Difference Methods for Double Change Covering Designs
A \textbf{double change covering design} is a $v$-set $X$ and an ordered list $\mathcal{L}$ of $b$ blocks of size $k$ where every $t$-set must occur in at least one block. Each pair of consecutive blocks differs by exactly two elements. A double change covering design is circular when the first and last blocks also differ by two elements. We give a recursive construction that uses 1-factorizations and expansion sets to increase $v$ by $\frac{v+k-2}{k-2}$. We construct ${\rm CDCCD}(2k-2,k,k-1)$ and ${\rm CDCCD}(2k-1,k,k-2)$ from single change covering designs. We use difference methods to construct tight ${\rm CDCCD(4k-5,k,4k-5)$, five infinite families for any $k\geq 3$, minimum ${\rm CDCCD}(c(4k-6)+1,k,c^2(4k-6)+c)$ when $c\leq 5$ and ${\rm CDCCD}(61,4,366)$. Finally, from these, we use the recursion to construct twelve additional minimum CDCCD.