Introducing the notation Xi = xi − 1, (1 6 i 6 m) (which will be used from now on) we have the following isomorphism Jp,m ≃ GF (p)[X1, X2, . . . , Xm]/(Xp 1 , Xp 2 , . . . Xp m). (1) The k-th power of the radical consists of reduced m-variable (non- constant) polynomials of degree at least k, where 0 6 k 6 t − 1, where t = m(p − 1) + 1. J k p,m = GRM(t − 1 − k, m) = 〈 m ∏ i=1 (Xi) ki | m ∑ i=1 ki > k (0 6 ki < p)〉. (2) Such a basis was exploited by Jennings [7]. By (2) the quotient space J k p,m/J k+1 p,m has a basis { m ∏ i=1 Xki i + J k+1 p,m , where 0 6 ki < p and m ∑ i=1 ki = k } . (3) Remark 1. It is known [15] that the dual code C⊥ of an ideal C in Ap,m coincides with the annihilator of C∗, where C∗ is the image of C by the involution ∗ defined on Ap,m by ∗ : g 7→ g−1 for all g ∈ G from Ap,m to itself. The annihilator of J k p,m is obviously J m(p−1)+1−k p,m . Thus the dual codes of GRM-codes are GRM-codes and GRM(k, m)⊥ = GRM(m(p − 1) − k − 1, m). It follows that for m = 2k + 1 and p = 2 the code GRM(k, m) is self-dual. 62 Construction of self-dual binary [22k, 22k−1, 2k]-codes 1. Construction of binary self-dual codes Let us consider the group algebra A2,m = GF (2)[x1, . . . xm]/(x2 1 − 1, x2 2 − 1, . . . x2 m − 1) ≃ GF (2)[Cm 2 ] as a vector space with basis xa1 1 xa2 2 . . . xam m , ai ∈ {0, 1}. (4) It is known ([7]) that the radical J2,m of this group algebra is generated by the monomials Xi = xi − 1 = xi + 1. Definition 1 ([5]). The code C in J2,m (see (1)) is said to be a monomial code if it is an ideal in A2,m generated by some monomials of the form Xk1 1 Xk2 2 . . . Xkm m , where 0 6 ki 6 1 (5) The codes we intend to study are monomial codes. For p = 2 using the usual polynomial product in the Boolean monomial Xk1 1 Xk2 2 . . . Xkm m (ki ∈ {0, 1}) we have Xk1 1 Xk2 2 . . . Xkm m = (x1 + 1)k1(x2 + 1)k2 . . . (xm + 1)km and the Hamming weight in the basis (4) of this monomial equals m ∏ i=1 (1+ki). Example. Let G be an elementary abelian group of order 2m, m > 2. Define the codes Cj as ideals in K[G] generated by Xj = xj − 1. These codes are binary self-dual [2m, 2m−1, 2] codes and they are self-dual since Cj = C⊥ j = 〈Xj〉. Further, this code is a direct sum of [2, 1, 2]-codes. The dimension of the code Cj is 2m−1, the same as the dimension of the radical of the group algebra GF (2)[H], where H is an elementary abelian 2-group of rank m − 1. The minimal distance of Cj is d = 2. This follows from the fact that the element Xj = xj + 1 is included in the basis of Cj . Thus, Cj is a self-dual [2m, 2m−1, 2]-code. By Remark 1 one can see that a power of the radical of a modular group algebra is self-dual if and only if the nilpotency index of the radical is even. In our case (when G is elementary abelian of order pm) the nilpotency index is even if and only if p = 2 and m is odd. If m is odd, the binary RM-codes with parameters [2m, 2m−1, 2 m+1 2 ] are self-dual and they are the m+1 2 -th powers of the radical A2,m. C. Hannusch, P. Lakatos 63 For m = 2k where k is an arbitrary integer, we have a new method to construct a doubly-even class of binary self-dual C codes with parameters [2m, 2m−1, 2k]. For this code C we have RM(k − 1, 2k) ⊂ C ⊂ RM(k, 2k). In the case of m = 4, we get two known extremal [16, 8, 4] codes (listed in [14]) and for m > 4 these codes are not extremal. A doubly-even (i.e. its minimum distance is divisible by 4) self-dual code is called