= Characteristic reachability for radial transport
{title2=$x u_x+y u_y=ku,\quad(x,y)=(a,1-a)e^s$}
The radial <transport equation> has projected <characteristic curves> $(x,y)=e^s(x_0,y_0)$, which remain on positive rays. Data on $x+y=1$ reach precisely the half-plane $x+y>0$, since $x+y=e^s$. A datum $u(a,1-a)=h(a)$ therefore gives $u(x,y)=(x+y)^k h(x/(x+y))$ in that half-plane. The opposite half-plane is not determined by these data. Extending a characteristic line through the origin would join distinct flow orbits across a point where the projected vector field vanishes.
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