The Glass Theorem
In 1973, Leon Glass proved a 2 dimensional discrete Poincare Hopf theorem for graphs. A thesis of Kate Perkins at Harvey Mudd has made me aware of this reference. Perkins relates it with a general Poincare Hopf theorem I have proven and which works for arbitrary graphs. I present the Glass theorem and its proof then relate it to a later theorem of mine which is a Poincare Hopf theorem for directed locally irrotational graphs. The later immediately allows to derive the Glass theorem too.
The drone footage at the beginning was done at the Arlington Reservoir. The drone footage at the end was done near Spy Pond in Arlington, an other major lake in Arlington.