extremal, if we have for its minimum distance d = 4 [ n 24 ] + 4, where n denotes the code length (see Definition 39 and Lemma 40 in [8]). To abbreviate the description of our codes, we shall refer to the mono- mial Xk1 1 . . . Xkm m as the m-tuple (k1, k2, . . . , km) ∈ {0, 1, . . . , p − 1}m of exponents. Using Plotkin’s construction of RM-codes (see Theorem 2 [13], Ch. 13, § 3) we obtain the following property of RM-codes. Lemma 1. If m is even and m = 2k, then RM(k−1, m) = J k+1 2,m contains a proper subspace which is isomorphic to RM(k − 1, m − 1). Proof. Recall, that the set of monomials in the basis (2) of J k+1 2,m is invariant under the permutations of the variables Xi, i.e. the set of binary m-tuples (k1, k2, . . . , km) assigned to the basis (2) is invariant under the permutation of all elements of the symmetric group Sm. Take the basis elements with km = 1. Then the monomials Xk1 1 . . . Xkm m of degree m can be projected by π : (k1, k2, . . . , km−1, 1) 7→ (k1, k2, . . . , km−1). In this way we get a basis of J k 2,m−1 ∼= RM(k − 1, m − 1). For m = 2k denote the set of all k-subsets of {1, 2, . . . , 2k} by X. The elements of X can be described by binary sequences (k1, k2, . . . , km) consisting of k ‘0‘-s and k ‘1‘-s in any order. Clearly, the cardinality of the set X is (2k k ) . We say that the subset Y of binary m-tuples in X is complement free if y ∈ Y implies 1 − y /∈ Y, where 1 = (1, 1, . . . , 1). Denote the set of monomials corresponding to the set of exponents in X by X . Denote the set with maximum number of pairwise orthogonal monomials in X by Y and their corresponding exponents in X by Y. Example. For m = 6 the quotient space J 3 2,m/J 4 2,m has a basis with (6 3 ) = 20 elements, where the binary 6-tuples corresponding to the coset 64 Construction of self-dual binary [22k, 22k−1, 2k]-codes representative monomials (the set X) are listed in pairs of complements: (1, 1, 1, 0, 0, 0) (0, 0, 0, 1, 1, 1) (1, 1, 0, 1, 0, 0) (0, 0, 1, 0, 1, 1) (1, 1, 0, 0, 1, 0) (0, 0, 1, 1, 0, 1) (1, 1, 0, 0, 0, 1) (0, 0, 1, 1, 1, 0) (1, 0, 1, 1, 0, 0) (0, 1, 0, 0, 1, 1) (1, 0, 1, 0, 1, 0) (0, 1, 0, 1, 0, 1) (1, 0, 1, 0, 0, 1) (0, 1, 0, 1, 1, 0) (1, 0, 0, 1, 1, 0) (0, 1, 1, 0, 0, 1) (1, 0, 0, 1, 0, 1) (0, 1, 1, 0, 1, 0) (1, 0, 0, 0, 1, 1) (0, 1, 1, 1, 0, 0) and we have 2 1 2 (6 3) = 210 complement-free sets. For example the following complement free sets Y and Y of 10 elements: Y Y (1, 1, 1, 0, 0, 0), X1X2X3 (0, 0, 1, 0, 1, 1), X3X5X6 (1, 1, 0, 0, 1, 0), X1X2X5 (0, 0, 1, 1, 1, 0), X3X4X5 (1, 0, 1, 1, 0, 0), X1X3X4 (0, 1, 0, 1, 0, 1), X2X4X6 (0, 1, 0, 1, 1, 0), X2X4X5 (0, 1, 1, 0, 0, 1), X2X3X6 (1, 0, 0, 1, 0, 1), X1X4X6 (1, 0, 0, 0, 1, 1), X1X5X6 Theorem 1. Let C be a binary code with RM(k−1, 2k) ⊂ C ⊂ RM(k, 2k) with the following basis of the factorspace C/RM(k − 1, 2k) { m ∏ i=1 Xki i + RM(k − 1, 2k), where ki ∈ {0, 1} and m ∑ i=1 ki = k } , (6) where the set of the exponents (k1, k2, . . . , km) is a maximal (with cardinal- ity 2 1 2 (2k k ) ) complement free subset of X. Then C forms a [22k, 22k−1, 2k] self-dual doubly-even code. Proof. Let G be an elementary abelian group of order 2m, where m = 2k, k > 2. By the group algebra representation of RM-codes and the definition of C we have the relation J k+1 2,m ⊂ C ⊂ J k 2,m. C. Hannusch, P. Lakatos 65 For m = 2k the set X is the set of coset representatives of the quotient space J k 2,m/J k+1 2,m , i.e. the set of monomials