This article is a contribution to the study of block-transitive automorphism groups of \(2\)-\((v,k,1)\) block designs. Let \(\mathcal{D}\) be a \(2\)-\((v,k,1)\) design admitting a block-transitive, point-primitive but not flag-transitive group \(G\) of automorphisms. Let \(k_1 = (k, v-1)\) and \(q = p^f\) for prime \(p\). In this paper we prove that if \(G\) and \(D\) are as above and \(q > {(2(k_rk-k_r+1)f)^{\frac{1}{4}}}\) then \(G\) does not admit a Chevalley group \(E_7(q)\) as its socle.

A graph \(G\) is called super edge-magic if there exists a bijection \(f\) from \(V(G) \cup E(G)\) to \(\{1, 2, \ldots, |V(G)| + |E(G)|\}\) such that \(f(u) + f(v) + f(uv) = C\) is a constant for any \(uv \in E(G)\) and \(f(V(G)) = \{1, 2, \ldots, |V(G)|\}\), \(f(E(G)) = \{|V(G)| + 1, |V(G)| + 2, \ldots, |V(G)| + |E(G)|\}\). R. M. Figueroa-Centeno et al. provided the following conjecture: For every integer \(n \geq 5\), the book \(B_n\) is super edge-magic if and only if \(n\) is even or \(n \equiv 5 \pmod 8\). In this paper, we show that \(B_n\) is super edge-magic for even \(n \geq 6\).

It was conjectured in \([10]\) that the upper bound for the strong chromatic index \(s'(G)\) of bipartite graphs is \(\Delta(G)^2+1\), where \(\Delta(G)\) is the largest degree of vertices in \(G\). In this note we study the strong edge coloring of some classes of bipartite graphs that belong to the class of partial cubes. We introduce the concept of \(\Theta\)-graph \(\Theta(G)\) of a partial cube \(G\), and show that \(s'(G) \leq \chi(\Theta(G))\) for every tree-like partial cube \(G\). As an application of this bound we derive that \(s'(G) \leq 2\Delta(G)\) if \(G\) is a \(p\)-expansion graph.

We introduce notions of \(k\)-chromatic uniqueness and \(k\)-chromatic equivalence in the class of all Sperner hypergraphs. They generalize the chromatic uniqueness and equivalence defined in the class of all graphs \([10]\) and hypergraphs \([2, 4, 8]\). Using some known facts, concerning a \(k\)-chromatic polynomial of a hypergraph \([5]\), a set of hypergraphs whose elements are \(3\)-chromatically unique is indicated. A set of hypergraphs characterized by a described \(3\)-chromatic polynomial is also shown. The application of the investigated notions can be found in \([5]\).

A graph-pair of order \(t\) is two non-isomorphic graphs \(G\) and \(H\) on \(t\) non-isolated vertices for which \(G \cup H \cong K_t\) for some integer \(t \geq 4\). Given a graph-pair \((G, H)\), we say \((G, H)\) divides some graph \(K\) if the edges of \(K\) can be partitioned into copies of \(G\) and \(H\) with at least one copy of \(G\) and at least one copy of \(H\). We will refer to this partition as a \((G, H)\)-multidecomposition of \(K\).

Let \(V\) denote the \(n\)-dimensional row vector space over a finite field \(\mathbb{F}_q\), and let \(W\) be a subspace of dimension \(n-d\). Let \(L(n,d) = \mathcal{P} \cup \{0\}\), where \({P} = \{A | A \text{ is a subspace of } V, A + W = V\}\). Partially ordered by ordinary or reverse inclusion, two families of finite atomic lattices are obtained. This article discusses their geometricity, and computes their characteristic polynomials.

A graph \(G\) is called \(H\)-equipackable if every maximal \(H\)-packing in \(G\) is also a maximum \(H\)-packing in \(G\). All \(M_2\)-equipackable graphs and \(P_3\)-equipackable graphs have been characterized. In this paper, \(P_k\)-equipackable paths, \(P_k\)-equipackable cycles, \(M_3\)-equipackable paths and \(M_3\)-equipackable cycles are characterized.

