We prove that if \(S\) is a quasiminimal generating set of a group \(\Gamma\) and \(F\) is an oriented forest with \(|S| > 2\) arcs, then the Cayley graph \({Cay}(\Gamma, S)\) can be decomposed into \(|\Gamma|\) arc-disjoint subdigraphs, each of which is isomorphic to \(F\).

The quantity \(g_2^{(k)}(v)\) is the minimum number of blocks in a family of blocks from a \(v\)-set that covers all \(\binom{v}{2}\) pairs exactly twice, given the restriction that the longest block in the covering family has length \(k\) (there may be many blocks of length \(k\)). We give certain results for the case \(k = 4\).

A set of edges \(D\) in a graph \(G\) is a dominating set of edges if every edge not in \(D\) is adjacent to at least one edge in \(D\). The minimum cardinality of an edge dominating set of \(G\) is the edge domination number of \(G\), denoted \(D_E(G)\). A graph \(G\) is edge domination critical, or \(EDC\), if for any vertex \(v\) in \(G\) we have \(D_E(G – v) = D_E(G) – 1\). Every graph \(G\) must have an induced subgraph \(F\) such that \(F\) is \(EDC\) and \(D_E(G) = D_E(F)\). In this paper, we prove that no tree with more than 2 vertices is \(EDC\), develop a forbidden subgraph characterization for the edge domination number of a tree, and we develop a construction that conserves the \(EDC\) property.

Let \(V\) be a finite set of order \(v\). A \((v,k,\lambda)\) covering design of index \(\lambda\) and block size \(k\) is a collection of \(k\)-element subsets, called blocks, such that every \(2\)-subset of \(V\) occurs in at least \(\lambda\) blocks. The covering problem is to determine the minimum number of blocks, \(\alpha(v, k, \lambda)\), in a covering design. It is well known that \(\alpha(v, k, \lambda) \geq \left\lceil\frac{v}{k} \lceil \frac{v-1}{k-1}.\lambda \rceil \right\rceil=\phi(v, k, \lambda)\), where \(\lceil x \rceil\) is the smallest integer satisfying \(x\leq\lceil x \rceil\). In this paper, we determine the value \(\alpha(v,5,\lambda)\), with few possible exceptions, for \(\lambda = 3\), \(v \equiv 2 \pmod{4}\) and \(\lambda = 9, 10, v\geq5\), and \(\lambda \geq 11\), \(v \equiv 2 \pmod{4}\).

Let \(G = (V, E)\) be a connected undirected graph. Suppose a fire breaks out at a vertex of \(G\) and spreads to all its unprotected neighbours in each time interval. Also, one vertex can be protected in each time interval. We are interested in the number of vertices that can be “saved”, that is, which will never be burned. An algorithm is presented to find the optimal solution in the 2-dimensional grid graphs and 3-dimensional cubic graphs. We also determined the upper and lower bounds of the maximum number of vertices that can be saved on the large product graphs. The problem of containing the fire with one firefighter or more is also considered.

Let \(C\) be the underlying graph of a configuration of \(l\) blocks in a path design of order \(v\) and block size \(3\), \((V, \mathcal{B})\). We say that \((V, \mathcal{B})\) is \((l,C)\)-ordered if it is possible to order its blocks in such a way that each set of \(l\) consecutive blocks has the same underlying graph \(C\). In this paper, we completely solve the problem of the existence of a \((2,C)\)-ordered path design \(P(v, 3, 1)\) for any configuration having two blocks.

Summary. In this paper, we present some inequalities on balanced arrays \((B-arrays)\) of strength five with two symbols.