Connected Turán number of trees

As a variant of the much studied Turán number, $ex(n,F)$, the largest number of edges that an $n$-vertex $F$-free graph may contain, we introduce the connected Turán number $ex_c(n,F)$, the largest number of edges that an $n$-vertex connected $F$-free graph may contain. We focus on the case where the forbidden graph is a tree. The celebrated conjecture of Erdős and Sós states that for any tree $T$, we have $ex(n,T)\le(|T|-2)\frac{n}{2}$. We address the problem how much smaller $ex_c(n,T)$ can be, what is the smallest possible ratio of $ex_c(n,T)$ and $(|T|-2)\frac{n}{2}$ as $|T|$ grows. We also determine the exact value of $ex_c(n,T)$ for small trees, in particular for all trees with at most six vertices. We introduce general constructions of connected $T$-free graphs based on graph parameters as longest path, matching number, branching number, etc.

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