Let \(G\) be a graph with \(r\) vertices of degree at least two. Let \(H\) be any graph. Consider \(r\) copies of \(H\). Then \(G \oplus H\) denotes the graph obtained by merging the chosen vertex of each copy of \(H\) with every vertex of degree at least two of \(G\). Let \(T_0\) and \(T^{A_1}\) be any two caterpillars. Define the first attachment tree \(T_1 = T_0 \oplus T^{A_1}\). For \(i \geq 2\), define recursively the \((i^{th})\) attachment tree \(T_i = T_{i-1} \oplus T^{A_i}\), where \(T_{i-1}\) is the \((i-1)^{th}\) attachment tree. Here one of the penultimate vertices of \(T^{A_1}\), \(i \geq 1\) is chosen for merging with the vertices of degree at least two of \(T_{i-1}\), for \(i \geq 1\). In this paper, we prove that for every \(i \geq 1\), the \(i\)th attachment tree \(T_i\) is graceful and admits a \(\beta\)-valuation. Thus it follows that the famous graceful tree conjecture is true for this infinite class of \((i^{th})\) attachment trees \(T’_is\), for all \(i \geq 1\). Due to the results of Rosa \([21]\) and El-Zanati et al. \([5]\) the complete graphs \(K_{2cm+1}\) and complete bipartite graphs \(K_{qm,pm}\), for \(c,p,m,q \geq 1\) can be decomposed into copies of \(i\)th attachment tree \(T_i\), for all \(i \geq 1\), where \(m\) is the size of such \(i\)th attachment tree \(T_i\).

A packing of \(K_n\) with copies of \(C_4\) (the cycle of length \(4\)), is an ordered triple \((V, \mathcal{C}, L)\), where \(V\) is the vertex set of the complete graph \(K_n\), \(C\) is a collection of edge-disjoint copies of \(C_4\), and \(L\) is the set of edges not belonging to a block of \(\mathcal{C}\). The number \(n\) is called the order of the packing and the set of unused edges \(L\) is called the leave. If \(C\) is as large as possible, then \((V, \mathcal{C}, L)\) is called a maximum packing MPC\((n, 4, 1)\). We say that an handcuffed design \(H(v, k, 1)\) \((W, P)\) is embedded into an MPC\((n, 4, 1)\) \((V, C, L)\) if \(W \subseteq V\) and there is an injective mapping \(f : \mathcal{P} \to \mathcal{C}\) such that \(P\) is a subgraph of \(f(P)\) for every \(P \in \mathcal{P}\). Let \(\mathcal{SH}(n, 4, k)\) denote the set of the integers \(v\) such that there exists an MPC\((n, 4, 1)\) which embeds an \(H(v, k, 1)\). If \(n \equiv 1 \pmod 8\) then an MPC\((n, 4, 1)\) coincides with a \(4\)-cycle system of order \(n\) and \(\mathcal{SH}(n, 4, k)\) is found by Milici and Quattrocchi, Discrete Math., \(174 (1997)\).

The aim of the present paper is to determine \(\mathcal{SH}(n, 4, k)\) for every integer \(n \not\equiv 1 \pmod 8\), \(n \geq 4\).

Given a sequence \(X = (x_1, x_2, \ldots, x_k)\), let \(Y = (y_1, y_2, \ldots, y_k)\) be a sequence obtained by rearranging the terms of \(X\). The total self-variation of \(Y\) relative to \(X\) is \(\zeta_X(Y) = \sum_{i=1}^k |y_i – x_i|\). On the other hand, let \(G = (V, E)\) be a connected graph and \(\phi\) be a permutation of \(V\). The total relative displacement of \(\phi\) is \(\delta_\phi(G) = \sum_{\{x \neq y\}\subset V} |d(x, y) – d(\phi(x), \phi(y))|\), where \(d(v, w)\) means the distance between \(v\) and \(w\) in \(G\). It’s clear that the total relative displacement of \(\phi\) is a total self-variation relative to the distance sequence of the graph.
In this paper, we determine the sequences which attain the maximum value of the total self-variation of all possible rearrangements \(Y\) relative to \(X\). Applying this result to the distance sequence of a graph, we find a best possible upper bound for the total relative displacement of a graph.