The eccentricity of a vertex \(v\) in a connected graph \(G\) is the distance between \(v\) and a vertex farthest from \(v\). For a vertex \(v\), we define the edge-added eccentricity of \(v\) as the minimum eccentricity of \(v\) in all graphs \(G+e\), taken over all edges \(e\) in the complement of \(G\). A graph is said to be edge-added stable (or just stable) if the eccentricity and the edge-added eccentricity are the same for all vertices in the graph. This paper describes properties of edge-added eccentricities and edge-added stable graphs.

In this paper, we find explicit formulas or generating functions for the cardinalities of the sets \(S_n(T,\tau)\) of all permutations in \(S_n\) that avoid a pattern \(\tau \in S_k\) and a set \(T, |T| \geq 2,\) of patterns from \(S_3\). The main body of the paper is divided into three sections corresponding to the cases \(|T| = 2, 3\) and \(|T| \geq 4\). As an example, in the fifth section, we obtain the complete classification of all cardinalities of the sets \(S_n(T,\tau)\) for \(k = 4\).

The concept of weakly associative lattices (i.e. relational systems with a reflexive and antisymmetric relation \(\leq\), in which for each pair of elements there exist a least upper and a greatest lower bound) was introduced in [3] and [5]. In [4] WU-systems are defined, i.e. weakly associative lattices with the unique bound property, and their equivalence with projective planes is described. In this paper we introduce WU\(_{\lambda}\)-systems, and discuss their relation to symmetric \(2\)-\((v,k,\lambda)\) designs equipped with a special “loop-free” mapping.

It is shown in this paper that every \(2\)-connected claw-free graph containing a \(k\)-factor has a connected \([k,k+1]\)-factor, where \(k \geq 2\).

Let \(G\) be a graph of order \(n\), and let \(n = \sum_{i=1}^{k}a^i\) be a partition of \(n\) with \(a_i \geq 2\). Let \(v_1, \ldots, v_k\) be given distinct vertices of \(G\). Suppose that the minimum degree of \(G\) is at least \(3k\). In this paper, we prove that there exists a decomposition of the vertex set \(V(G) = \bigcup_{i=1}^k A_i\) such that \(|A_i| = a_i\), \(v_i \in A_i\), and the subgraph induced by \(A_i\) contains no isolated vertices for all \(i, 1 \leq i \leq k\).

Let \(G\) be a graph of order \(n \geq 4k\) and let \(S\) be the graph obtained from \(K_4\) by removing two edges which have a common vertex. In this paper, we prove the following theorem:
A graph \(G\) of order \(n \geq 4k\) with \(\sigma_2(G) \geq n+k\) has \(k\) vertex-disjoint \(S\).This theorem implies that a graph \(G\) of order \(n = 4k\) with \(\sigma_2(G) \geq 5k\) has an \(S\)-factor.

The reconstruction number \(rn(G)\) of graph \(G\) is the minimum number of vertex-deleted subgraphs of \(G\) required in order to identify \(G\) up to isomorphism. Myrvold and Molina have shown that if \(G\) is disconnected and not all components are isomorphic then \(rn(G) = 3\), whereas, if all components are isomorphic and have \(c\) vertices each, then \(rn(G)\) can be as large as \(c + 2\). In this paper we propose and initiate the study of the gap between \(rn(G) = 3\) and \(rn(G) = c + 2\). Myrvold showed that if \(G\) consists of \(p\) copies of \(K_c\), then\(rn(G) = c + 2\). We show that, in fact, this is the only class of disconnected graphs with this value of \(rn(G)\). We also show that if \(rn(G) \geq c + 1\) (where \(c\) is still the number of vertices in any component), then, again, \(G\) can only be copies of \(K_c\). It then follows that there exist no disconnected graphs \(G\) with \(c\) vertices in each component and \(rn(G) = c + 1\). This poses the problem of obtaining for a given \(c\), the largest value of \(t = t(c)\) such that there exists a disconnected graph with all components of order \(c\), isomorphic and not equal to \(K_c\), and is such that \(rn(G) = t\).

We take a special \(1\)-factorization of \(K_{n,n}\), and investigate the subgraphs suborthogonal to the \(1\)-factorization. Some interesting results are obtained, including an identity involving \(n^n\) and \(n!\) and a property of permutations.

An extended Mendelsohn triple system of order \(v\) (EMTS(\(v\))) is a collection of cyclically ordered triples of the type \([x,y,z], [x,x,y]\), or \([x,x,x]\) chosen from a \(v\)-set, such that each ordered pair (not necessarily distinct) belongs to exactly one triple. If such a design with parameters \(v\) and \(a\) exist, then they will have \(b_{v,a}\) blocks, where \(b_{v,a} = (v^2 + 2a)/3\). In this paper, we show that there are two (not necessarily distinct) EMTS(\(v\))’s with common triples in the following sets:

\(\{0,1,2,\ldots,b_v-4,b_v-2,b_v\}\), if \(v \neq 6\); and

\(\{0,1,2,\ldots,b_v-4,b_v-2\}\), if \(v = 6\),

where \(b_v\) is \(b_{v,v-1}\) if \(v \equiv 2 \pmod{3}\); \(b_{v,v}\) if \(v \not\equiv 2 \pmod{3}\).

Dudeney’s round table problem was proposed about one hundred years ago. It is already solved when the number of people is even, but it is still unsettled except for only a few cases when the number of people is odd.

In this paper, a solution of Dudeney’s round table problem is given when \(n = p+2\), where \(p\) is an odd prime number such that \(2\) is the square of a primitive root of \(\mathrm{GF}(p)\), and \(p \equiv 3 \pmod{4}\).