We give formulas, in terms of graph theoretical invariants, for the minimum distance and the generalized Hamming weights of the linear code generated by the rows of the incidence matrix of a signed graph over a finite field, and for those of its dual code. Then we determine the regularity of the ideals of circuits and cocircuits of a signed graph, and prove an algebraic formula in terms of the multiplicity for the frustration index of an unbalanced signed graph.
IntroductionThe generalized Hamming weights (GHWs) of a linear code are parameters of interest in many applications [12,16,20,27,31,37,42,43,45] and they have been nicely related to the graded Betti numbers of the ideal of cocircuits of the matroid of a linear code [19,20], to the nullity function of the dual matroid of a linear code [42], and to the enumerative combinatorics of linear codes [3,18,22,23]. Because of this, their study has attracted considerable attention, but determining them is in general a difficult problem. The notion of generalized Hamming weight was introduced by Helleseth, Kløve and Mykkeltveit in [17] and was first used systematically by Wei in [42]. For convenience we recall this notion. Let K = F q be a finite field and let C be an [m, k]-linear code of length m and dimension k, that is, C is a linear subspace of K m with k = dim K (C). Let 1 ≤ r ≤ k be an integer. Given a linear subspace D of C, the support of D, denoted χ(D), is the set of nonzero positions of D, that is, χ(D) := {i : ∃ (a 1 , . . . , a m ) ∈ D, a i = 0}.The r-th generalized Hamming weight of C, denoted δ r (C), is given by δ r (C) := min{|χ(D)| : D is a subspace of C, dim K (D) = r}.As usual we call the set {δ 1 (C), . . . , δ k (C)} the weight hierarchy of the linear code C. The 1st Hamming weight of C is the minimum distance δ(C) of C, that is, one haswhere ω(x) is the Hamming weight of the vector x, i.e., the number of non-zero entries of x. To determine the minimum distance is essential to find good error-correcting codes [23].The notion of generalized Hamming weights for linear codes was extended to matroids by Britz, Johnsen, Mayhew and Shiromoto [6, p. 332] as we now explain.Let M be a matroid with ground set E, rank function ρ, nullity function η, and let M * be its dual matroid. The r-th generalized Hamming weight of M , denoted d r (M ), is given by d r (M ) := min{|X| : X ⊆ E and η(X) = r} for 1 ≤ r ≤ η(E).2010 Mathematics Subject Classification. Primary 94B05; Secondary 94C15, 05C40, 05C22; 13P25.