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Stratifold

## Summary

In differential topology, a branch of mathematics, a stratifold is a generalization of a differentiable manifold where certain kinds of singularities are allowed. More specifically a stratifold is stratified into differentiable manifolds of (possibly) different dimensions. Stratifolds can be used to construct new homology theories. For example, they provide a new geometric model for ordinary homology. The concept of stratifolds was invented by Matthias Kreck. The basic idea is similar to that of a topologically stratified space, but adapted to differential topology.

## Definitions

Before we come to stratifolds, we define a preliminary notion, which captures the minimal notion for a smooth structure on a space: A differential space (in the sense of Sikorski) is a pair ${\displaystyle (X,C),}$ where X is a topological space and C is a subalgebra of the continuous functions ${\displaystyle X\to \mathbb {R} }$ such that a function is in C if it is locally in C and ${\displaystyle g\circ \left(f_{1},\ldots ,f_{n}\right):X\to \mathbb {R} }$ is in C for ${\displaystyle g:\mathbb {R} ^{n}\to \mathbb {R} }$ smooth and ${\displaystyle f_{i}\in C.}$ A simple example takes for X a smooth manifold and for C just the smooth functions.

For a general differential space ${\displaystyle (X,C)}$ and a point x in X we can define as in the case of manifolds a tangent space ${\displaystyle T_{x}X}$ as the vector space of all derivations of function germs at x. Define strata ${\displaystyle X_{i}=\{x\in X:T_{x}X}$ has dimension i${\displaystyle \}.}$ For an n-dimensional manifold M we have that ${\displaystyle M_{n}=M}$ and all other strata are empty. We are now ready for the definition of a stratifold, where more than one stratum may be non-empty:

A k-dimensional stratifold is a differential space ${\displaystyle (S,C),}$ where S is a locally compact Hausdorff space with countable base of topology. All skeleta should be closed. In addition we assume:

The suspension
1. The ${\displaystyle \left(S_{i},C|_{S_{i}}\right)}$ are i-dimensional smooth manifolds.
2. For all x in S, restriction defines an isomorphism of stalks ${\displaystyle C_{x}\to C^{\infty }(S_{i})_{x}.}$
3. All tangent spaces have dimension ≤ k.
4. For each x in S and every neighbourhood U of x, there exists a function ${\displaystyle \rho :U\to \mathbb {R} }$ with ${\displaystyle \rho (x)\neq 0}$ and ${\displaystyle {\text{supp}}(\rho )\subset U}$ (a bump function).

A n-dimensional stratifold is called oriented if its (n − 1)-stratum is empty and its top stratum is oriented. One can also define stratifolds with boundary, the so-called c-stratifolds. One defines them as a pair ${\displaystyle (T,\partial T)}$ of topological spaces such that ${\displaystyle T-\partial T}$ is an n-dimensional stratifold and ${\displaystyle \partial T}$ is an (n − 1)-dimensional stratifold, together with an equivalence class of collars.

An important subclass of stratifolds are the regular stratifolds, which can be roughly characterized as looking locally around a point in the i-stratum like the i-stratum times a (n − i)-dimensional stratifold. This is a condition which is fulfilled in most stratifold one usually encounters.

## Examples

There are plenty of examples of stratifolds. The first example to consider is the open cone over a manifold M. We define a continuous function from S to the reals to be in C if and only if it is smooth on ${\displaystyle M\times (0,1)}$ and it is locally constant around the cone point. The last condition is automatic by point 2 in the definition of a stratifold. We can substitute M by a stratifold S in this construction. The cone is oriented if and only if S is oriented and not zero-dimensional. If we consider the (closed) cone with bottom, we get a stratifold with boundary S.

Other examples for stratifolds are one-point compactifications and suspensions of manifolds, (real) algebraic varieties with only isolated singularities and (finite) simplicial complexes.

## Bordism theories

An example of a bordism relation

In this section, we will assume all stratifolds to be regular. We call two maps ${\displaystyle S,S'\to X}$ from two oriented compact k-dimensional stratifolds into a space X bordant if there exists an oriented (k + 1)-dimensional compact stratifold T with boundary S + (−S') such that the map to X extends to T. The set of equivalence classes of such maps ${\displaystyle S\to X}$ is denoted by ${\displaystyle SH_{k}X.}$ The sets have actually the structure of abelian groups with disjoint union as addition. One can develop enough differential topology of stratifolds to show that these define a homology theory. Clearly, ${\displaystyle SH_{k}({\text{point}})=0}$ for ${\displaystyle k>0}$ since every oriented stratifold S is the boundary of its cone, which is oriented if ${\displaystyle \dim(S)>0.}$ One can show that ${\displaystyle SH_{0}({\text{point}})\cong \mathbb {Z} .}$ Hence, by the Eilenberg–Steenrod uniqueness theorem, ${\displaystyle SH_{k}(X)\cong H_{k}(X)}$ for every space X homotopy-equivalent to a CW-complex, where H denotes singular homology. For other spaces these two homology theories need not be isomorphic (an example is the one-point compactification of the surface of infinite genus).

There is also a simple way to define equivariant homology with the help of stratifolds. Let G be a compact Lie group. We can then define a bordism theory of stratifolds mapping into a space X with a G-action just as above, only that we require all stratifolds to be equipped with an orientation-preserving free G-action and all maps to be G-equivariant. Denote by ${\displaystyle SH_{k}^{G}(X)}$ the bordism classes. One can prove ${\displaystyle SH_{k}^{G}(X)\cong H_{k-\dim(G)}^{G}(X)}$ for every X homotopy equivalent to a CW-complex.

## Connection to the theory of genera

A genus is a ring homomorphism from a bordism ring into another ring. For example, the Euler characteristic defines a ring homomorphism ${\displaystyle \Omega ^{O}({\text{point}})\to \mathbb {Z} /2[t]}$ from the unoriented bordism ring and the signature defines a ring homomorphism ${\displaystyle \Omega ^{SO}({\text{point}})\to \mathbb {Z} [t]}$ from the oriented bordism ring. Here t has in the first case degree 1 and in the second case degree 4, since only manifolds in dimensions divisible by 4 can have non-zero signature. The left hand sides of these homomorphisms are homology theories evaluated at a point. With the help of stratifolds it is possible to construct homology theories such that the right hand sides are these homology theories evaluated at a point, the Euler homology and the Hirzebruch homology respectively.

## Umkehr maps

Suppose, one has a closed embedding ${\displaystyle i:N\hookrightarrow M}$ of manifolds with oriented normal bundle. Then one can define an umkehr map ${\displaystyle H_{k}(M)\to H_{k+\dim(N)-\dim(M)}(N).}$ One possibility is to use stratifolds: represent a class ${\displaystyle x\in H_{k}(M)}$ by a stratifold ${\displaystyle f:S\to M.}$ Then make ƒ transversal to N. The intersection of S and N defines a new stratifold S' with a map to N, which represents a class in ${\displaystyle H_{k+\dim(N)-\dim(M)}(N).}$ It is possible to repeat this construction in the context of an embedding of Hilbert manifolds of finite codimension, which can be used in string topology.

## References

• M. Kreck, Differential Algebraic Topology: From Stratifolds to Exotic Spheres, AMS (2010), ISBN 0-8218-4898-4
• The stratifold page
• Euler homology