A digraph \(D\) is reversible if it is isomorphic to the digraph obtained by reversing all arcs of \(D\). A digraph is subreversible if adding any arc between two non-adjacent vertices results in a reversible digraph. We characterize all subreversible digraphs which do not contain cycles of length 3 or 4.

In this paper we prove that, except for the 4-cycle and the 5-cycle, every 2-connected \(K(1,3)\)-free graph of diameter at most two is pancyclic.

The well-known clique tree representation for chordal graphs is extended to multidimensional representations for arbitrary graphs in which the number of vertices in the representation, minus the number of edges, plus the number of distinguished cycles, minus the number of distinguished polyhedra, and so on, always equals one. This approach generalizes both chordal graphs and cycle spaces of graphs. It also leads to a `dimension’ parameter that is shown to be no greater than the boxicity, chromatic number, and tree-width parameters.

An \(e=1\) function is a function \(f: V(G) \rightarrow [0,1]\) such that every non-isolated vertex \(u\) is adjacent to some vertex \(v\) such that \(f(u) + f(v) = 1\), and every isolated vertex \(w\) has \(f(w) = 1\). A theory of \(e=1\) functions is developed focussing on minimal and maximal \(e=1\) functions. Relationships are traced between \(e=1\) parameters and some well-known domination parameters, which lead to results about classical and fractional domination parameters.

We formulate the construction of 1-rotational difference families as a combinatorial optimization problem. A tabu search algorithm is used to find an optimal solution to the optimization problem for various 1-rotational difference family parameters. In particular, we construct two new 1-rotational difference families which lead to an equal number of new 1-rotational RBIBDs with parameters: \((36, 9, 8)\) and \((40, 10, 9)\). Our algorithm also was able to construct six non-isomorphic \((36, 9, 8)\) and three \((40, 10, 9)\) RBIBDs

For two vertices \(u\)  and \(v\) of a connected graph \(G\) , the set \(H(u,v)\) consists of all those vertices lying on a \(u-v\) geodesic in \(G\) . Given a set \(S\) of vertices of \(G\) , the union of all sets \(H(u,v)\) for \(u,v\in S\) is denoted by \(H(S)\) . A convex set \(S\) satisfies \(H(S)=S\) . The convex hull \([S]\) is the smallest convex set containing \(S\) . The hull number \(h(G)\) is the minimum cardinality among the subsets \(S\) of \(V(G)\) with \([S]=V(G)\) . A set \(S\) is a geodetic set if \(H(S)=V(G)\) ; while \(S\) is a hull set if \([S]=V(G)\) . The minimum cardinality of a geodetic set of \(G\) is the geodetic number \(g(G)\) . A subset \(T\) of a minimum hull set \(S\) is called a forcing subset for \(S\) if \(S\) is the unique minimum hull set containing \(T\) . The forcing hull number \(f(S,h)\) of \(S\) is the minimum cardinality among the forcing subsets of \(S\) , and the forcing hull number \(f(G,h)\) of \(G\) is the minimum forcing hull number among all minimum hull sets of \(G\) . The forcing geodetic number \(f(S,g)\)  of a minimum geodetic set \(S\) in \(G\) and the forcing geodetic number \(f(G,g)\) of \(G\) are defined in a similar fashion. The forcing hull numbers of several classes of graphs are determined. It is shown that for integers \(a,b\) with \(0\leq a\leq b\) , there exists a connected graph \(G\) such that \(f(G,h)=a\) and \(h(G)=b\) . We investigate the realizability of integers \(a,b\geq0\) that are the forcing hull and forcing geodetic numbers, respectively, of some graph.

Let \(C\) be a perfect 1-error-correcting code of length \(15\). We show that a quotient \(H(C)\) of the minimum distance graph of \(C\) constitutes an invariant for \(C\) more sensible than those studied up to the present, namely the kernel dimension and the rank. As a by-product, we get a nonlinear Vasil’ev code \(C\) all of whose associated Steiner triple systems are linear. Finally, the determination of \(H(C)\) for known families of \(C\)’s is presented.

A computer search shows that there does not exist a nested BIB design \(\text{NB}(10, 15, 2, 3)\).

We construct several families of simple 4-designs, which are closely related to Alltop’s series with parameters \(4-(2^f+1,5,5)\), \(f\) odd. More precisely, for every \(q=2^f\), where \(gcd(f,6)=1\), \(f\geq5\), we construct designs with the following parameters:

\[4-(q+1,6,\lambda),\, \text{where}\, \lambda\in\{60,70,90,100,150,160\},\]
\[4-(q+1,8,35),\]
\[4-(q+1,9,\lambda),\, \text{where}\, \lambda\in\{63,147\}.\]

Eulerian numbers may be defined recursively and have applications to many branches of mathematics. We derive some congruence and divisibility properties of Eulerian numbers.