Matar mosques a canonades

Abstract

The story told here starts with an innocuous little geometry problem, posed in a September 2006 blog entry by R. Nandakumar, an engineer from Calcutta, India: This little problem is a «sparrow», tantalizing, not as easy as one could perhaps expect, and Recreational Mathematics: of no practical use. I will sketch, however, how this little problem connects to very serious mathematics: For the modelling of this problem we employ insights from a key area of Applied Mathematics, the Theory of Optimal Transportation. This will set up the stage for application of a major tool from Very Pure Mathematics, known as Equivariant Obstruction Theory. This is a «cannon», and well have some fun firing it at the sparrow. On the way to a solution, combinatorial properties of a very classical geometric object, the permutahedron, turn out to be essential. These will, at the end of the story, lead us back to India, with some time travel that takes us one hundred years into the past: For the last step in our (partial) solution of the sparrows problem we need a simple property of the numbers in Pascals triangle, which was first observed by Balak Ram, in Madras 1909. But even if the existence problem is solved, the little geometry problem is not: If the solution exists, how do you find one? This problem will be left to you. Instead, I will comment on the strained relationship between cannons and sparrows, and to this avail quote a poem by Hans Magnus Enzensberger.