Past exam of the mathematics course of the University of Cambridge / 2008 / ii / Paper 3 / 14A / Solution

For a positively oriented simple closed curve on which the vector field is nonzero, its Poincaré index is the total rotation of the vector field along that curve divided by $2\pi$, equivalently the winding number of its normalized direction. The Poincaré index of an isolated fixed point is that of a sufficiently small enclosing curve containing no other fixed point. The index of a planar periodic orbit is $+1$.