So, I know that one can apply the strong LP duality theorem to specific instances of maximum flow problems to recover some nontrivial theorems in combinatorics, such as Hall's theorem, Koenig's theorem and Menger's theorem. It's neat that these theorems, all equivalent to one another, can be seen to follow from a single framework.
I was wondering if there are other such interesting applications of LP duality (or more general kinds of duality) in combinatorics, perhaps of a different flavor? Is there a good reference for these topics? The more surprising the applications, the better.
Best Answer
How about Boosting1 and the Hardcore Lemma, as described in this paper?
The Hardcore Lemma can be proved via LP duality:
Their proof of their main result "uses duality of linear programming (or, equivalently, the finite dimensional Hahn-Banach Theorem) in the form of the min-max theorem for two-player zero-sum games."
Another summary of Impagliazzo's Hardcore Lemma:
1 Boosting constitutes "a family of machine learning algorithms which convert weak learners to strong ones." ... "[M]ost boosting algorithms consist of iteratively learning weak classifiers with respect to a distribution and adding them to a final strong classifier."