[+] Separators in Hminorfree graphs. 
As we
have seen and will see in other lectures, small separators have
several important applications, ranging from shortest paths to
approximation schemes. They are one of the core tools that allow for
faster algorithms on certain graph classes.
In this lecture, we focus on studying a result of Alon, Seymour, and Thomas, stating that every Hminorfree graph admits a small separator and such a separator can be found in polynomial (in fact, subquadratic) time. To this end, we introduce the notion of a haven and show that every graph with a haven of large order must contain a large clique as a minor. We will also briefly discuss how the concept of a knitted HPartition introduced in the last lecture can be used to find a slightly larger separator in linear time. 
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