Large sets of balanced incomplete block (\(BIB\)) designs and resolvable \(BIB\) designs are discussed. Some recursive constructions of such large sets are given. Some existence results, in particular for practical \(k\), are reviewed.

We consider point-line geometries having three points on every line, having three lines through every point (\(bi\)-\(slim\; geometries\)), and containing triangles. We give some (new) constructions and we prove that every flag-transitive such geometry either belongs to a certain infinite class described by Coxeter a long time ago, or is one of three well-defined sporadic ones, namely, The Möbius-Kantor geometry on \(8\) points, The Desargues geometry on \(10\) points,A unique infinite example related to the tiling of the real Euclidean plane in regular hexagons.We also classify the possible groups.

Let \(G\) be a simple graph such that \(\delta(G) \geq \lfloor\frac{|V(G)|}{2}\rfloor + k\), where \(k\) is a non-negative integer, and let \(f: V(G) \to \mathbb{Z}^+\) be a function having the following properties (i)\(\frac{d_G(x)}{2}-\frac{k+1}{2}\leq f(x)\leq \frac{d_G(x)}{2}+\frac{k+1}{2}\) for every \(x \in V(G)\), (ii)\(\sum\limits_{x\in V(G)}f(x)=|E(G)|\). Then \(G\) has an orientation \(D\) such that \(d^+_D(x) = f(x)\), for every \(x \in V(G)\).

The so-called multi-restricted numbers generalize and extend the role of Stirling numbers and Bessel numbers in various problems of combinatorial enumeration. Multi-restricted numbers of the second kind count set partitions with a given number of parts, none of whose cardinalities may exceed a fixed threshold or “restriction”. The numbers are shown to satisfy a three-term recurrence relation. Both analytic and combinatorial proofs for this relation are presented. Multi-restricted numbers of both the first and second kinds provide connections between the orbit decompositions of subsets of powers of a finite group permutation representation, in which the number of occurrences of elements is restricted. An exponential generating function for the number of orbits on such restricted powers is given in terms of powers of partial sums of the exponential function.

A class of graphs called generalized ladder graphs is defined. A sufficient condition for pairs of these graphs to be chromatically equivalent is proven. In addition, a formula for the chromatic polynomial of a graph of this type is proven. Finally, the chromatic polynomials of special cases of these graphs are explicitly computed.

Let \(k \geq 3\) be odd and \(G = (V(G), E(G))\) be a \(k\)-edge-connected graph. For \(X \subseteq V(G)\), \(e(X)\) denotes the number of edges between \(X\) and \(V(G) – X\). We here prove that if \(\{s_i, t_i\} \subseteq X_i \subseteq V(G)\), \(i = 1, 2\), \(X_1 \cap X_2 = \emptyset\), \(e(X_1) \leq 2k-2\) and \(e(X_2) < 2k-1\), then there exist paths \(P_1\) and \(P_2\) such that \(P_i\) joins \(s_i\) and \(t_i\), \(V(P_i) \subseteq X_i\) (\(i = 1, 2\)) and \(G – E(P_1 \cup P_2)\) is \((k-2)\)-edge-connected. And in fact, we give a generalization of this result and some other results about paths not containing given edges.

Optimal binary linear codes of length \(18\) containing the \([6, 5, 2]\otimes[ 3, 1, 3]\) product code are presented. It is shown that these are \([18, 9, 5]\) and \([18, 8, 6]\) codes. The soft-decision maximum-likelihood decoding complexity of these codes is determined. From this point of view, these codes are better than the \([18, 9, 6]\) code.

It is shown that the voltage-current duality in topological graph theory can be obtained as a consequence of a combinatorial description of the pair (an embedded graph, the embedded dual graph)without any reference to derived graphs and derived embeddings. In the combinatorial description the oriented edges of an embedded graph are labeled by oriented edges of the embedded dual graph.

We extend the work of Currie and Fitzpatrick [1] on circular words avoiding patterns by showing that, for any positive integer \(n\), the Thue-Morse word contains a subword of length \(n\) which is circular cube-free. This proves a conjecture of V. Linek.

Let \(G\) be a simple graph with the average degree \(d_{ave}\) and the maximum degree \(\Delta\). It is proved, in this paper, that \(G\) is not critical if \(d_{ave} \leq \frac{103}{12}\) and \(\Delta \geq 12\). It also improves the current result by L.Y. Miao and J.L. Wu [7] on the number of edges of critical graphs for \(\Delta \geq 12\).