Decomposable Twofold Triple Systems with Non-Hamiltonian 2-Block Intersection Graphs

The $2$-block intersection graph ($2$-BIG) of a twofold triple system (TTS) is the graph whose vertex set is composed of the blocks of the TTS and two vertices are joined by an edge if the corresponding blocks intersect in exactly two elements. The $2$-BIGs are themselves interesting graphs: each component is cubic and $3$-connected, and a $2$-BIG is bipartite exactly when the TTS is decomposable to two Steiner triple systems. Any connected bipartite $2$-BIG with no Hamilton cycle is a further counter-example to a disproved conjecture posed by Tutte in 1971. Our main result is that there exists an integer $N$ such that for all $v\geq N$, if $v\equiv 1$ or $3\mod{6}$ then there exists a TTS($v$) whose $2$-BIG is bipartite and connected but not Hamiltonian. Furthermore, $13<N\leq 663$. Our approach is to construct a TTS($u$) whose $2$-BIG is connected bipartite and non-Hamiltonian and embed it within a TTS($v$) where $v>2u$ in such a way that, after a single trade, the $2$-BIG of the resulting TTS($v$) is bipartite connected and non-Hamiltonian.

On some parametric classifications of quasi-symmetric 2-designs

Quasi-symmetric $2$-designs with block intersection numbers $x$ and $y$, where $y=x+4$ and $x > 0$ are considered. If $D(v, b, r, k, \lambda; x, y)$ is a quasi-symmetric $2$-design with above condition, then it is shown that the number of such designs is finite, whenever $3\leq x \leq 68$. Moreover, the non-existence of triangle free quasi-symmetric $2$-designs under these parameters is obtained.