# Andy's Math/CS page

## Wednesday, May 30, 2007

### Flooding and Kleitman's Theorem

In their classic, and highly entertaining, survey book on probabilistic methods in combinatorics, Alon and Spencer pose the following question: suppose we generate a random graph on n vertices, independently including each edge with probability 1/2. Suppose we do not look at the graph right away, but are informed that the graph we got contains a Hamiltonian path, that is, a path visiting each vertex exactly once.

Does this knowledge make it more or less likely that the graph is nonplanar?

Of course, it should make it more likely. Please stop and verify that this ought to be true. Can you prove it?

Results that prove facts like this are called 'correlation inequalities', and the simplest one that applies here is called Kleitman's Theorem. (In Alon and Spencer, Kleitman's Theorem is derived from more general results, but I encourage you to try to prove it directly.)

Say a subset A of the Boolean cube is monotone if, whenever x is in A and y >= x (bitwise domination), then y is in A too. We may also say that A is 'upwards closed'.

Kleitman's Theorem states that, given two monotone subsets A, B of the Boolean cube, if we pick a random cube element z, the chance of z being in B cannot decrease if we condition on the event that z is in A.

So, in the example, let the bits of the cube represent the presence/absence of edges in a graph of fixed size. Let A and B be the properties of Hamiltonicity and nonplanarity, respectively. Then Kleitman yields the intuitive fact we were looking for.

***

Now I am going to describe how Kleitman's result solves the previous flooded-city puzzle I posed last time. In fact, trying to reprove Kleitman's Theorem was what led me to pose the flooding puzzle in the first place, although I now tend to think of Kleitman's as the logically prior result.

To do this requires an easy generalization of Kleitman's Theorem, where we replace the boolean cube in the statement with {0, 1, ... m}^n. So, e.g., vector (3, 4, 5) dominates (3, 3, 4), but is incomparable with (4, 0, 0). In fact, we'll only need this result for n = 2.

Create a bipartite graph U x F, with unflooded houses on the left and flooded houses on the right. Add an edge from an unflooded house h to a flooded house h' if there exists an h-to-h' path with all steps running north or east.

Given a set S of unflooded houses, let N(S) denote the flooded houses adjacent to some house of S in the bipartite graph.

In the example above, the green-circled houses are S and the orange-circled flooded houses are N(S).

Each unflooded house contains a unit amount of water to be delivered, while each flooded house needs an amount |U|/|F| of water delivered, which can only be delivered from a house that is adjacent in the graph we defined. Then, by a version of Hall's Theorem, not difficult to derive from the original formulation, the delivery plan we seek exists if and only if for every subset S of U, the total water-need of N(S) meets or exceeds the amount of water available from S.

Suppose that the desired allocation does not exist. Then by Hall's Theorem, there exists some subset S of U such that the water supplied by S exceeds the need of N(S).

We take such an S and iteratively add into S all the unflooded north- and east-neighbors of houses in S. This doesn't increase the size of N(S), but it does increase the amount of water supplied by S, so the resulting modified S is still a witness to the impossibility of our desired allocation.

We repeat this until the union of S and N(S) is upwards-closed. Call the resulting set S'. Since S' supplies more water than N(S') demands, the ratio |S'|/|N(S')| exceeds |U|/|F|.

If S were as in the example above, S' would be the purple-circled set of houses below:

This means that, if we pick a house at random from within the union of S' and N(S'), we are less likely to get a flooded house than if we picked a house uniformly at random from the whole city. This means that, in picking uniformly from the whole city, the (upwards-closed) event of getting a flooded house is negatively correlated with the upwards-closed event of getting a house from the union of S' and N(S')--a contradiction to the generalized Kleitman's Theorem.

We conclude that the desired delivery plan exists, and can be found in polynomial time using known algorithms for bipartite matching.

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