Does Poincare-Hopf index theorem generalizes in any way to non compact manifolds ? In particular, I am interested in the case of a smooth vector field on a cylinder $\mathbb{T}\_1\times\mathbb{R}$? If so, are there some additional assumption that one has to impose on a vector field considered (maybe it should vanish outside some compact set or decay very fast "at infinity"?). Sorry if the question is silly - I know the Hodge index theorem only from very elementary sources (Arnold's book on ODEs and Wikipedia). 


Motivation (physical digression): A friend of mine tries to model action of a cardiac tissue in a heart and its soundings. One of his aims is to understand phenomena called "spiral waves" (they are believed to be partially responsible for hearth attacks). I don't know the details but those "spiral waves" can be described by some ODE defined on a domain which is closely related to the real geometry of considered tissue. From the information about indexes of singular points of a corresponding vector field it is possible to deduce some qualitative information about occurrence of this phenomenon. 



Is there a Poincare-Hopf Index theorem for non compact manifolds?
Every noncompact manifold admits nonzero vector fields, or more generally,
vector fields with any specified set of isolated zeros along with the behavior near
that zero. 


However, if you have information of the behavior of a vector field
near infinity, or just in a neighborhood of the boundary of a compact set, there
is an index theorem. Perhaps this is the case with your $\mathbb T^! \times \mathbb R$.


In the particular case of a cylinder, there is a simple way to calculate the index.
Take any compact subcylinder delimited by two circles. Map the cylinder to the plane
minus the origin. Around each of the curves, the vector field has a turning number:
as you go around the curve counterclockwise, the vector field turns by some number
of rotations (counting counterclockwise as positive. The index of the
vector field in the compact subannulus is the difference: the number of turns on
the outer boundary minus the number of turns on the inner boundary.


One way to describe a general formula is this:
 let $N^n$ be manifold, and let
$M, \partial M \subset N$ be a compact submanifold. Let $X$ be a vector field that is
is nonvanishing in a neighborhood of $\partial M$. 


Choose an outward normal vector field $U$ along $\partial M$; now arrange $X$ so
that its direction coincides with $U$ only in isolated points, so if we project
$X$ to $N$ along $U$, it is a vector field with isolated singularities. Let $i\_+(X)$
be the sum of the Poincaré-Hopf indices over all singularities where $X$ is oriented
outward. Then the Poincaré-Hopf index $i(X)$ of $X$ in $M$ equals the Euler characteristic
of $M$ minus $i\_+(X)$.


Here's one proof: triangulate a neighborhood of $M$ so that $\partial M$ is a subcomplex, and so that $X$ is *transverse* to the triangulation except
near the singularities, in the sense that in any simplex, the foliation defined by
$X$ is topologically equivalent to the kernel of a linear map in
general position of the simplex to $\mathbb R^{n-1}$. Put a $+1$ at the barycenter 
of each triangle of even dimension, and a $-1$ at the barycenter of each triangle
of odd dimension. Think of $X$ as a wind that blows these numbers along, so
that after an instant, all numbers (except for exceptions near the zeros of $X$)
are inside an $n$-simplex. In any typical simplex, all the signs cancel out.
However, along the boundary, some of the numbers are blown away and lost.
To regularize the situation, modify $X$ by pushing in the negative normal direction.
Now $X$ points inward everywhere except in a neighborhood of points where
it coincides with the outward normal. Thus everything cancels out except for
local contributions given by $i(X)$ and $i\_+(X)$.


