Dominant tournament families

For a tournament $H$ with $h$ vertices, its typical density is $h!2^{-\left(\genfrac{}{}{0px}{}{h}{2}\right)}\mathrm{/}aut(H)$, i.e. this is the expected density of $H$ in a random tournament. A family $\mathcal{F}$ of $h$-vertex tournaments is {\em dominant} if for all sufficiently large $n$, there exists an $n$-vertex tournament $G$ such that the density of each element of $\mathcal{F}$ in $G$ is larger than its typical density by a constant factor. Characterizing all dominant families is challenging already for small $h$. Here we characterize several large dominant families for every $h$. In particular, we prove the following for all $h$ sufficiently large: (i) For all tournaments $H^{\ast }$ with at least $5\log h$ vertices, the family of all $h$-vertex tournaments that contain $H^{\ast }$ as a subgraph is dominant. (ii) The family of all $h$-vertex tournaments whose minimum feedback arc set size is at most $\frac{1}{2}\left(\genfrac{}{}{0px}{}{h}{2}\right)-h^{3\mathrm{/}2}\sqrt{\ln h}$ is dominant. For small $h$, we construct a dominant family of $6$ (i.e. $50\mathrm{\%}$ of the) tournaments on $5$ vertices and dominant families of size larger than $40\mathrm{\%}$ for $h=6,7,8,9$. For all $h$, we provide an explicit construction of a dominant family which is conjectured to obtain an absolute constant fraction of the tournaments on $h$ vertices. Some additional intriguing open problems are presented.