satisfying (6). Clearly, two monomials Xk1 1 Xk2 2 . . . Xkm m and X l1 1 X l2 2 . . . X lm m are or- thogonal, i.e. their product is zero, if for some i : 1 6 i 6 m we have ki = li. Thus, the elements in the radical corresponding to these monomials are orthogonal if their exponent m-tuples belong to a complement free set. The m-tuples (k1, k2 . . . km) have to be complement free in Y, otherwise the corresponding monomials in Y are not orthogonal. Clearly Y is a complement free subset of X (given by (4)) with cardinality 1 2 (2k k ) = (2k−1 k−1 ) . By definition, C = 〈 J k+1 2,m ⋃ Y 〉 is a subspace of the radical J2,m of the group algebra A2,m generated by the union of J k+1 2,m and Y. For the dimension of C we have dim(C) = dim(RM(k−1, m))+ 1 2 ( 2k k ) = 1+ k−1 ∑ i=1 ( 2k i ) + 1 2 ( 2k k ) = 22k−1. It follows that C is self-dual. Since a binary self-dual code contains a word of weight 2 if and only if the generator matrix has two equal columns, we have our self-dual code to be doubly-even. Each monomial in Y has the same weight 2k, that is the minimal distance of C. Using the identities for the monomials involved in the basis of our codes xi(xj + 1) = (xi + 1)(xj + 1) + (xj + 1) and (xi + 1)2 = 0, we easily obtain that C (which is subspace of J2,m) is an ideal in the group algebra GF (2)[G]. Theorem 2. Let Y and Y be sets defined above and let C be the code defined in Theorem 1. Suppose that ki = 0 for some i : 1 6 i 6 m in each element of the subset Y, (i.e. the variable Xi is missing in each monomial of Y). Then we have the isomorphism C ≃ RM(k − 1, 2k − 1) ⊕ RM(k − 1, 2k − 1). Proof. The elements of Y are of the form Xk1 1 . . . Xkm m = (x1 + 1)k1(x2 + 1)k2 . . . (xm + 1)km , where m ∑ i=1 ki = k 66 Construction of self-dual binary [22k, 22k−1, 2k]-codes and their weight is 2k. Project the set of monomials with ki = 0 in C = 〈 J k+1 2,m ⋃ Y 〉 onto the monomials Xk1 1 , . . . , X ki−1 i−1 , X ki+1 i+1 , . . . , Xkm m . The image C1 of this projection is a self-dual RM(k − 1, 2k − 1)-code with parameters [22k−1, 22k−2, 2k]. By Lemma 1 the elements of the basis of Jk+1 2,m with ki = 1 generate a subspace C2 which is isomorphic to RM(k − 1, 2k − 1). The intersection of C1 and C2 is empty. Therefore C ≃ C1⊕C2 and the statement follows. Remark 2. In particular, by Theorem 1 we get [16, 8, 4] self-dual codes for m = 4. These codes are extremal doubly-even codes. Using the SAGE computer algebra software we may check easily the classification of binary self-dual codes listed in [14]. There are two cases: 1) If ki = 0 for some i : 1 6 i 6 m in each element of the set Y , then we get the direct sum E8 ⊕ E8, where E8 is the extended Hamming code. 2) otherwise we get an indecomposable [16, 8, 4] code (which is denoted by E16 in [14]). These codes are formally self-dual. Both classes have the following weight function: z16 + 28z12 + 198z8 + 28z4 + 1 Remark 3. It is known that for each odd m > 1 there exists a self-dual affine-invariant code of length 2m over GF (2), which is not a self-dual RM-code [4]. The factor space J k p,m/J k+1 p,m is an irreducible AGL(m, GF (p)) module. Thus the code C is not affine invariant (see [1] Theorem 4.17 ) as the powers of the radical of Ap,m are. The code C cannot be an extended cyclic code by Corollary 1 in [4]. Remark 4. Using the inclusion-exclusion principle a formula can be given for the dimension of the RM(k + 1, m)-code (see for