The present paper studies bisectable trees, i.e., trees whose edges can be colored by two colors so that the induced monochromatic subgraphs are isomorphic. It is proved that the number of edges that have to be removed from a tree with maximum degree three to make it bisectable can be bounded by an absolute constant.

We study the maximal intersection number of known Steiner systems and designs obtained from codes. By using a theorem of Driessen, together with some new observations, we obtain many new designs.

Taking as blocks some subspace pairs in a finite unitary geometry, we construct a number of new Balanced Incomplete Block (BIB) designs and Partially Balanced Incomplete Block (PBIB) designs, and also give their parameters.

The support size of a factorization is the sum over the factors of the number of distinct edges in each factor. The spectrum of support sizes of \(l\lambda\)-factorizations of \(\lambda K_n\) and \(\lambda K_{n,n}\) are completely determined for all \(\lambda\) and \(n\).

Latin interchanges have been used to establish the existence of critical sets in Latin squares, to search for subsquare-free Latin squares, and to investigate the intersection sizes of Latin squares. Donald Keedwell was the first to study Latin interchanges in their
own right. This paper builds on Keedwell’s work and proves new results about the existence of Latin interchanges.

A balanced ternary design of order nine with block size three, index two, and \(\rho_2 = 1\) is a collection of multi-subsets of size \(3\) (of type \(\{x, y, z\}\) or \(\{x, x, y\}\)) called blocks, chosen from a \(9\)-set, in which each unordered pair of distinct elements occurs twice, possibly in one block, and in which each element is repeated in just one block. So there are precisely \(9\) blocks of type \(\{x, x, y\}\). We denote such a design by \((9; 1; 3, 2)\) BTD. In this note, we describe the procedures we have used to
determine that there are exactly \(1475\) non-isomorphic \((9; 1; 3, 2)\) BTDs.

A graph \(G\) is said to be \emph{hypohamiltonian} if \(G\) is not Hamiltonian but for each \(v \in V(G)\), the vertex-deleted subgraph \(G – v\) is Hamiltonian. In this paper, we show that there is no hypohamiltonian graph on \(17\) vertices and thereby complete the answer to the question, “For which values of \(n\) do there exist hypohamiltonian graphs
on \(n\) vertices?”. In addition, we present an exhaustive list of hypohamiltonian graphs on fewer than \(18\) vertices and extend previously obtained results for cubic hypohamiltonian graphs.

We consider the problem of constructing pairwise balanced designs of order \(v\) with a hole of size \(k\). This problem was addressed by Hartman and Heinrich who gave an almost complete solution. To date, there remain fifteen unresolved cases. In this paper, we construct designs settling all of these.

All non-Hamiltonian cubic \(2\)-edge-connected graphs, including all snarks, on \(16\) or fewer vertices are listed, along with some of their properties. Questions concerning the existence of graphs with certain properties are posed.