Let \(D\) denote any balanced ternary design with block size three, index two, and \(\rho_2 = 1\) (that is, with each element occurring repeated in just one block). This paper shows that there exists such a design \(D\) on \(V\) elements containing exactly \(k\) pairs of repeated blocks if and only if \(V \equiv 0 \pmod{3}\), \[0\leq k \leq t_V = \frac{1}{6}V(V-3), \; \; k\neq t_V – 1, \text{and} (k,V)\neq(1,6)\].

For each integer \(v \geq 0\) and each \(\lambda \in \{4, 5, 7, 8\}\), the possible numbers of distinct blocks in a triple system of order \(v\) and index \(\lambda\) is determined. This essentially completes the determination of possible support sizes for triple systems with \(\lambda \leq 8\).

If \(n\) is an integer, \(n \geq 2\), and \(u\) and \(v\) are vertices of a graph \(G\), then \(u\) and \(v\) are said to be \(K_n\)-adjacent vertices of \(G\) if there is a subgraph of \(G\), isomorphic to \(K_n\), containing \(u\) and \(v\). A total \(K_n\)-dominating set of \(G\) is a set \(D\) of vertices such that every vertex of \(G\) is \(K_n\)-adjacent to a vertex of \(D\). The total \(K_n\)-domination number \(\gamma_{K_n}^t(G)\) of \(G\) is the minimum cardinality among the total \(K_n\)-dominating sets of vertices of \(G\). It is shown that, for \(n \in \{3, 4, 5\}\), if \(G\) is a graph with no \(K_n\)-isolated vertex, then \(\gamma_{K_n}^t(G) \leq (2p)/{n}\). Further, \(K_n\)-connectivity is defined and it is shown that, for \(n \in \{3, 4\}\), if \(G\) is a \(K_n\)-connected graph of order \(\geq n + 1\), then \(\gamma_{K_n}^t(G) \leq (2p)/(n + 1)\). We establish that the upper bounds obtained are best possible.

Let \(D\) and \(\overline{D}^d\) be two designs such that there is a joint embedding \(D’\) and \(\overline{D}’\) of \(D\) and \(\overline{D}\) in a finite projective plane \(\pi\) of order \(n\) such that the points of \(D’\) and the lines of \(\overline{D}’\) are mutually all of the exterior elements of each other. We show that there is a tactical decomposition \(T\) of \(\pi\), two of the tactical configurations of which are \(D’\) and \(\overline{D}’\), and determine combinatorial restrictions on \(n\) and the parameters of \(D\) and \(\overline{D}^d\). We also determine the entries of the incidence matrices of \(T\).

The Josephus problem is concerned with anticipating which will be the last elements left in the ordered set \(\{1, 2, \ldots, n\}\) as successive \(m\)th elements (counting cyclically) are eliminated. We study the set of permutations of \(\{1, 2, \ldots, n\}\) which arise from the different orders of elimination as \(m\) varies, and give a criterion based on the Chinese Remainder Theorem for deciding if a given permutation can be interpreted as arising as a given order of elimination for some step size \(m\) in a Josephus problem.

In a secret sharing scheme, a dealer has a secret. The dealer gives each participant in the scheme a share of the secret. There is a set \(\Gamma\) of subsets of the participants with the property that any subset of participants that is in \(\Gamma\) can determine the secret. In a perfect secret sharing scheme, any subset of participants that is not in \(\Gamma\) cannot obtain any information about the secret. We will say that a perfect secret sharing scheme is ideal if all of the shares are from the same domain as the secret. Shamir and Blakley constructed ideal threshold schemes, and Benaloh has constructed other ideal secret sharing schemes. In this paper, we construct ideal secret sharing schemes for more general access structures which include the multilevel and compartmented access structures proposed by Simmons.

It is shown that the collection of all \(\dbinom{12}{5}\) quintuples chosen from a set of twelve points can be partitioned into twelve mutually disjoint \(4-(11,5,1)\) designs in precisely \(24\) non-isomorphic ways. The results obtained are then used to show that the collection of all \(\dbinom{13}{6}\) hextuples chosen from a set of thirteen points cannot be partitioned into thirteen mutually disjoint \(5-(12,6,1)\) designs.

The set of Lyndon words of length \(n\), \(\Lambda_n\), is the set obtained by choosing those strings of length \(n\) over any finite alphabet \(\Sigma\) of cardinality \(\sigma\) which are lexicographically least in the primitive or aperiodic equivalence classes determined by cyclic permutation. It is well-known that \(\Lambda_n\) is a maximal synchronizable code with bounded synchronization delay for fixed word length \(n\). If the Lyndon words of length \(n\) are represented as vertices of the \(n\)-cube, we show that they form a connected set for arbitrary alphabets. Indeed, we show that between any two Lyndon words, there is a path consisting of at most \(2n\) Lyndon words in the \(n\)-cube. Further, we show that there always exists a path of \(n(\sigma – 1) – 1\) Lyndon words in the \(n\)-cube.

The conjugation relation among the subspaces of a finite unitary geometry and its properties are studied. Then they are used to find some enumeration formulas for the subspaces of the unitary geometry, to prove a type of transitivity of the unitary group, to construct Partially Balanced Incomplete Block (PBIB) designs, and to establish the isomorphism between some known PBIB designs.