In 1932 von Neumann proposed classifying the statistical behavior of physical systems. The idea was to take a diffeomorphism of a compact manifold and describe what one might observe as random (as in coin flipping) or predictable (as in a translation on a compact group), or even better have a dictionary  in which one could look up the precise behavior.

Remarkable progress was made on this problem; benchmarks include the Halmos-von Neumann theorem on discrete spectrum and the work of Kolmogorov on Entropy that culminated in the Ornstein classification of Bernoulli shifts. One genre of applications of this theory were the results of Furstenberg on Szemeredi’s theorem and eventually the work of  Green and Tao.

Still the problem resisted a complete solution. Strange examples of completely determined systems that showed completely random statistical behavior began to surface. Starting in the 1990’s anti-classification theorems began to appear. These results showed, in a rigorous way, that complete invariants for measure preserving systems cannot exist. Moreover the isomorphism relation itself is completely intractable. Very recently these results were extended to measure preserving diffeomorphisms of the 2-torus.

What is the maximum number of rational points on a curve of genus g over a finite field of size q?  What is the distribution of rational point counts for degree d plane curves over a fixed finite field?  We discuss these and several related questions and show how to use curves over finite fields to construct interesting error-correcting codes.

We continue our discussion of perfect and scattered subsets in the generalized Cantor space. This week we finish the proof of the fact that \kappa-closed forcings don't add branches to \kappa-scattered subsets of 2^{\kappa}. We then introduce a collection of topologies over 2^{\lambda} whose restrictions to P_{\kappa}\lambda have some desirable properties. These topologies will rely on the notion of a P_{\kappa}\lambda-forest, which is a natural generalization of a tree. 

The enteric nervous system (ENS) in our gastrointestinal tract, nicknamed the ``second brain'', is responsible for normal gut function and peristaltic contraction. Embryonic development of the ENS involves the colonization of the gut wall from one end to the other by a population of proliferating neural crest cells. Failure of these cells to invade the whole gut results in the relatively common, potentially fatal condition known as Hirschsprung disease. We have collaborated with developmental biologists for over ten years, addressing various aspects of ENS development and disease. Both continuum and discrete models have provided insight into the key biological processes required for complete colonization and have generated experimentally testable predictions.