Real-time blow-up and connection graphs of rational vector fields on the Riemann sphere

Abstract

Inspired by pioneering work of Ky\^uya Masuda in the 1980s, only much more recent PDE studies address global boundedness versus finite-time blow-up in complex time. The two phenomena are related by passage from real to purely imaginary time. As a most simplistic ODE example, we study scalar rational vector fields \begin{equation*} \label{*} \dot{w}=P(w)/Q(w), \tag{*} \end{equation*} for complex polynomials $P,Q$. We impose mild generic nondegeneracy conditions, including simplicity of poles and hyperbolicity of zeros. Generically, the real-time dynamics become gradient-like Morse. Poles play the role of hyperbolic saddle points. At poles, however, solutions may blow up in finite time. On the Riemann sphere $w\in\widehat{\mathbb{C}}$, we classify the resulting global dynamics up to $C^0$ orbit equivalence, in real time. This relies on a global description of the connection graph of blow-up orbits, from sources towards saddles/poles, in forward time. Time reversal identifies the dual graph of blow-down orbits. We show that the blow-up and blow-down graphs of (*) realize all finite multi-graphs on $\mathbb{S}^2$, equivalently. The purely polynomial case $Q=1$ realizes all planar trees, alias diagrams of non-intersecting circle chords. The anti-holomorphic cousin $P=1$ realizes all noncrossing trees with vertices restricted to circles. This classification provide combinatorial counts for the number of global phase portraits, which only depend on the degrees of $P$ and $Q$, respectively.