Optimally building spanning graphs in semirandom graph processes

Abstract: The semirandom graph process constructs a graph $G$ in a series of rounds, starting with the empty graph on $n$ vertices. In each round, a player is offered a vertex $v$ chosen uniformly at random, and chooses an edge on $v$ to add to $G$. The player's aim is to make $G$ satisfy some property as quickly as possible. Our interest is in the property that $G$ contain a given $n$-vertex graph $H$ with maximum degree $\Delta$. In 2021, Ben-Eliezer, Gishboliner, Hefetz and Krivelevich showed that there is a semirandom strategy that achieves this, with probability tending to 1 as $n$ tends to infinity, in $(1 + o_\Delta(1)) \frac{3 \Delta n}{2}$ rounds, where $o_\Delta(1)$ is a function that tends to $0$ as $\Delta$ tends to infinity. We improve this to $(1 + o_\Delta(1)) \frac{\Delta n}{2}$, which can be seen to be asymptotically optimal in $\Delta$. We show the same result for a variant of the semirandom graph process, namely the semirandom tree process introduced by Burova and Lichev, where in each round the player is offered the edge set of a uniformly chosen tree on $n$ vertices, and chooses one edge to keep.