Part 1: About this post
This post to be updated throughout the day today, and finished by this evening. UPDATE: Finished with pictures by this weekend.
Based on a conversation I had a few weeks ago, I thought it worthwhile to give an outline the inductive method for calculating of intersection homology I was using last year.
Briefly, we allow closed chains living in the smooth part of the stratified space, and need only conisder whether they should be allowed to “cap off” to the lower strata, which is determined inductively and based on dimension: an already allowable chain, living in the cone over a lower strata is allowed to cap down to the strata if it is the product of an allowable lower strata chain, and the cone of a link of dimension better than half the dimension of the link.
More elaboration on what that means, and some examples later today.
Part 2: Stratified spaces
We consider a topological space
such that
- each
is closed,
is a manifold of dimension k, and
latex X_{n-2}$.
We also may write the space in terms of open pieces
where
.
We also require that each strata
is covered by open sets in
such that each open set
is of the form
where L (called the “link”) is a stratified space depending only on the strata (or possibly on the component of the strata) and
indicates the open cone
.
Part 3: Admissible (co)chains
First, any closed chain that lives entirely in the “smooth” part of our stratified space,
is called admissible. A chain,
, that intersects
will be called admissible if it can be written as the product
where
is an admissible chain (defined inductively) for the space
, and
is a chain in L with sufficiently large dimension (small co-dimension). Sufficiently large dimension isn’t mysterious; for most cases (the standard case I think) we require it to have half the dimension of the link.
Part 4: Eg. Banana Space

The banana space is the torus with one of its belts pinched to a point. So called because one way of drawing it looks like a banana bending around so its tips meet. Also you may call it a circle with two of its antipodes identified.

It has two stratum. One the singular point, and the other of dimension 2 (everything else).

The link, L, over the singular point consists of two circles (one on each side of the banana). No 1-chains can hit the singularity. Only two chains can meet the singularity.
Part 5: Eg. Three Complex 2-Planes

Next we consider the case of three complex hyperplanes complex 3-space. Or rather the one-point compactification (for technical reasons we like working on compact spaces only).
Here the “smooth part” consists of three copies of
, the next strata consists of three copies of
at their intersections, and the final strata consists of two points, the origin and point of compactification.
Over any point in the
the link is two circles, one for each hyperplane. Once again, 1-chains cannot cross the singular stratum. Also no 2-chain can touch unless it wraps around the singular part.
[I need to dig up in my notes, I don't remember what happens near the origin.]
Part 6: Eg. Suspended 3-Torus
This example is the simplest where the link is more than one dimensional. In this case the smooth part is just a thickened three torus. The singular stratum consists of two points, one at each end of the suspension. The link around either of these points is simply a 3-torus.
We allow ourselves to cap off a chain in the 3-torus only if its dimension is 2 or 3. In other words a 1-chain (which is the cone of a 0-chain) is not allowed, only certain 3-chains (the cones of 2-chains) and 4-chains are allowed.
When we take the (co)homology of the resulting intersection (co)chain complex we will get: