Mathematics of a Serial Killer

Someone sent me a link to this story about a mathematical model of a particular serial killer’s behavior. Two things struck me about it:

  1. How much it sounded like the kind of bizarre model you’d see on Charline on Numb3rs come up with in order to crack the case.
  2. That Cosma Shalizi would hate the model, since it’s the kind of a casual use of power laws he regularly criticizes. And here’s his analysis of the paper. He points out that, as in many other cases, a lognormal distribution provides a better fit.

Best Possible Rejection Letter

Andrew Gelman quotes from the best possible rejection letter from a journal (sent to Charles Babbage):

It is no inconsiderable degree of reluctance that I decline the offer of any Paper from you. I think, however, you will upon reconsideration of the subject be of the opinion that I have no other alternative. The subjects you propose for a series of Mathematical and Metaphysical Essays are so profound, that there is perhaps not a single subscriber to our Journal who could follow them.

I encourage all journals to adopt this as the standard form letter for rejection.

Back, From Outer Space!

And you just walked in to find me here with that sad look upon my face.

The computer that hosted Ars Math (which was at a dedicated web-site hosting company) died a horrible death a couple of months ago. For a while, it looked like several years of posts had been lost. They could be recovered by cutting-and-pasting from the Internet Archive, but I found the idea so depressing that I didn’t do anything about recovering the site. Fortunately, we were able to extract the posts from the database anyway. Someone offered to help out with the WordPress hosting, so we’re back online!

At the moment the last couple of years of comments are missing, but we’re still investigating what happened to them.

Shapley-Folkman-Starr Theorem

While economic theory sometimes uses advanced mathematics, such as Brouwer’s fixed point theorem, it’s less common for economic theory to lead to new mathematical developments. The Shapley-Folkman-Starr Theorem is an example of the latter. Roughly, the theorem states that the (Minkowski) sum of a large number of arbitrary sets in a finite-dimensional vector space will be close to convex. Starr was an economics undergraduate who was working on a term paper on approximating non-convex optimization problems with convex ones. This led to collaboration with Shapley (a game theorist), and Folkman (a mathematician), and the eponymous theorem.

Machine Learning Book

Machine learning is an exciting new area of applied mathematics and computer science. It’s philosophically interesting because it requires to make a precise mathematical formulation of what it means to learn, and to be able to distinguish concepts from each other. Nils Nilsson has an early draft of a textbook on the subject.

Scientific Swindler

I came across this story about a 19th century swindler who targeted geologists. He would pose (convincingly) as an expert, and exploit the collegiality of geologists to “borrow” money, or valuable specimens, equipment, and books. You would never hear a similar story about mathematicians because they have nothing worth stealing.