The v-Numbers and Linear Presentations of Ideals of Covers of Graphs

Let G be a graph and let J=Ic(G) J = I c ( G ) be its ideal of covers. The aims of this work are to study the v-number v(J) v ( J ) of J and to study when J is linearly presented using combinatorics and commutative algebra. We classify when v(J) v ( J ) attains its minimum and maximum possible values in terms of the vertex covers of the graph that satisfy the exchange property. If the cover ideal of a graph has a linear presentation, we express its v-number in terms of the covering number of the graph. If G is unmixed, the graph GJ G J of J is the graph whose vertices are the minimal vertex covers of G and whose edges are the pairs ,C'\ { C , C ′ } such that |C∪ C'|=|C|+1 | C ∪ C ′ | = | C | + 1 . We show necessary and sufficient conditions for the graph GJ G J of J to be connected. Then, for unmixed König graphs, we classify when J is linearly presented using graph theory, and show some results on Cohen–Macaulay König graphs. If G is unmixed, it is shown that the columns of the linear syzygy matrix of J are linearly independent if and only if GJ G J has no strong 3-cycles. One of our main theorems shows that if G is unmixed and has no induced 4-cycles, then J is linearly presented. For unmixed graphs without 3- and 5-cycles, we classify combinatorially when J is linearly presented.