Robert R. Bruner
After a quick introduction to the projective plane, we show
that extending a differential equation to the projective plane
is a quick and effective way to study the asymptotic behavior
of its solutions. The simplest approach leads to equations
with ugly singularities, so we also show how to use the method
of rescaling time to desingularize them.
The main theorem is as follows:
Theorem: Let x' = f(x,y), y' = g(x,y) be a polynomial differential
equation in R2 of degree N. Then either
- there are at most N+1 possible slopes "at infinity" for the unbounded trajectories, or
- there are at most N-1 slopes which are omitted: all other slopes "at infinity" actually occur.
The theorem is proved by exhibiting the polynomial whose roots are the
possible slopes in case (1), or the possibly omitted slopes, in case (2).
It is simple to apply and gives information that would otherwise be difficult
to extract.
The only real prerequisite will be differentiation and integration
of functions of one variable. The approach to differential equations
will be qualitative and intuitive, so the talk could also serve as a
good introduction to the geometric point of view on differential equations.
The audience will be left with an open ended list of examples and applications
to explore.