${\rm{TS}}(v,\lambda)$ with cyclic 2-intersecting Gray codes: $v\equiv 0$ or $4\pmod{12}$

Abstract

A ${\rm{TS}}(v,\lambda)$ is a pair $(V,\mathcal{B})$ where $V$ contains $v$ points and $\mathcal{B}$ contains $3$-element subsets of $V$ so that each pair in $V$ appears in exactly $\lambda$ blocks. A $2$-block intersection graph ($2$-BIG) of a ${\rm{TS}}(v,\lambda)$ is a graph where each vertex is represented by a block from the ${\rm{TS}}(v,\lambda)$ and each pair of blocks $B_i,B_j\in \mathcal{B}$ are joined by an edge if $|B_i\cap B_j|=2$. Using constructions for ${\rm{TS}}(v,\lambda)$ given by Schreiber, we show that there exists a ${\rm{TS}}(v,\lambda)$ for $v\equiv 0$ or $4\pmod{12}$ whose $2$-BIG is Hamiltonian.