When is the kernel of a linear operator closed?

[AxiomOfChoice]

If you consider a bounded linear operator between two Hausdorff topological vector spaces, isn't the kernel *always* closed? I mean, if you assume singleton sets are closed, then the set \{0\} in the image is closed, so that means T^{-1}(\{0\}) is closed, right (since T is assumed continuous)? I keep finding these weird provisos all over the Internet (see, for instance, http://en.wikipedia.org/wiki/Kernel_(linear_algebra )) that require the target space to be finite-dimensional, and I don't see why this is a necessary hypothesis.

 

[micromass]

You are right, if T:V\rightarrow W is continuous and if W is Hausdorff, then T^{-1}({0}) is always closed.

Why the weird proviso's like finite-dimensional? Well, because the result in the wiki is an iff statement. So the statement in the theorem also says that if T^{-1}(\{0\}) is closed, then T is continuous. This is not true in general but only if W is finite dimensional!