A nice theoretical explanation of the crossover behavior of the band gap in carbon nanotubes.

This older version of the paper is more useful in my research than the newer version, which was published in PRL. 

I read that Euler found the first solution to the three body problem in 1765: three point masses all in a line. In 1772, Lagrange found a second: three point masses at the corners of an equilateral triangle. Apparently there have been no new analytic solutions since then.

It looks like chapter 5 of this book might discuss these.