A Celebration of Independence

Yesterday marked the second anniversary of my blog, and today is the 239th birthday of the U.S. In celebration of Independence Day, I want to explain what matroids are. Matroids were invented by Hassler Whitney (and independently by Takeo Nakasawa) to abstract the notion of linear independence from vector spaces to a much more general setting that includes acyclicity in graphs. Other pioneering early work on matroids was done by Garrett Birkhoff and Saunders MacLane. Matroid theory is a rich subject about which we will only scratch the surface here. In particular, there are many different (“cryptomorphic“) ways to present the matroid axioms which all turn out to be (non-obviously) equivalent to one another. We will focus on just a couple of ways of looking at matroids, emphasizing their connections to tropical geometry. Continue reading

The Mathematics of Marriage

linking-wedding-ringsIt’s been a while since my last blog post — one reason being that I recently got married.  In honor of that occasion, and my return to math blogging, here is a post on Hall’s Marriage Theorem.

Consider the following game of solitaire: you deal a deck of cards into 13 piles of 4 cards each, and your goal is to select one card from each pile so that no value (Ace through King) is repeated.  It is a beautiful mathematical fact that this can always been done, no matter how the cards were originally dealt!

We will deduce this from a more general result due to Philip Hall commonly known as Hall’s Marriage Theorem.  Suppose you are given finite sets A_1, A_2,\ldots, A_n and you wish to find distinct elements x_1 \in A_1,\ldots, x_n \in A_n.  (In our solitaire example, take A_j to be the values of the cards in the j^{\rm th} pile.)  Such a collection is called a transversal or SDR (system of distinct representatives).  Under what conditions is this possible?  Well, for a transversal to exist it is necessary that for each subset J \subset \{ 1,\ldots, n \}, the set A_J:= \bigcup_{j \in J} A_j contains at least \#J elements.  Hall’s theorem asserts that these conditions are also sufficient. Continue reading