Homology of a disk bundle over a surface with boundary

algebraic-topologyfiber-bundlessmooth-manifolds

Let $\Sigma_g$, a surface of genus $g$ with a single boundary component, be properly embedded in $M= \mathbb{CP}^2 \setminus \mathbb{B}^4$. Let $X=\nu\, \Sigma_g$ be the tubular neighborhood of the surface with $\chi(\nu\, \Sigma_g) = n$. In particular, we know that $X$ is a $\mathbb{D}^2$-bundle over the surface $\Sigma_g$. We know that such tubular neighborhood exists: how can I describe it? Is it true that $X$ is the closure of $\nu\,\overset{\circ}{\Sigma_g}$ (i.e. is the closure of the tubular neighborhood of the surface with the boundary removed)?

In general, is there a way to compute the homology groups of $X$?

I know that I can compute the homotopy groups of $X$ via the serre fibration long exact sequence, but I have some trouble with using the actual serre's maps the fibration long exact sequence.

Best Answer

Let $D(E) \to B$ be the unit disc bundle of a vector bundle $E$ (with respect to some metric), and denote the image of the zero section $\sigma$ by $Z$.

The continuous map $H : D(E)\times[0,1] \to D(E)$ given by $(e, t) \mapsto (1-t)e$ satisfies

  • $H(\,\cdot\,, 0) = \operatorname{id}_{D(E)}$,
  • $H(e, 1) \in Z$ for all $e \in E$, and
  • $H(z, t) = z$ for all $z \in Z$ and $t \in [0,1]$.

Therefore $H$ is a strong deformation retraction of $D(E)$ onto $Z$. Note that $\pi|_Z : Z \to B$ is a homeomorphism (with inverse given by $\sigma$), so $D(E)$ is homotopy equivalent to $B$.

In your case, it follows that $H_*(X) = H_*(\Sigma_g\setminus D^2)$. As $\Sigma_g\setminus D^2$ deformation retracts onto a bouquet of $2g$ circles, we see that $H_0(X) \cong \mathbb{Z}$, $H_1(X) \cong \mathbb{Z}^{2g}$, and $H_k(X) = 0$ for all other $k$.

Beyond the homotopy type, note that the normal bundle of $\Sigma_g\setminus D^2$ in $\mathbb{CP}^2\setminus D^4$ is an orientable rank two vector bundle over $\Sigma_g\setminus D^2$. As such, it is determined up to isomorphism by its Euler class in $H^2(\Sigma_g\setminus D^2) = 0$. So the normal bundle, and the corresponding disc bundle, is trivial. Therefore $X$ is diffeomorphic to $D^2\times(\Sigma_g\setminus D^2)$.