Expository notes

• Lagrange's four square theorem via convex geometry (6/21/2023)
  Every nonnegative integer can be written as a sum of four squares. This note derives this from Minkowski's theorem on lattice points in centrally symmetric convex sets.
• The Ritt–Raudenbush basis theorem (2/19/2023)
  An analogue of the Hilbert basis theorem in differential algebra.
• Dickson invariants (3/17/2011)
  Generators for the ring of invariants of the general linear group over a finite field; roughly speaking these are a q-analogue of elementary symmetric polynomials.