example in [1] Theorem 5.5). If p = 2 and 0 6 k 6 m, then we have dim C = 1 2 (2k k ) + m ∑ i=k+1 2k ∑ j=0 (−1)j (2k j )(2k−2j+i−1 i−2j ) = m ∑ i=k+1 (2k i ) + 1 2 (2k k ) , where i − 2j > 0. The codes constructed in the current paper are worth to be studied further. Already for k = 2 we get two non-isomorphic codes with the same parameters. It would be interesting to determine all classes of codes C. Hannusch, P. Lakatos 67 up to isomorphism for each arbitrary integer k and to determine their automorphism group. The code C in Theorem 1 is not affine-invariant and first computations show that the automorphism group of C with ki = 0 differs from the automorphism group of C with ki = 1 for some 1 6 i 6 m. We can formulate the following open questions about the code C of Theorem 1: 1) Does there exist a classification for all complement-free sets for arbitrary even m? 2) How many non-equivalent (in any sense) self-dual binary codes exist for fixed m and p? 3) Compare the automorphism groups of the codes C defined in Theo- rem 1 with the automorphism group of RM-codes. 4) Find decoding algorithms for C. References [1] Assmus, E.F. Key, J.K., Polynomial codes and finite geometries, Chapter in Hand- book of Coding Theory, edited by V. Pless and W. C. Huffman, Elsevier, 1995. [2] Berman, S.D., On the theory of group code, Kibernetika, 3(1) (1967), 31–39. [3] Charpin, P., Codes cycliques étendus et idA©aux principaux d’une alge‘bre modu- laire, C.R. Acad. Sci. Paris, 295(1) (1982), 313–315. [4] Charpin, P, Levy-Dit-Vehel, F., On Self-Dual Affine-lnvariant Codes Journal Combiunatorial Theory, Series A 67 (1994), 223–244. [5] Drensky, V., Lakatos, P., Monomial ideals, group algebras and error correcting codes, Lecture Notes in Computer Science, Springer Verlag, 357 (1989), 181–188. [6] Hannusch, C., Lakatos, P., Construction of self-dual radical 2-codes of given distance, Discrete Math., Algorithms and Applications, 4(4) (2012). [7] Jennings, S. A., The structure of the group ring of a p-group over modular fields, Trans. Amer. Math. Soc. 50 (1941), 175–185. [8] Joyner, D., Kim, J.-L., Selected unsolved problems in Coding Theory, Birkha̋user, 2011. [9] Kasami, T. , Lin, S, Peterson, W.W., New generalisations of the Reed-Muller codes, IEEE Trans. Inform. Theory II-14 (1968) 189–199. [10] Kelarev, A. V.; Yearwood, J. L.; Vamplew, P. W., A polynomial ring construction for the classification of data, Bull. Aust. Math. Soc. 79 , 2 (2009) 213–225. [11] Landrock, P., Manz, O., Classical codes as ideals in group algebras, Designs, Codes and Cryptography, 2(3) (1992), 273–285. [12] Muller, D. E., Application of boolean algebra to switching circuit design and to error detection, IRE Transactions on Electronic Computers, 3:6–12 (1954). [13] MacWilliams, F.J., Sloane, N.J.A., The Theory of Error-Correcting Codes, North Holland, Amsterdam, 1983. 68 Construction of self-dual binary [22k, 22k−1, 2k]-codes [14] Pless, V., A classification of self-orthogonal codes over GF(2), Discrete Mathematics 3 (1972), 209–246. [15] MacWilliams, F.J., Codes and Ideals in group algebras, Univ. of North Carolina Press, 1969. Contact information C. Hannusch, P. Lakatos Institute of Mathematics, University of Debrecen, 4010 Debrecen, pf.12, Hungary E-Mail(s): carolin.hannusch@science.unideb.hu, lakatosp@science.unideb.hu Web-page(s): www.mat.unideb.hu Received by the editors: 21.09.2015 and in final form 16.12.2015.